Loading...
HomeMy WebLinkAbout20190753 Martino Exterior SSPF Corr 8-7-19 S:tratourti Spriny., Preservation Foundation August 7,2019 Ms.Tamie Ehinger,Chair F 0 R e7elf Design Review Commission PRESERVATION City Hall 474 Broadway , Saratoga Springs,NY 12866 RE: 419-423 Broadway-Exterior Alterations Dear Tamie: The Saratoga Springs Preservation Foundation reviewed the application to make exterior alterations at 419-423 Broadway Street. Board of Directors The building located at 419-423 Broadway was constructed circa 1840 in the Greek Revival Style with engaged square columns. Substantial changes were made to the exterior in the 1870's-an Adam N. Favro Italianate cornice and three-story porch was added(see historic photograph). After 1900,the President three-story porch was removed and the store fronts were brought to the sidewalk and the Colonial James Gold Revival two story porch was added above the storefronts. Vice President The Foundation is pleased with the proposed changes to restore the historic appearance of the Linda Harvey-Opiteck Secretary building. The Foundation is pleased that the windows will be replaced with aluminum clad wood windows with two-over-two windows. Dmitriy Yerrnolayev Treasurer However,the Foundation has concerns about how the brick will be stripped,cleaned,and sealed. Matthew Veitch The owner should follow the guidance provided in the Cleaning and Water Repellent Treatments Past President for Historic Masonry Buildings and Repointing Mortar Joints for Historic Masonry Buildings National Preservation Briefs, see enclosed. Caroline Cardene Giovanna D'Orazio It appears Brennan Drake in photographs that historically the masonry was painted(see enclosed). The building Sandra Fox had several different significant time periods: Greek Revival,Italianate,and Colonial Revival.The John Haller Foundation recommends, if possible,that the owner see if there are any historic layers of paint that Liz Israel may indicate the historic paint colors.The color scheme should be consistent with paint colors that Samantha Kercull were Colonial Revival,should that be the period of significance that the owner wishes to restore Richard King the building. The Foundation respectfully requests the Design Review Commission request a Dorothy Rogers-Bullis paint sample of the"white-that will be used as a bright white may not be appropriate. Cindy Spence Lastly,the Foundation enthusiastically supports the owner moving forward with restoring the James Kettlewell porch as shown in the circa 1920's photograph. The building owner may want to consider seeking emeritus federal and state rehabilitation tax credits for the work being proposed in phase one and phase two. Executive Director The Foundation appreciates the significant investment that the owner is making to preserve their Samantha Bosshart historic building. Membership& Thank you in advance for your thoughtful consideration. Programs Director Nicole Babie Sincerely, 1(14. Favrof Samantha Bosshart President Executive Director Cc: Vincent DiMartino, Owner Robert West,Agent Bradley Birge,Administrator of the Office of Planning and Economic Development 1 12 Spring Street, Suite 203 Saratoga Springs, NY 12866 P 518-387-5030 F5 I 8-581-1448 ,.rte_ ,..•".ager r i4..:... '_: :. :S r•: y` .:_ _ 7cS - n••t _ '`',� ate. 3. "1 - v � •.r i.1 - rte,.• , '�* ..fler' ''1.•' ;. -''4414 01111 . , 1!.i'1,I .• Rill'' . .. . .$ r g {. - t,''' 'f'::' 4 ? n y J i.: 411 . ' t*,....;',';.•-,-:,,,. .4...,,.. ti . . . r ' • • -,,;-.i. r ,x , y- ^ i i 17- _ . •" Il i . .1.,..„;_r......._.7.7,,_. y' r ° • s rI. r' a� J tiIP ..,-,;: r . . - -;,'?.,.. °,„' . -.--. ., 7. ..- •ni . . .. , . _L..___,,,. . ,,t..-,',-',./..' •,;,:4:-':,----------t-- '-'-----.. ---- .:-',_ f_ ... --,.. ---, -..-,.,---.7.7.-- -- .-3 „ j..,,,,,,„ .. -„,... ,..: i, . .._ .: _ .,..,if.... ,, 1. ,- - ''' ,...., iiir,'-'4 .1,,, - - _ .. . --Nqiit..14.- oi,274-14.,,,.,*".,,.0. .. . . .. ,.. ;,.. ..,,,,4,.., _...,„,... .,.. . .:.;' . ' _ ' . ' ,.....:,_ . 1,_ ,•„n., 4.47,7,.........................,44 4, ,F............. .7"..... , . . , ,.,.,. ...._..._ ,.,,,L,, ...„, L '1,...-:'001.1'-' : '''''..), t t i I ,'4',,-.: .1:4 ri 1 '41,,,IP:,-4 ' ''''' ' cl - '''':'''''''''''''''.''''''*.'.,,: :., , L i. 4if. �j � ` • i.. `k •A�•'I# B gyp!r yP y� '4, '. 1 . , ,,,,,1 . . . r,.; •' °411114 -;411t16 ' i '', _ :-,_ , 7-'. .. / jlliVIOPII, ., , ..'"IIVII,, . ..., _ s • 44 . 7" ii 1* • iJ . /1 n _. d, ...4 --,,,l-t. ..., .,-- . -,. ,...,,: - . .,.. /fir...:, ._ , 7,. '-'"'•,6 4. '.3.9' ',.',— , ,::,,,....'-,:,'. t. 7, ,...:i.i...----. i‘• - ' , .... ,,,,Vja...,,.,. i . i (• ... 'I,,. I'.I' i . ..,— 4 4- .,-f , ..,„. .„-i, • .,,,... ..,,• , . . r:- ° i., - • 4, 111 1\--..''' 1 of tt 1 ^ R } 1- tia- F.- I' 13/4 II: 74'.:- ..-- ,,.`:... '...7".1\2-,' _ i - - ._ . 1' "•.Tg. .fir } -: *- lift. ' e A . .i/ i,.'•f - -� ell... t''... '- '� - wY ,4,„ONC*14Acill!411;,PPOW91MiiicIRM,KAPPNWt..... :-. .,-e r ie 4. , A - riv4a,40, A 44,-s,,....illik •,... - •• .„-,, • , •L. -'`. _ "•'_ -/_, q �. Aso` +mak, Air -..,,, in- • ____ _ . •-•• ;.„„, -. , , , ,- ...f. I ' ' AN In III [ . ,r , ...._ _ -,,,,,i.....r_. -.--.-,- :1,47, .,Nrcimaltiortaim r---111021,4111111--110111*--11013:101-113,Mitv.15-- -- " 4 +""n f ,. `;. '1"°`-. gel(• 4' ...-s, 4' -_ �I I 21/40K00.115 ,r,�\l ri�iN E� 10120 d /,`7 /A`l r4'I ;;'. . .. .., ��[ • .. VIII on . ar mg +r r ..i. iw _ e *w ' . ! 4 '�.( .4 }. .a . 1 * * • r• e ± r - , a a. ---. . ..,_, mr. ti 24 _ • 116#714..7- • . ''''' -4, 4:-, :- - . - ,,t- rrM.■r �� , } -w r, ii, ii-. -.. _ u _ It.::. , 4, ..„1„ -,-.,,4,....:7_,:,.....: .7. -r-, -, . - _ _ a<�...r r- - f'ems-b"a _. - 1 w�. • • r .�■/ `phi{#' 11111.',...:4:. -, 7,-...r -• - -...7-------,,,,'• --= --:_-,ate, ,.�-' N.a, a,m ;ter ,,, .-.._.- _ •M,,.--.q-•• , .---.7...4::7.-� • -9':., [ R E S T A Li R A N T ir j. a....: 1... jralerr7trrirli .7,-, ..---•- ----- i.,`..;;f: :.: . 14-1 w " . A i Jl+rv.i - •'Ai•i♦.F.ai. Y :4 - _ _ _ . s. r� s""- r ,. .. - k R it A 1 Pr 4 *r C . L .. . „, , ' -:4., --- .::-.4 Att” .„ - i i ....., .,,_...T.., 1.1 •All ..: i-:.-_7...,' �` ,,. • • Q 1.'`' -'; " 'i�Y 74 r'rr,.f ._- • 4141“1"17 -7': --•6,1i;.-'''' ' : -7 - - -:-. r r e a -- . LtA W #,...Iiiira 6 La•--,j 0. vi ---''-liritt,--,..•.,.:7.,• fill' ha,. t i --r,_ . .. ,r r ,P ,..,'•--.,-" ir e i •_ �"�''M 'r�„;4 Iii a . R V _ .,.' . A i. )111 V - Aim, , , ...tr01 .....1 ,r -- , fes. +* • • 1a .,,,„xiii....,. t ,". ., ..,47.-.4.407. it ......i._,-,-,..., —a,.,.xi . , *4,,. : 1 .. ' ti ..4..�rt. ..r1 risir "' ,' ...,,,,n,_ 4.'44100,...,' '' Y - '.4_41,7 ' tit e. �, • .,,.a , i3t't'' • I ..), ,a, •ii,. r � ......:**,44.4...„ ,,,',iiiillittA th • ' i 11 C. ..PI NEL. . ..;t;', .. ., ,,,,,,,,,-., ' % ' : •• • ;# , .,: -— ...,.:J.. . . .,.. . , , ., .. . , , , , _. . .. —-i, —4:,...,,,„„,.,-,,,.---, ___ .L --,.. . ,. . ._ ri,..,„..,_ . lit '-7..... '. . ' , -,116'.1 r 1 Mielb 11 i •,i' , .,,, \, ,, ii.I:, ._ .....„.„......,7., ..,,,,...4 ...4.,.. . ii,.. . . -$. ,, , ..„, .., is 1 . ., , 1„ , . 1 _ —,,,,...:4,../(1, I.,. ir... :1,,,,, ,, , it _ v F .., , ., \ f ;-7: Alit At it \ ..t .-. .s. 4 4 ,,, •. / , i ..,.....,. ,•• 1„,--if• .- :77_ ..., ,,,,, 04. '. i4i:,..,....„ • -. , 110 .._ ,.. ixtt. . . . ' , * �f 1 ■1. ni � illor'w _.. * %. 0. - 07 ,. 4 t 4 �* lib � �-�. 'til L 111. ; I • .1,01 a . j , . linr.t. lel , .. ; I juk... ...40....._iiiimmi . . it r. . .1,L.;._' '''''.7111 i Ea , , ., . ,,,,,,.., . ,- ,.., -..,,,, 31,..-.. -1,... , , 4,... , • R -1-461V6t, . .. .. ,,, . 'NEP ... r.-.I.' 4 .,i...... !03114 . . 'ga. 4111 .• .... al' lib 41.1 w 41, 10...'11.11111111r- - ' ' '114..•:„.....,,.,....... . lialliori -,a, ... . illix........ _tr. i -..... .. i ta - .. +kms •. + w 44' . : -'1_,„. - ''1' -. ,t‘ ..... ,.. e..... IT ..4.-.., -..., ',iir- il. r . t .. ., -. f L-,,., , . • . . , f , • a r„, , 1 Gt... -416 ti s. ting y 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings National Park S-3i ite Technical Preservation Su.S.[epai i77ant°of iti interior Home > How to Preserve > Preservation Briefs > 2 Repointing Mortar Joints Some of the web versions of the Preservation Briefs differ somewhat from the printed versions. Many illustrations are new and in color; Captions are simplified and some complex charts are omitted.To order hard copies of the Briefs,see Jrinted PublicationsG'. PRESERVATION BRIEFS 2 Repointing Mortar Joints in Historic Masonry Buildings Robert C. Mack, FAIA, and John P. Speweik Historical Background 9 4 Identifying the Problem Before Repointing, Finding an Appropriate Mortar Match Properties of Mortar Mortar Analysis Soft mortar for Components of Mortar repointing.Photo:John P.Speweik. Mortar Type and Mix Budgeting and Scheduling, Contractor Selection Execution of the Work Visually Examining the Mortar and the Masonry Units Summary and References Reading List Download the PDF El Masonry—brick, stone,terra-cotta, and concrete block—is found on nearly every historic building. Structures with all-masonry exteriors come to mind immediately, but most other buildings at least have masonry foundations or chimneys. Although generally considered "permanent," masonry is subject to deterioration, especially at the mortar joints. Repointing, also known simply as "pointing"or—somewhat inaccurately—"tuck pointing"*, is the process of removing deteriorated mortar from the joints of a masonry wall and replacing it with new mortar. Properly done, repointing restores the visual and physical integrity of the masonry. Improperly done, repointing not only detracts from the appearance of the building, but may also cause physical damage to the masonry units themselves. The purpose of this Brief is to provide general guidance on appropriate materials and methods for repointing historic masonry buildings and it is intended to benefit building owners, architects, and contractors. The Brief should serve as a guide to prepare specifications for repointing historic masonry buildings. It should also help develop sensitivity to the particular needs of historic masonry, and to assist historic building owners in working cooperatively with architects, architectural conservators and historic preservation consultants, and contractors. Although specifically intended for historic buildings, the guidance is appropriate for other masonry buildings as well. This publication updates Preservation Briefs 2: Repointing Mortar Joints in Historic Brick Buildings to include all types of historic unit masonry. The scope of the earlier Brief has also been expanded to acknowledge that the many buildings constructed in the first half of the 20th century are now https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 1/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings historic and eligible for listing in the National Register of Historic Places, and that they may have been originally constructed with portland cement mortar. *Tuckpointing technically describes a primarily decorative application of a raised mortar joint or lime putty joint on top of flush mortar joints. Historical Background Mortar consisting primarily of lime and sand has been used as an integral part of masonry structures for thousands of years. Up until about the mid-19th century, lime or quicklime (sometimes called lump lime) was delivered to construction sites, where it had to be slaked, or combined with water. Mixing with water caused it to boil and resulted in a wet lime putty that was left to mature in a pit or wooden box for several weeks, up to a year. Traditional mortar was made from lime putty, or slaked lime, combined with local sand, generally in a ratio of 1 part lime putty to 3 parts sand by volume. Often other ingredients, such as crushed marine shells (another source of lime), brick dust, clay, natural cements, pigments, and even animal hair were also added to mortar, but the basic formulation for lime putty and sand mortar remained unchanged for centuries until the advent of portland cement or its forerunner, Roman cement, a natural, hydraulic cement. Portland cement was patented in Great Britain in 1824. It was named after the stone from Portland in Dorset which it resembled when hard. This is a fast-curing, hydraulic cement which hardens under water. Portland cement was first manufactured in the United States in 1872, although it was imported before this date. But it was not in common use throughout the country until the early 20th century. Up until the turn of the century portland cement was considered primarily an additive, or "minor ingredient" to help accelerate mortar set time. By the 1930s, however, most masons used a mix of equal parts portland cement and lime putty. Thus, the mortar found in masonry structures built between 1873 and 1930 can range from pure lime and sand mixes to a wide variety of lime, portland cement, and sand combinations. In the 1930s more new mortar products intended to hasten and simplify masons' work were introduced in the U.S. These included masonry cement, a premixed, bagged mortar which is a combination of portland cement and ground limestone, and hydrated lime, machine-slaked lime that eliminated the necessity of slaking quicklime into putty at the site. Identifying the Problem Before Repointing The decision to repoint is most often related to some obvious sign of deterioration, such as disintegrating mortar, cracks in mortar joints, loose bricks or stones, damp walls, or damaged plasterwork. It is, however, erroneous to assume that repointing alone will solve deficiencies that result from other problems. The root cause of the deterioration—leaking roofs or gutters, differential settlement of the building, capillary action causing rising damp, or extreme weather exposure—should always be dealt with prior to beginning work. Without appropriate repairs to eliminate the source of the problem, mortar deterioration will continue and any repointing will have been a waste of time and �. - money. Use of Consultants Because there are so manypossible causes for deterioration in historic buildings, s g it may be desirable to retain a consultant, such as a historic architect or 11 architectural conservator, to analyze the building. In addition to determining the most appropriate solutions to the problems, a consultant can prepare specifications which reflect the particular requirements of each job and can Masons practice using lime putty mortar to repair provide oversight of the work in progress. Referrals to preservation consultants historic marble.Photo: NPS files. frequently can be obtained from State Historic Preservation Offices, the American Institute for Conservation of Historic and Artistic Works (AIC), the Association for Preservation Technology (APT), and local chapters of the American Institute of Architects (AIA). Finding an Appropriate Mortar Match Preliminary research is necessary to ensure that the proposed repointing work is both physically and visually appropriate to the building. Analysis of unweathered portions of the historic mortar to which the new mortar will be matched can suggest appropriate mixes for the repointing mortar so that it will not damage the building because it is excessively strong or vapor impermeable. Examination and analysis of the masonry units—brick, stone or terra cotta—and the techniques used in the original construction will assist in maintaining the building's historic appearance. A simple, non- technical, evaluation of the masonry units and mortar can provide information concerning the relative strength and permeability of each—critical https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 2/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings factors in selecting the repointing mortar—while a visual analysis of the historic mortar can provide the information necessary for developing the new mortar mix and . _ T � -,.• application techniques. •aR• rf - Although not crucial to a successful repointing pro ect for projects involving pro erties *�� of special historic significance, a mortar analysis by a qualified laboratory can be useful •a!'' a by providing information on the original ingredients. However, there are limitations with rT- r such an analysis, and replacement mortar specifications should not be based solely on a f laboratory analysis. Analysis requires interpretation and there are important factors • which affect the condition and performance of the mortar that cannot be established -_ - .," through laboratory analysis. These may include: the original water content, rate of This late 19th century granite has recently been repointed with the joint curing, weather conditions during original construction, the method of mixing and profile and mortar color carefully placing the mortar, and the cleanliness and condition of the sand. The most useful matched to the original.Photo: NPS files. information that can come out of laboratory analysis is the identification of sand by gradation and color. This allows the color and the texture of the mortar to be matched with some accuracy because sand is the largest ingredient by volume. In creating a repointing mortar that is compatible with the masonry units, the objective is to achieve one that matches the historic mortar as closely as possible, so that the new material can coexist with the old in a sympathetic, supportive and, if necessary, sacrificial capacity. The exact physical and chemical properties of the historic mortar are not of major significance as long as the new mortar conforms to the following criteria: • The new mortar must match the historic mortar in color,texture and tooling. (If a laboratory analysis is undertaken, it may be possible to match the binder components and their proportions with the historic mortar, if those materials are available.) • The sand must match the sand in the historic mortar. (The color and texture of the new mortar will usually fall into place if the sand is matched successfully.) • The new mortar must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units. • The new mortar must be as vapor permeable and as soft or softer (measured in compressive strength) than the historic mortar. (Softness or hardness is not necessarily an indication of permeability; old, hard lime mortars can still retain high permeability.) Mortar Analysis Methods for analyzing mortars can be divided into two broad categories: wet chemical and instrumental. Many laboratories that analyze historic mortars use a simple wet-chemical method � called acid digestion, whereby a sample of the mortar is crushed and then mixed with a dilute acid. The acid dissolves all the carbonate-containing minerals not only in the binder, but also in the aggregate (such as oyster shells, coral sands, or other carbonate-based materials), as well as any other acid-soluble materials. The sand and fine-grained acid-insoluble material is left behind. There are several variations on the simple acid digestion test. One involves collecting the carbon dioxide gas given off as the carbonate is digested by the acid; based on the gas volume the carbnate content of the mortar can be accurately determined (Jedrzejewska, 1960). Simple acid digestion methods .i are rapid, inexpensive, and easy to perform, but the information they provide about the original This mortar is the composition of a mortar is limited to the color and texture of the sand. The gas collection method proper consistency for provides more information about the binder than a simple acid digestion test. repointing historic brick. Photo:John P.Speweik. Instrumental analysis methods that have been used to evaluate mortars include polarized light or thin-section microscopy, scanning electron microscopy, atomic absorption spectroscopy, X-ray diffraction, and differential thermal analysis. All instrumental methods require not only expensive, specialized equipment, but also highly-trained experienced analysts. However, instrumental methods can provide much more information about a mortar. Thin-section microscopy is probably the most commonly used instrumental method. Examination of thin slices of a mortar in transmitted light is often used to supplement acid digestion methods, particularly to look for carbonate-based aggregate. For example, the new ASTM test method, ASTM C 1324-96 "Test Method for Examination and Analysis of Hardened Mortars" which was designed specifically for the analysis of modern lime-cement and masonry cement mortars, combines a complex series of wet chemical analyses with thin-section microscopy. The drawback of most mortar analysis methods is that mortar samples of known composition have not been analyzed in order to evaluate the method. Historic mortars were not prepared to narrowly defined specifications from materials of uniform quality; they contain a wide array of locally derived materials combined at the discretion of the mason. While a https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 3/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings particular method might be able to accurately determine the original proportions of a lime-cement-sand mortar prepared from modern materials, the usefulness of that method for evaluating historic mortars is questionable unless it has been tested against mortars prepared from materials more commonly used in the past. Properties of Mortar Mortars for repointing should be softer or more permeable than the masonry units and no harder or more impermeable than the historic mortar to prevent damage to the masonry units. It is a common error to assume that hardness or high strength is a measure of appropriateness, particularly for lime-based historic mortars. Stresses within a wall caused by expansion, contraction, moisture migration, or settlement must be accommodated in some manner; in a masonry wall, these stresses should be relieved by the mortar rather than by the masonry units. A mortar that is stronger in compressive strength than the masonry units will not "give," thus causing stresses to be relieved through the masonry units—resulting in permanent damage to the masonry, such as cracking and spalling, that cannot be repaired easily. While stresses can also break the bond between the mortar and the masonry units, permitting water to penetrate the resulting hairline cracks, this is easier to correct in the joint through repointing than if the break occurs in the masonry units. Permeability, or rate of vapor transmission, is also critical. High lime mortars are more • permeable than denser cement mortars. Historically, mortar acted as a bedding material—not . unlike an expansion joint—rather than a "glue" for the masonry units, and moisture was able to migrate through the mortar joints rather than the masonry units. When moisture evaporates 4 from the masonry it deposits any soluble salts either on the surface as efflorescence or below -- the surface as subflorescence. While salts deposited on the surface of masonry units are usually relatively harmless, salt crystallization within a masonry unit creates pressure that can cause parts ofthe outer surface to spall off or delaminate. If the mortar does not permitmoisture or moisture vapor to migrate out of the wall and evaporate, theresult will be damage to the : , , masonry units. This early 19th century building is being repointed with lime Components of Mortar mortar. Photo:Travis McDonald. Sand Sand is the largest component of mortar and the material that gives mortar its distinctive color, texture and cohesiveness. Sand must be free of impurities, such as salts or clay. The three key characteristics of sand are: particle shape, gradation and void ratios. When viewed under a magnifying glass or low-power microscope, particles of sand generally have either rounded edges, such as found in beach and river sand, or sharp, angular edges, found in crushed or manufactured sand. For repointing mortar, rounded or natural sand is preferred for two reasons. It is usually similar to the sand in the historic mortar and provides a better visual match. It also has better working qualities or plasticity and can thus be forced into the joint more easily, forming a good contact with the remaining historic mortar and the surface of the adjacent masonry units. Although manufactured sand is frequently more readily available, it is usually possible to locate a supply of rounded sand. The gradation of the sand (particle size distribution) plays a very important role in the durability and cohesive properties of a mortar. Mortar must have a certain percentage of large to small particle sizes in order to deliver the optimum performance. Acceptable guidelines on particle size distribution may be found in ASTM C 144 (American Society for Testing and Materials). However, in actuality, since neither historic nor modern sands are always in compliance with ASTM C 144, matching the same particle appearance and gradation usually requires sieving the sand. A scoop of sand contains many small voids between the individual grains. A mortar that performs well fills all these small voids with binder (cement/lime combination or mix) in a balanced manner. Well-graded sand generally has a 30 per cent void ratio by volume. Thus, 30 per cent binder by volume generally should be used, unless the historic mortar had a different binder: aggregate ratio. This represents the 1:3 binder to sand ratios often seen in mortar specifications. For repointing, sand generally should conform to ASTM C 144 to assure proper gradation and freedom from impurities; some variation may be necessary to match the original size and gradation. Sand color and texture also should match the original as closely as possible to provide the proper color match without other additives. Lime Mortar formulations prior to the late-19th century used lime as the primary binding material. Lime is derived from heating limestone at high temperatures which burns off the carbon dioxide, and turns the limestone into quicklime. There are three types of limestone—calcium, magnesium, and dolomitic—differentiated by the different levels of magnesium carbonate they https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 4/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings contain which impart specific qualities to mortar. Historically, calcium lime was used for mortar rather than the dolomitic lime (calcium magnesium carbonate) most often used today. But it is also important to keep in mind the fact that the historic limes, and other components of mortar, varied a great deal because they were natural, as opposed to modern lime which is manufactured and, therefore, standardized. Because some of the kinds of lime, as well as other components of mortar, that were used historically are no longer readily available, even when a conscious effort is made to replicate a "historic" mix, this may not be achievable due to the differences between modern and historic materials. Lime, itself, when mixed with water into a paste is very plastic and creamy. It will remain workable and soft indefinitely, if stored in a sealed container. Limey (calcium hydroxide) hardens by carbonation absorbing carbon dioxide primarily from the air, converting itself to calcium carbonate. Once a lime and sand mortar is mixed and placed in a wall, it begins the process of carbonation. If lime mortar is left to dry too rapidly, carbonation of the mortar will be reduced, resulting in _ poor adhesion and poor durability. In addition, lime mortar is slightly water soluble and thus is able to re-seal any hairline cracks that may develop during the life of the mortar. Lime mortar is soft, porous, and changes little in volume e' 011141 during temperature fluctuations thus making it a good choice for historic a . ` buildings. Because of these qualities, high calcium lime mortar may be Caulking was inappropriately used here in place of considered for many repointing projects, not just those involving historic mortar on the top of the wall.As a result,it has not been durable.Photo: NPS files. buildings. For repointing, lime should conform to ASTM C 207, Type S, or Type SA, Hydrated Lime for Masonry Purposes. This machine-slaked lime is designed to assure high plasticity and water retention. The use of quicklime which must be slaked and soaked by hand may have advantages over hydrated lime in some restoration projects if time and money allow. Lime Putty Lime putty is slaked lime that has a putty or paste-like consistency. It should conform to ASTM C 5. Mortar can be mixed using lime putty according to ASTM C 270 property or proportion specification. Portland Cement More recent, 20th-century mortar has used portland cement as a primary binding material. A straight portland cement and sand mortar is extremely hard, resists the movement of water, shrinks upon setting, and undergoes relatively large thermal movements. When mixed with water, portland cement forms a harsh, stiff paste that is quite unworkable, becoming hard very quickly. (Unlike lime, portland cement will harden regardless of weather conditions and does not require wetting and drying cycles.) Some portland cement assists the workability and plasticity of the mortar without adversely affecting the finished project; it also provides early strength to the mortar and speeds setting. Thus, it may be appropriate to add some portland cement to an essentially lime-based mortar even when repointing relatively soft 18th or 19th century brick under some circumstances when a slightly harder mortar is required. The more portland cement that is added to a mortar formulation the harder it becomes—and the faster the initial set. For repointing, portland cement should conform to ASTM C 150. White, non- staining portland cement may provide a better color match for some historic mortars than the more commonly available grey portland cement. But, it should not be assumed, however, that white portland cement is always appropriate for all historic buildings, since the original mortar may have been mixed with grey cement. The cement should not have more than 0.60 per cent alkali to help avoid efflorescence. Masonry Cement Masonry cement is a preblended mortar mix commonly found at hardware and home repair stores. It is designed to produce mortars with a compressive strength of 750 psi or higher when mixed with sand and water at the job site. It may contain hydrated lime, but it always contains a large amount of portland cement, as well as ground limestone and other workability agents, including air-entraining agents. Because masonry cements are not required to contain hydrated lime, and generally do not contain lime, they produce high strength mortars that can damage historic masonry. For this reason, they generally are not recommended for use on historic masonry buildings. Lime Mortar (pre-blended) Hydrated lime mortars, and pre-blended lime putty mortars with or without a matched sand are commercially available. Custom mortars are also available with color. In most instances, pre-blended lime mortars containing sand may not provide an exact match; however, if the project calls for total repointing, a pre-blended lime mortar may be worth considering as long as the mortar is compatible in strength with the masonry. If the project involves only selected, "spot" repointing, then it may be better to carry out a mortar analysis which can provide a custom pre-blended lime mortar with a matching sand. https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 5/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings In either case, if a preblended lime mortar is to be used, it should contain Type S or SA hydrated lime conforming to ASTM C 207. Water Water should be potable—clean and free from acids, alkalis, or other dissolved organic materials. Other Components Historic components In addition to the color of the sand, the texture of the mortar is of critical importance in duplicating historic mortar. Most mortars dating from the mid-19th century on—with some exceptions—have a fairly homogeneous texture and color. Some earlier mortars are not as uniformly textured and may contain lumps of partially burned lime or "dirty lime", shell (which often provided a source of lime, particularly in coastal areas), natural cements, pieces of clay, lampblack or other pigments, or even animal hair. The visual characteristics of these mortars can be duplicated through the use of similar materials in the repointing mortar. Replicating such unique or individual mortars will require writing new specifications for each project. If possible, suggested sources for special materials should be included. For example, crushed oyster shells can be obtained in a variety of sizes from poultry supply dealers. Pigments Some historic mortars, particularly in the late 19th century, were tinted to match or contrast with the brick or stone. Red pigments, sometimes in the form of brick dust, as well as brown, and black pigments were commonly used. Modern pigments are available which can be added to the mortar at the job site, but they should not exceed 10 per cent by weight of the portland cement in the mix, and carbon black should be limited to 2 per cent. Only synthetic mineral oxides, which are alkali-proof and sun-fast, should be used to prevent bleaching and fading. Modern Components Admixtures are used to create specific characteristics in mortar, and whether they should be used will depend upon the individual project. Air entraining agents, for example, help the mortar to resist freeze-thaw damage in northern climates. Accelerators are used to reduce mortar freezing prior to setting while retarders help to extend the mortar life in hot climates. Selection of admixtures should be made by the architect or architectural conservator as part of the specifications, not something routinely added by the masons. Generally, modern chemical additives are unnecessary and may, in fact, have detrimental effects in historic masonry projects. The use of antifreeze compounds is not recommended. They are not very effective with high lime mortars and may introduce salts, which may cause efflorescence later. A better practice is to warm the sand and water, and to protect the completed work from freezing. No definitive study has determined whether air-entraining additives should be used to resist frost action and enhance plasticity, but in areas of extreme exposure requiring high-strength mortars with lower permeability, air-entrainment of 10-16 percent may be desirable (see formula for"severe weather exposure" in Mortar Type and Mix). Bonding agents are not a substitute for proper joint preparation, and they should generally be avoided. If the joint is properly prepared, there will be a good bond between the new mortar and the adjacent surfaces. In addition, a bonding agent is difficult to remove if smeared on a masonry surface. Mortar Type and Mix Mortars for repointing projects, especially those involving historic buildings, typically are custom mixed in order to ensure the proper physical and visual qualities. These materials can be combined in varying proportions to create a mortar with the desired performance and durability. The actual specification of a particular mortar type should take into consideration all of the factors affecting the life of the building including: current site conditions, present condition of the masonry, function of the new mortar, degree of weather exposure, and skill of the mason. Thus, no two repointing projects are exactly the same. Modern materials specified for use in repointing mortar should conform to specifications of the American Society for Testing and Materials (ASTM) or comparable federal specifications, and the resulting mortar should conform to ASTM C 270, Mortar for Unit Masonry. Specifying the proportions for the repointing mortar for a specific job is not as difficult as it might seem. Five mortar types, each with a corresponding recommended mix, have been established by ASTM to distinguish high strength mortar from soft flexible mortars. The ASTM designated them in decreasing order of approximate general strength as Type M (2,500 psi), Type S (1,800 psi), Type N (750 psi),Type 0 (350 psi) and Type K (75 psi). (The letters identifying the types are from the words MASON WORK using every other letter.) Type K has the highest lime content of the mixes that contain portland cement, although it is seldom used today, except for some historic preservation projects. The designation "L" in the accompanying chart identifies a straight lime and sand mix. Specifying the appropriate ASTM mortar by proportion of https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 6/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings ingredients, will ensure the desired physical properties. Unless specified otherwise, measurements or - proportions for mortar mixes are always given in the following order: cement-lime-sand. Thus, a Type K mix, for example, would be referred to as 1-3-10, or 1 part cement to 3 parts lime to 10 ff parts sand. Other requirements to create the desired visual qualities should be included in the R r specifications. The strength of a mortar can vary. If mixed with higher amounts of portland cement, a harder - mortar is obtained. The more lime that is added, the softer and more plastic the mortar becomes, -w increasing its workability. A mortar strong in compressive strength might be desirable for a hard stone (such as granite) pier holding up a bridge deck, whereas a softer, more permeable lime mortar would be preferable for a historic wall of soft brick. Masonry deterioration caused by salt deposition Here,a hammer and results when the mortar is less permeable than the masonry unit. A strong mortar is still more chisel are being correctly permeable than hard, dense stone. However, in a wall constructed of soft bricks where the masonry used to prepare a joint for repointing. Photo:John P. unit itself has a relatively high permeability or vapor transmission rate, a soft, high lime mortar is Speweik. necessary to retain sufficient permeability. Budgeting and Scheduling Repointing is both expensive and time consuming due to the extent of handwork and special materials required. It is preferable to repoint only those areas that require work rather than an entire wall, as is often specified. But, if 25 to 50 per cent or more of a wall needs to be repointed, repointing the entire wall may be more cost effective than spot repointing. Total repointing may also be more sensible when access is difficult, requiring the erection of expensive scaffolding (unless the majority of the mortar is sound and unlikely to require replacement in the foreseeable future). Each project requires . judgement based on a variety of factors. Recognizing this at the outset will help to • z prevent many jobs from becoming prohibitively expensive. In scheduling, seasonal aspects need to be considered first. Generally speaking, I wall temperatures between 40 and 95 degrees F (8 and 38 degrees C) will prevent76, freezing or excessive evaporation of the water in the mortar. Ideally, repointing should be done in shade, away from strong sunlight in order to slow the drying 417 • l process, especially during hot weather. If necessary, shade can be provided for When repairing this stone wall,the mason large-scale projects with appropriate modifications to scaffolding. matched the raised profile of the original tuckpointing.Photo: NPS files. The relationship of repointing to other work proposed on the building must also be recognized. For example, if paint removal or cleaning is anticipated, and if the mortar joints are basically sound and need only selective repointing, it is generally better to postpone repointing until after completion of these activities. However, if the mortar has eroded badly, allowing moisture to penetrate deeply into the wall, repointing should be accomplished before cleaning. Related work, such as structural or roof repairs, should be scheduled so that they do not interfere with repointing and so that all work can take maximum advantage of erected scaffolding. Building managers also must recognize the difficulties that a repointing project can create.The process is time consuming, and scaffolding may need to remain in place for an extended period of time. The joint preparation process can be quite noisy and can generate large quantities of dust 1 which must be controlled, especially at air intakes to protect human health, and also where it might damage operating machinery. Entrances may be blocked from time to time making access difficult urine for both building tenants and visitors. Clearly, building managers will need to coordinate the I'' ,. • repointing work with other events at the site Contractor Selection i Contractor Selection The ideal way to select a contractor is to ask knowledgeable owners of recently A mechanical grinder repointed historic buildings for recommendations. Qualified contractors then can provide lists of improperly used to cut out other repointing projects for inspection. More commonly, however, the contractor for a repointing the horizontal joint and incompatible repointing project is selected through a competitive bidding process over which the client or consultant has only have seriously damaged limited control. In this situation it is important to ensure that the specifications stipulate that masons the 19th century brick. Photo: NPS files. must have a minimum of five years' experience with repointing historic masonry buildings to be eligible to bid on the project. Contracts are awarded to the lowest responsible bidder, and bidders who have performed poorly on other projects usually can be eliminated from consideration on this basis, even if they have the lowest prices. https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 7/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings The contract documents should call for unit prices as well as a base bid. Unit pricing forces the contractor to determine in advance what the cost addition or reduction will be for work which varies from the scope of the base bid. If, for example, the contractor has fifty linear feet less of stone repointing than indicated on the contract documents but thirty linear feet more of brick repointing, it will be easy to determine the final price for the work. Note that each type of work—brick repointing, stone repointing, or similar items—will have its own unit price. The unit price also should reflect quantities; one linear foot of pointing in five different spots will be more expensive than five contiguous linear feet. Execution of the Work Test Panels These panels are prepared by the contractor using the same techniques that will be used on the remainder of the project. Several panel locations—preferably not on the front or other highly visible location of the building—may be necessary to include all types of masonry,joint styles, mortar colors, and other problems likely to be encountered on the job. If cleaning tests, for example, are also to be undertaken, they should be carried out in the ,N same location. Usually a 3 foot by 3 foot area is sufficient for brickwork, while a somewhat larger area may be required for stonework. These panels establish an acceptable standard of :.iit.i work and serve as a benchmark for evaluating and accepting subsequent work on the building. Joint Preparation r Old mortar should be removed to a minimum depth of 2 to 2-1/2 times the width of the joint to V ``'' ensure an adequate bond and to prevent mortar"popouts." For most brick joints, this will iA • require removal of the mortar to a depth of approximately Q to 1 inch; for stone masonry with wide joints, mortar may need to be removed to a depth of several inches. Any loose or disintegrated mortar beyond this minimum depth also should be removed. Unskilled repointing has Although some damage may be inevitable, careful joint preparation can help limit damage to negatively impacted the character of this late-19th century building. masonry units. The traditional manner of removing old mortar is through the use of hand Photo: NPS files. chisels and mash hammers. Though labor-intensive, in most instances this method poses the least threat for damage to historic masonry units and produces the best final product. The most common method of removing mortar, however, is through the use of power saws or grinders. The use of power tools by unskilled masons can be disastrous for historic masonry, particularly soft brick. Using power saws on walls with thin joints, such as most brick walls, almost always will result in damage to the masonry units by breaking the edges and by overcutting on the head, or vertical joints. However, small pneumatically-powered chisels generally can be used safely and effectively to remove mortar on historic buildings as long as the masons maintain appropriate control over the equipment. Under certain circumstances, thin diamond-bladed grinders may be used to cut out horizontal joints only on hard portland cement mortar common to most early-20th century masonry buildings. Usually, automatic tools most successfully remove old mortar without damaging the masonry units when they are used in combination with hand tools in preparation for repointing. Where horizontal joints are uniform and fairly wide, it may be possible to use a power masonry saw to assist the removal of mortar, such as by cutting along the middle of the joint; final mortar removal from the sides of the joints still should be done with a hand chisel and hammer. Caulking cutters with diamond blades can sometimes be used successfully to cut out joints without damaging the masonry. Caulking cutters are slow; they do not rotate, but vibrate at very high speeds, thus minimizing the possibility of damage to masonry units. Although mechanical tools may be safely used in limited circumstances to cut out horizontal joints in preparation for repointing, they should never be used on vertical joints because of the danger of slipping and cutting into the brick above or below the vertical joint. Using power tools to remove mortar without damaging the surrounding masonry units also necessitates highly skilled masons experienced in working on historic masonry buildings. Contractors should demonstrate proficiency with power tools before their use is approved. Using any of these power tools may also be more acceptable on hard stone, such as quartzite or granite, than on terra cotta with its glass-like glaze, or on soft brick or stone. The test panel should determine the acceptability of power tools. If power tools are to be permitted, the contractor should establish a quality control program to account for worker fatigue and similar variables. Mortar should be removed cleanly from the masonry units, leaving square corners at the back of the cut. Before filling, the joints should be rinsed with a jet of water to remove all loose particles and dust. At the time of filling, the joints should be damp, but with no standing water present. For masonry walls—limestone, sandstone and common brick—that are extremely absorbent, it is recommended that a continual mist of water be applied for a few hours before repointing begins. Mortar Preparation https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 8/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings Mortar components should be measured and mixed carefully to assure the uniformity of visual and physical characteristics. Dry ingredients are measured by volume and thoroughly mixed before the addition of any water. Sand must be added in a damp, loose condition to avoid over sanding. Repointing mortar is typically pre-hydrated by adding water so it will just hold together, thus allowing it to stand for a period of time before the final water is added. Half the water should be added, followed by mixing for approximately 5 minutes. The remaining water should then be added in small portions until a mortar of the desired consistency is reached. The total volume of water necessary may vary from batch to batch, depending on weather conditions. It is important to keep the water to a minimum for two reasons: first, a drier mortar is cleaner to work with, and it can be compacted tightly into the joints; second, with no excess water to evaporate, the mortar cures without shrinkage cracks. Mortar should be used within approximately 30 minutes of final mixing, and "retempering," or adding more water, should not be permitted. Using Lime Putty to Make Mortar Mortar made with lime putty and sand, sometimes referred to as roughage or course stuff, should be measured by volume, and may require slightly different proportions from those used with hydrated lime. No additional water is usually needed to achieve a workable consistency because enough water is already contained in the putty. Sand is proportioned first, followed by the lime putty, then mixed for five minutes or until all the sand is thoroughly coated with the lime putty. But mixing, in the familiar sense of turning over with a hoe, sometimes may not be sufficient if the best possible performance is to be obtained from a lime putty mortar. Although the old practice of chopping, beating and ramming the mortar has largely been forgotten, recent field work has confirmed that lime putty and sand rammed and beaten with a wooden mallet or ax handle, interspersed by chopping with a hoe, can significantly improve workability and performance. The intensity of this action increases the overall lime/sand contact and removes any surplus water by compacting the other ingredients. It may also be advantageous for larger projects to use a mortar pan mill for mixing. Mortar pan mills which have a long tradition in Europe produce a superior lime putty mortar not attainable with today's modern paddle and drum type mixers. For larger repointing projects the lime putty and sand can be mixed together ahead of time and stored indefinitely, on or off site, which eliminates the need for piles of sand on the job site. This mixture, which resembles damp brown sugar, must be protected from the air in sealed containers with a wet piece of burlap over the top or sealed in a large plastic bag to prevent evaporation and premature carbonation. The lime putty and sand mixture can be recombined into a workable plastic state months later with no additional water. If portland cement is specified in a lime putty and sand mortar—Type 0 (1:2:9) or Type K (1:3:11)—the portland cement should first be mixed into a slurry paste before adding it to the lime putty and sand. Not only will this ensure that the portland cement is evenly distributed throughout the mixture, but if dry portland cement is added to wet ingredients it tends to "ball up,"jeopardizing dispersion. (Usually water must be added to the lime putty and sand anyway once the portland cement is introduced.) Any color pigments should be added at this stage and mixed for a full five minutes. The mortar should be used within 30 minutes to 1S2 hours and it should not be retempered. Once portland cement has been added the mortar can no longer be stored. Filling the Joint Where existing mortar has been removed to a depth of greater than 1 inch, these deeper areas should be filled first, compacting the new mortar in several layers. The back of the entire joint should be filled successively by applying approximately 1/4 inch of mortar, packing it well into the back corners. This application may extend along the wall for several feet. As soon as the mortar has reached thumb-print hardness, another 1/4 inch layer of mortar—approximately the same thickness—may be applied. Several layers will be needed to fill the joint flush with the outer surface of the masonry. It is important to allow each layer time to harden before the next layer is applied; most of the mortar shrinkage occurs during the hardening process and layering thus minimizes overall shrinkage. When the final layer of mortar is thumb-print hard, the joint should be tooled to match the historic joint. Proper timing of the tooling is important for uniform color and appearance. If tooled when too soft, the color will be lighter than expected, and hairline cracks may occur; if tooled when too hard, there may be dark streaks called "tool burning," and good closure of the mortar against the masonry units will not be achieved. If the old bricks or stones have worn, rounded edges, it is best to recess the final mortar slightly from the face of the masonry. This treatment will help avoid a joint which is visually wider than the actual joint; it also will avoid creation of a large, thin featheredge which is easily damaged, thus admitting water. After tooling, excess mortar can be removed from the edge of the joint by brushing with a natural bristle or nylon brush. Metal bristle brushes should never be used on historic masonry. Curing Conditions https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 9/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings The preliminary hardening of high-lime content mortars—those mortars that contain more lime by volume than portland cement, i.e., Type 0 (1:2:9),Type K (1:3:11), and straight lime/sand, Type "L" (0:1:3)—takes place fairly rapidly as water in the mix is lost to the porous surface of the masonry and through evaporation. A high lime mortar (especially Type "L") left to dry out too rapidly can result in chalking, poor adhesion, and poor durability. Periodic wetting of the repointed area after the mortar joints are thumb-print hard and have been finish tooled may significantly accelerate the carbonation process. When feasible, misting using a hand sprayer with a fine nozzle can be simple to do for a day or two after repointing. Local conditions will dictate the frequency of wetting, but initially it may be as often as every hour and gradually reduced to every three or four hours. Walls should be covered with burlap for the first three days after repointing. (Plastic may be used, but it should be tented out and not placed directly against the wall.) This helps keep the walls damp and protects them from direct sunlight. Once carbonation of the lime has begun, it will continue for many years and the lime will gain strength as it reverts back to calcium carbonate within the wall. Aging the Mortar Even with the best efforts at matching the existing mortar color, texture, and • •i• materials/ usuallythere will be a visible difference between the old and new work, ter . 7 r i - partly because the new mortar has been matched to the unweathered portions of the � _ historic mortar. Another reason for a slight mismatch may be that the sand is more ` '' exposed in old mortar due to the slight erosion of the lime or cement. Although spot • '- - ri,114 repointing is generally preferable and some color difference should be acceptable, if the difference between old and new mortar is too extreme, it may be advisable in - some instances to repoint an entire area of a wall, or an entire feature such as a bay, .$ to minimize the difference between the old and the new mortar. If the mortars have This 18th century pediment and surrounding been properly matched, usually the best way to deal with surface color differences is wall exhibit distinctively different mortar to let the mortars age naturally. Other treatments to overcome these differences, joints.Photo: NPS files. including cleaning the non-repointed areas or staining the new mortar, should be carefully tested prior to implementation. Staining the new mortar to achieve a better color match is generally not recommended, but it may be appropriate in some instances. Although staining may provide an initial match, the old and new mortars may weather at different rates, leading to visual differences after a few seasons. In addition, the mixtures used to stain the mortar may be harmful to the masonry; for example, they may introduce salts into the masonry which can lead to efflorescence. Cleaning the Repointed Masonry If repointing work is carefully executed, there will be little need for cleaning other than to remove the small amount of mortar from the edge of the joint following tooling. This can be done with a stiff natural bristle or nylon brush after the mortar has dried, but before it is initially set (1-2 hours). Mortar that has hardened can usually be removed with a wooden paddle or, if necessary, a chisel. Further cleaning is best accomplished with plain water and natural bristle or nylon brushes. If chemicals must be used, they should be selected with extreme caution. Improper cleaning can lead to deterioration of the masonry units, deterioration of the mortar, mortar smear, and efflorescence. New mortar joints are especially susceptible to damage because they do not become fully cured for several months. Chemical cleaners, particularly acids, should never be used on dry masonry. The masonry should always be completely soaked once with water before chemicals are applied. After cleaning, the walls should be flushed again with plain water to remove all traces of the chemicals. Several precautions should be taken if a freshly repointed masonry wall is to be cleaned. First, the mortar should be fully hardened before cleaning. Thirty days is usually sufficient, depending on weather and exposure; as mentioned previously, the mortar will continue to cure even after it has hardened. Test panels should be prepared to evaluate the effects of different cleaning methods. Generally, on newly repointed masonry walls, only very low pressure (100 psi) water washing supplemented by stiff natural bristle or nylon brushes should be used, except on glazed or polished surfaces, where only soft cloths should be used.** New construction "bloom" or efflorescence occasionally appears within the first few months of repointing and usually disappears through the normal process of weathering. If the efflorescence is not removed by natural processes, the safest way to remove it is by dry brushing with stiff natural or nylon bristle brushes followed by wet brushing. Hydrochloric (muriatic) acid, is generally ineffective, and it should not be used to remove efflorescence. It may liberate additional salts, which, in turn, can lead to more efflorescence. Surface grouting is sometimes suggested as an alternative to repointing brick buildings, in particular. This process involves the application of a thin coat of cement-based grout to the mortar joints and the mortar/brick interface. To be effective, the grout must extend slightly onto the face of the masonry units, thus widening the joint visually. The change in the joint appearance can alter the historic character of the structure to an unacceptable degree. In addition, although https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 10/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings masking of the bricks is intended to keep the grout off the remainder of the face of the bricks, some level of residue, called "veiling," will inevitably remain. Surface grouting cannot substitute for the more extensive work of repointing, and it is not a recommended treatment for historic masonry. "Additional information on masonry cleaning is presented in Preservation Briefs 1: Assessing Cleaning and Water- Repellent Treatments for Historic Masonry Buildings, Robert C. Mack, FAIR, and Anne E. Grimmer, Washington, D.C.: Technical Preservation Services, National Park Service, U.S. Department of the Interior, 2000; and Keeping it Clean: Removing Exterior Dirt, Paint, Stains& Graffiti from Historic Masonry Buildings, Anne E. Grimmer, Washington, D.C.: Technical Preservation Services, National Park Service, U.S. Department of the Interior, 1988. Visually Examining the Mortar and the Masonry Units A simple in situ comparison will help determine the hardness and condition of the mortar and the masonry units. Begin by scraping the mortar with a screwdriver, and gradually tapping harder with a cold chisel and mason's hammer. Masonry units can be tested in the same way beginning, even more gently, by scraping with a fingernail. This relative analysis which is derived from the 10-point hardness scale used to describe minerals, provides a good starting point for selection of an appropriate mortar. It is described more fully in "The Russack System for Brick&Mortar Description" referenced in Reading List at the end of this Brief. Mortar samples should be chosen carefully, and picked from a variety of locations on the building to find unweathered mortar, if possible. Portions of the building may have been repointed in the past while other areas may be subject to conditions causing unusual deterioration. There may be several colors of mortar dating from different construction periods or sand used from different sources during the initial construction. Any of these situations can give false readings to the visual or physical characteristics required for the new mortar. Variations should be noted which may require developing more than one mix. 1. Remove with a chisel and hammer three or four unweathered samples of the mortar to be matched from several locations on the building. (Set the largest sample aside--this will be used later for comparison with the repointing mortar). Removing a full representation of samples will allow selection of a "mean" or average mortar sample. 2. Mash the remaining samples with a wooden mallet, or hammer if necessary, until they are separated into their constituent parts. There should be a good handful of the material. 3. Examine the powdered portion—the lime and/or cement matrix of the mortar. Most particularly, note the color. There is a tendency to think of historic mortars as having white binders, but grey portland cement was available by the last quarter of the 19th century, and traditional limes were also sometimes grey. Thus, in some instances, the natural color of the historic binder may be grey, rather than white. The mortar may also have been tinted to create a colored mortar, and this color should be identified at this point. 4. Carefully blow away the powdery material (the lime and/or cement matrix which bound the mortar together). 5. With a low power(10 power) magnifying glass, examine the remaining sand and other materials such as lumps of lime or shell. 6. Note and record the wide range of color as well as the varying sizes of the individual grains of sand, impurities, or other materials. Other Factors to Consider Color Regardless of the color of the binder or colored additives, the sand is the primary material that gives mortar its color. A surprising variety of colors of sand may be found in a single sample of historic mortar, and the different sizes of the grains of sand or other materials, such as incompletely ground lime or cement, play an important role in the texture of the repointing mortar. Therefore, when specifying sand for repointing mortar, it may be necessary to obtain sand from several sources and to combine or screen them in order to approximate the range of sand colors and grain sizes in the historic mortar sample. Pointing Style Close examination of the historic masonry wall and the techniques used in the original construction will assist in maintaining the visual qualities of the building. Pointing styles and the methods of producing them should be examined. It is important to look at both the horizontal and the vertical joints to determine the order in which they were tooled and whether they were the same style. Some late-19th and early-20th century buildings, for example, have horizontal joints that were raked https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 11/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings back while the vertical joints were finished flush and stained to match the bricks, thus creating the illusion of horizontal bands. Pointing styles may also differ from one facade to another; front walls often received greater attention to mortar detailing than side and rear walls. Tuckpointing is not true repointing but the application of a raised joint or lime putty joint on top of flush mortar joints. Penciling is a purely decorative, painted surface treatment over a mortar joint, often in a contrasting color. Masonry Units The masonry units should also be examined so that any replacement units will match the historic masonry. Within a wall there may be a wide range of colors, textures, and sizes, particularly with hand -made brick or rough -cut, locally -quarried stone. Replacement units should blend in with the full range of masonry units rather than a single brick or stone. Matching Color and Texture of the Repointing Mortar New mortar should match the unweathered interior portions of the historic mortar. The simplest way to check the match is to make a small sample of the proposed mix and allow it to cure at a temperature of approximately 70 degrees F for about a week, or it can be baked in an oven to speed up the curing; this sample is then broken open and the surface is compared with the surface of the largest "saved" sample of historic mortar. If a proper color match cannot be achieved through the use of natural sand or colored aggregates like crushed marble or brick dust, it may be necessary to use a modern mortar pigment. During the early stages of the project, it should be determined how closely the new mortar should match the historic mortar. Will "quite close" be sufficient, or is "exactly" expected? The specifications should state this clearly so that the contractor has a reasonable idea how much time and expense will be required to develop an acceptable match. The same judgment will be necessary in matching replacement terra cotta, stone or brick. If there is a known source for replacements, this should be included in the specifications. If a source cannot be determined prior to the bidding process, the specifications should include an estimated price for the replacement materials with the final price based on the actual cost to the contractor. Mortar Types (Measured by volume) Designation Cement Hydrated Lime or Lime Putty Sand M 1 1/4 3- 3 3/4 S 1 1/2 4-4 1/2 N 1 1 5-6 O 1 2 8-9 K 1 3 10-12 "L" 0 1 2 1/4-3 Suggested Mortar Types for Different Exposures Exposure Masonry Material Sheltered Moderate Severe Very durable: granite, hard -cored brick, etc. O N S Moderately durable: limestone, durable stone, molded brick K O N Minimally durable:soft hand -made brick "L" K O Summary and References For the Owner/Administrator The owner or administrator of a historic building should remember that repointing is likely to be a lengthy and expensive process. First, there must be adequate time for evaluation of the building and investigation into the cause of problems. Then, there will be time needed for preparation of the contract documents. The work itself is precise, time-consuming and https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 12/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings noisy, and scaffolding may cover the face of the building for some time. Therefore, the owner must carefully plan the work to avoid problems. Schedules for both repointing and other activities will thus require careful coordination to avoid unanticipated conflicts. The owner must avoid the tendency to rush the work or cut corners if the historic building is to retain its visual integrity and the job is to be durable. For the Architect/Consultant Because the primary role of the consultant is to ensure the life of the building, a knowledge of historic construction techniques and the special problems found in older buildings is essential. The consultant must assist the owner in planning for logistical problems relating to research and construction. It is the consultant's responsibility to determine the cause of the mortar deterioration and ensure that it is corrected before the masonry is repointed. The consultant must also be prepared to spend more time in project inspections than is customary in modern construction. For the Masons Successful repointing depends on the masons themselves. Experienced masons understand the special requirements for work on historic buildings and the added time and expense they require. The entire masonry crew must be willing and able to perform the work in conformance with the specifications, even when the specifications may not be in conformance with standard practice. At the same time, the masons should not hesitate to question the specifications if it appears that the work specified would damage the building. Conclusion A good repointing job is meant to last, at least 30 years, and preferably 50- 100 years. Shortcuts and poor craftsmanship result not only in diminishing the historic character of a building, but also in a job that looks bad, and will require future repointing sooner than if the work had been done correctly. The mortar joint in a historic masonry building has often been called a wall's "first line of defense." Good repointing practices guarantee the long life of the mortar joint, the wall, and the historic structure. Although careful maintenance will help preserve the freshly repointed mortar joints, it is important to remember that mortar joints are intended to be sacrificial and will probably require repointing some time in the future. Nevertheless, if the historic mortar joints proved durable for many years, then careful repointing should have an equally long life, ultimately contributing to the preservation of the entire building. Useful Addresses Brick Institute of America 11490 Commerce Park Drive Reston, VA 22091 National Lime Association 200 N. Glebe Road, Suite 800 Arlington, VA 22203 Portland Cement Association 5420 Old Orchard Road Skokie, IL 60077 Acknowledgments Robert C. Mack, FAIA, is a principal in the firm of MacDonald & Mack, Architects, Ltd., an architectural firm that specializes in historic buildings in Minneapolis, Minnesota. John P. Speweik, CSI, Toledo, Ohio, is a 5th -generation stonemason, and principal in U.S. Heritage Group, Inc., Chicago, Illinois, which does custom historic mortar matching. Anne E. Grimmer, Senior Architectural Historian, National Park Service, was responsible for developing and coordinating the revision of this Preservation Brief, incorporating professional comments, and the technical editing. The authors and the editor wish to thank the following for the professional and technical review they provided: Mark Macpherson and Ron Peterson, Masonry Restoration Contractors, Macpherson -Towne Company, Minneapolis, MN; Lorraine Schnabel, Architectural Conservator, John Milner Associates, Inc., Philadelphia, PA; Lauren B. Sickels-Taves, Ph.D., Architectural Conservator, Biohistory International, Huntington Woods, MI; and the following National Park Service professional staff, including: E. Blaine Cliver, Chief, Historic American Buildings Survey/Historic American Engineering Record; Douglas C. Hicks, Deputy Superintendent, Historic Preservation Training Center, Frederick, MD; Chris McGuigan, Supervisory Exhibits Specialist, Historic Preservation Training Center, Frederick, MD; Charles E. Fisher, Sharon C. Park, FAIA, John Sandor, Technical Preservation Services Branch, Heritage Preservation Services, and Kay D. Weeks, Heritage Preservation Services. The original version of this brief, Repointing Mortar Joints in Historic Brick Buildings, was written by Robert C. Mack in 1976, and was revised and updated in 1980 by Robert C. Mack, de Teel Patterson Tiller, and James S. Askins. https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 13/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings This publication has been prepared pursuant to the National Historic Preservation Act of 1966, as amended, which directs the Secretary of the Interior to develop and make available information concerning historic properties. Technical Preservation Services (TPS), National Park Service prepares standards, guidelines, and other educational materials on responsible historic preservation treatments for a broad public. October 1998 Reading List Ashurst, John & Nicola. Practical Building Conservation. Vol. 3: Mortars, Plasters and Renders. New York: Halsted Press, a Division of John Wiley & Sons, Inc., 1988. Cliver, E. Blaine. "Tests for the Analysis of Mortar Samples." Bulletin of the Association for Preservation Technology. Vol. 6, No. 1 (1974), pp. 68-73. Coney, William B., AIA. Masonry Repointing of Twentieth -Century Buildings. Illinois Preservation Series. Number 10. Springfield, IL: Division of Preservation Services, Illinois Historic Preservation Agency, 1989. Davidson, J.I. "Masonry Mortar." Canadian Building Digest. CBD 163. Ottawa, ONT: Division of Building Research, National Research Council of Canada, 1974. Ferro, Maximillian L., AIA, RIBA. "The Russack System for Brick and Mortar Description: A Field Method for Assessing Masonry Hardness." Technology and Conservation. Vol. 5, No. 2 (Summer 1980), pp. 32-35. Hooker, Kenneth A. "Field Notes on Repointing." Aberdeen's Magazine of Masonry Construction. Vol. 4, No. 8 (August 1991), pp. 326-328. Jedrzejewska, H. "Old Mortars in Poland: A New Method of Investigation." Studies in Conservation. Vol. 5, No. 4 (1960), pp. 132-138. "Lime's Role in Mortar." Aberdeen's Magazine of Masonry Construction. Vol. 9, No. 8 (August 1996), pp. 364-368. Phillips, Morgan W. "Brief Notes on the Subjects of Analyzing Paints and Mortars and the Recording of Moulding Profiles: The Trouble with Paint and Mortar Analysis." Bulletin of the Association for Preservation Technology. Vol. 10, No. 2 (1978), pp. 77-89. Preparation and Use of Lime Mortars: An Introduction to the Principles of Using Lime Mortars. Scottish Lime Centre for Historic Scotland. Edinburgh: Historic Scotland, 1995. Schierhorn, Carolyn. "Ensuring Mortar Color Consistency." Aberdeen's Magazine of Masonry Construction. Vol. 9, No. 1 (January 1996), pp. 33-35. "Should Air -Entrained Mortars Be Used?" Aberdeen's Magazine of Masonry Construction. Vol. 7, No. 9 (September 1994), pp. 419-422. Sickels-Taves, Lauren B. "Creep, Shrinkage, and Mortars in Historic Preservation." Journal of Testing and Evaluation, JTEVA. Vol. 23, No. 6 ( November 1995), pp. 447-452. Speweik, John P. The History of Masonry Mortar in America, 1720-1995. Arlington, VA: National Lime Association, 1995. Speweik, John P. "Repointing Right: Why Using Modern Mortar Can Damage a Historic House." Old -House Journal. Vol. XXV, No. 4 (July -August 1997), pp. 46-51. Technical Notes on Brick Construction. Brick Institute of America, Reston, VA. "Moisture Resistance of Brick Masonry: Maintenance." 7F. February 1986. "Mortars for Brick Masonry." 8 Revised II. November 1989. "Standard Specification for Portland Cement -Lime Mortar for Brick Masonry." 8A Revised. September 1988. "Mortar for Brick Masonry -Selection and Controls." 8B Reissued. September 1988. (July/August 1976). "Guide Specifications for Brick Masonry, Part V Mortar and Grout." 11E Revised. September 1991. "Bonds and Patterns in Brickwork." 30 Reissued. September 1988. https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 14/15 8/7/2019 Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings nps.gov E PERI E N CE YOU R A MERICA" https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm 15/15 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings National Park S - i -vice Technical Preservation Sarevr U. S. Deparmient of titre imteriar Home > How to Preserve > Preservation Briefs > 1 Cleaning and Water -Repellent Treatments Some of the web versions of the Preservation Briefs differ somewhat from the printed versions. Many illustrations are new and in color; Captions are simplified and some complex charts are omitted. To order hard copies of the Briefs, see -Irinted Publications G'. PRESERVATION BRIEFS 1 Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings Robert C. Mack, FAIA, and Anne E. Grimmer Preparing for a Cleaning Project Understanding the Building Materials Cleaning Methods and Materials Planning a Cleaning Project Water -Repellent Coatings and Waterproof Coatings Summary and References Reading List Download the PDF Appropriate cleaning of historic masonry. Photo: NPS files. Inappropriate cleaning and coating treatments are a major cause of damage to historic masonry buildings. While either or both treatments may be appropriate in some cases, they can be very destructive to historic masonry if they are not selected carefully. Historic masonry, as considered here, includes stone, brick, architectural terra cotta, cast stone, concrete and concrete block. It is frequently cleaned because cleaning is equated with improvement. Cleaning may sometimes be followed by the application of a water-repellent coating. However, unless these procedures are carried out under the guidance and supervision of an architectural conservator, they may result in irrevocable damage to the historic resource. Ninety years of accumulated dirt and pollutants are being removed from this historic theater using an appropriate chemical cleaner, applied in The purpose of this Brief is to provide information on the variety of cleaning methods and materials that are available for use on the exterior of historic masonry buildings, and to provide guidance in selecting the most appropriate method or combination of methods. The difference between water-repellent coatings and waterproof coatings is explained, and the purpose of each, the suitability of their application to historic masonry buildings, and the possible consequences of their inappropriate use are discussed. The Brief is intended to help develop sensitivity to the qualities of historic masonry that makes it so special, and to assist historic building owners and property managers in working cooperatively with architects, architectural conservators, and contractors. Although specifically intended for historic buildings, the information is applicable to all masonry buildings. This publication updates and expands Preservation Briefs 1: The Cleaning and Waterproof Coating of Masonry Buildings. The Brief is not meant to be a cleaning manual or a guide for preparing specifications. Rather, it provides general information to raise awareness of the many factors involved in selecting cleaning and water-repellent treatments for historic masonry buildings. https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 1/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings stages. Photo: Richard Wagner, AIA.Preparing for a Cleaning P Reasons for Cleaning First, it is important to determine whether it is appropriate to clean the masonry. The objective of cleaning a historic masonry building must be considered carefully before arriving at a decision to clean. There are several major reasons for cleaning a historic masonry building: improve the appearance of the building by removing unattractive dirt or soiling materials, or non -historic paint from the masonry; retard deterioration by removing soiling materials that may be damaging the masonry; or provide a clean surface to accurately match repointing mortars or patching compounds, or to conduct a condition survey of the masonry. Identify What is to be Removed The general nature and source of dirt or soiling material on a building must be identified to remove it in the gentlest means possible --that is, in the most effective, yet least harmful, manner. Soot and smoke, for example, require a different cleaning agent to remove than oil stains or metallic stains. Other common cleaning problems include biological growth such as mold or mildew, and organic matter such as the tendrils left on masonry after removal of ivy. Consider the Historic Appearance of the Building If the proposed cleaning is to remove paint, it is important in each case to learn whether or not unpainted masonry is historically appropriate. And, it is necessary to consider why the building was painted. Was it to cover bad repointing or unmatched repairs? Was the building painted to protect soft brick or to conceal deteriorating stone? Or, was painted masonry simply a fashionable treatment in a particular historic period? Many buildings were painted at the time of construction or shortly thereafter; retention of the paint, therefore, may be more appropriate historically than removing it. And, if the building appears to have been painted for a long time, it is also important to think about whether the paint is part of the character of the historic building and if it has acquired significance over time. Consider the Practicalities of Cleaning or Paint Removal Some gypsum or sulfate crusts may have become integral with the stone and, if cleaning could result in removing some of the stone surface, it may be preferable not to clean. Even where unpainted masonry is appropriate, the retention of the paint may be more practical than removal in terms of long range preservation of the masonry. In some cases, however, removal of the paint may be desirable. For example, the old paint layers may have built up to such an extent that removal is necessary to ensure a sound surface to which the new paint will adhere. Study the Masonry Although not always necessary, in some instances it can be beneficial to have the coating or paint type, color, and layering on the masonry researched before attempting its removal. Analysis of the nature of the soiling or of the paint to be removed from the masonry, as well as guidance on the appropriate cleaning method, may be provided by professional consultants, including architectural conservators, conservation scientists, and preservation architects. The State Historic Preservation Office (SHPO), local historic district commissions, architectural review boards, and preservation -oriented websites may also be able to supply useful information on masonry cleaning techniques. Understanding the Building Materials The construction of the building must be considered when developing a cleaning program because inappropriate cleaning can have a deleterious effect on the masonry as well as on other building materials. The masonry material or materials must be correctly identified. It is sometimes difficult to distinguish one type of stone from another; for example, certain sandstones can be easily confused with limestones. Or, what appears to be natural stone may not be stone at all, but cast stone or concrete. Historically, cast stone and architectural terra cotta were frequently used in combination with natural stone, especially for trim elements or on upper stories of a building where, from a distance, these substitute materials looked like real stone. Other features on historic buildings that appear to be stone, such as decorative cornices, entablatures and window hoods, may not even be masonry, but metal. Identify Prior Treatments The decorative trim on this Previous treatments of the building and its surroundings should be researched and building brick builing is architectural terra-cotta intended to maintenance records should be obtained, if available. Sometimes if streaked or spotty areas do simulate the limestone not seem to get cleaner following an initial cleaning, closer inspection and analysis may be foundation. Photo: NPS files. warranted. The discoloration may turn out not to be dirt but the remnant of a water-repellent coating applied long ago which has darkened the surface of the masonry over time. Successful removal may require testing several cleaning agents to find something that will dissolve and remove the coating. Complete removal may not always be https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 2/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings possible. Repairs may have been stained to match a dirty building, and cleaning may make these differences apparent. De- icing salts used near the building that have dissolved can migrate into the masonry. Cleaning may draw the salts to the surface, where they will appear as efflorescence (a powdery, white substance), which may require a second treatment to be removed. Allowances for dealing with such unknown factors, any of which can be a potential problem, should be included when investigating cleaning methods and materials. Just as more than one kind of masonry on a historic building may necessitate multiple cleaning approaches, unknown conditions that are encountered may also require additional cleaning treatments. Any cleaning method should be tested before using it on historic masonry. Photo: NPS files. Choose the Appropriate Cleaner The importance of testing cleaning methods and materials cannot be over emphasized. Applying the wrong cleaning agents to historic masonry can have disastrous results. Acidic cleaners can be extremely damaging to acid -sensitive stones, such as marble and limestone, resulting in etching and dissolution of these stones. Other kinds of masonry can also be damaged by incompatible cleaning agents, or even by cleaning agents that are usually compatible. There are also numerous kinds of sandstone, each with a considerably different geological composition. While an acid -based cleaner may be safely used on some sandstones, others are acid -sensitive and can be severely etched or dissolved by an acid cleaner. Some sandstones contain water-soluble minerals and can be eroded by water cleaning. And, even if the stone type is correctly identified, stones, as well as some bricks, may contain unexpected impurities, such as iron particles, that may react negatively with a particular cleaning agent and result in staining. Thorough understanding of the physical and chemical properties of the masonry will help avoid the inadvertent selection of damaging cleaning agents. Other building materials also may be affected by the cleaning process. Some chemicals, for example, may have a corrosive effect on paint or glass. The portions of building elements most vulnerable to deterioration may not be visible, such as embedded ends of iron window bars. Other totally unseen items, such as iron cramps or ties which hold the masonry to the structural frame, also may be subject to corrosion from the use of chemicals or even from plain water. The only way to prevent problems in these cases is to study the building construction in detail and evaluate proposed cleaning methods with this information in mind. However, due to the very likely possibility of encountering unknown factors, any cleaning project involving historic masonry should be viewed as unique to that particular building. Cleaning Methods and Materials Masonry cleaning methods generally are divided into three major groups: water, chemical, and abrasive. Water methods soften the dirt or soiling material and rinse the deposits from the masonry surface. Chemical cleaners react with dirt, soiling material or paint to effect their removal, after which the cleaning effluent is rinsed off the masonry surface with water. Abrasive methods include blasting with grit, and the use of grinders and sanding discs, all of which mechanically remove the dirt, soiling material or paint (and, usually, some of the masonry surface). Abrasive cleaning is also often followed with a water rinse. Laser cleaning, although not discussed here in detail, is another technique that is used sometimes by conservators to clean small areas of historic masonry. It can be quite effective for cleaning limited areas, but it is expensive and generally not practical for most historic masonry cleaning projects. Although it may seem contrary to common sense, masonry cleaning projects should be carried out starting at the bottom and proceeding to the top of the building always keeping all surfaces wet below the area being cleaned. The rationale for this approach is based on the principle that dirty water or cleaning effluent dripping from cleaning in progress above will leave streaks on a dirty surface but will not streak a clean surface as long as it is kept wet and rinsed frequently. Water Cleaning Water cleaning methods are generally the gentlest means possible, and they can be used safely to remove dirt from all types of historic masonry.* There are essentially four kinds of water-based methods: soaking; pressure water washing; water washing supplemented with non-ionic detergent; and steam, or hot -pressurized water cleaning. Once water cleaning has been completed, it is often necessary to follow up with a water rinse to wash off the loosened soiling material from the masonry. *Water cleaning methods may not be appropriate to use on some badly deteriorated masonry because water may exacerbate the deterioration, or on gypsum or alabaster, which are very soluble in water. Soaking Prolonged spraying or misting with water is particularly effective for cleaning limestone and marble. It is also a good method for removing heavy accumulations of soot, sulfate crusts or gypsum crusts that tend to form in protected areas of a building not regularly washed by rain. Water is distributed to lengths of punctured hose or pipe with non-ferrous fittings https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 3/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings hung from moveable scaffolding or a swing stage that continuously mists the surface of the masonry with a very fine spray. A timed on-off spray is another approach to using this cleaning technique. After one area has been cleaned, the apparatus is moved on to another. Soaking is often used in combination with water washing and is also followed by a final water rinse. Soaking is a very slow method—it may take several days or a week—but it is a very gentle method to use on historic masonry. Water Washing Washing with low-pressure or medium -pressure water is probably one of the most commonly used methods for removing dirt or other pollutant soiling from historic masonry buildings. Starting with a very low pressure (100 psi or below), even using a garden hose, and progressing as needed to slightly higher pressure—generally no higher than 300-400 psi—is always the recommended way to begin. Scrubbing with natural bristle or synthetic bristle brushes—never metal which can abrade the surface and leave metal particles that can stain the masonry—can help in cleaning areas of the masonry that are especially dirty. Water Washing with Detergents Non-ionic detergents—which are not the same as soaps—are synthetic organic compounds that are especially effective in removing oily soil. (Examples of some of the numerous proprietary non-ionic detergents include Igepal by GAF, Tergitol by Union Carbide and Triton by Rohm & Haas.) Thus, the addition of a non-ionic Low -to -medium -pressure steam (hot - pressurized water washing) is a gentle method of softening heavy soiling deposits and cleaning historic marble. Photo: NPS files. detergent, or surfactant, to a low- or medium -pressure water wash can be a useful aid in the cleaning process. (A non-ionic detergent, unlike most household detergents, does not leave a solid, visible residue on the masonry.) Adding a non-ionic detergent and scrubbing with a natural bristle or synthetic bristle brush can facilitate cleaning textured or intricately carved masonry. This should be followed with a final water rinse. Steam/Hot-Pressurized Water Cleaning Steam cleaning is actually low-pressure hot water washing because the steam condenses almost immediately upon leaving the hose. This is a gentle and effective method for cleaning stone and particularly for acid -sensitive stones. Steam can be especially useful in removing built-up soiling deposits and dried-up plant materials, such as ivy disks and tendrils. It can also be an efficient means of cleaning carved stone details and, because it does not generate a lot of liquid water, it can sometimes be appropriate to use for cleaning interior masonry. Potential Hazards of Water Cleaning Despite the fact that water-based methods are generally the most gentle, even they can be damaging to historic masonry. Before beginning a water cleaning project, it is important to make sure that all mortar joints are sound and that the building is watertight. Otherwise water can seep through the walls to the interior, resulting in rusting metal anchors and stained and ruined plaster. Some water supplies may contain traces of iron and copper which may cause masonry to discolor. Adding a chelating or complexing agent to the water, such as EDTA (ethylene diamine tetra -acetic acid), which inactivates other metallic ions, as well as softens minerals and water hardness, will help prevent staining on light-colored masonry. Any cleaning method involving water should never be done in cold weather or if there is any likelihood of frost or freezing because water within the masonry can freeze, causing spalling and cracking. Since a masonry wall may take over a week to dry after cleaning, no water cleaning should be permitted for several days prior to the first average frost date, or even earlier if local forecasts predict cold weather. Most important of all, it is imperative to be aware that using water at too high a pressure, a practice common to "power washing" and "water blasting", is very abrasive and can easily etch marble and other soft stones, as well as some types of brick. In addition, the distance of the nozzle from the masonry surface and the type of nozzle, as well as gallons per minute (gpm), are also important variables in a water cleaning process that can have a significant impact on the outcome of the project. This is why it is imperative that the cleaning be closely monitored to ensure that the cleaning operators do not raise the pressure or bring the nozzle too close to the masonry in an effort to "speed up" the process. The appearance of grains of stone or sand in the cleaning effluent on the ground is an indication that the water pressure may be too high. Chemical Cleaning Chemical cleaners, generally in the form of proprietary products, are another material frequently used to clean historic masonry. They can remove dirt, as well as paint and other coatings, metallic and plant stains, and graffiti. Chemical cleaners used to remove dirt and soiling include acids, alkalies and organic compounds. Acidic cleaners, of course, https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 4/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings should not be used on masonry that is acid sensitive. Paint removers are alkaline, based on organic solvents or other chemicals. Chemical Cleaners to Remove Dirt Both alkaline and acidic cleaning treatments include the use of water. Both cleaners are also likely to contain surfactants (wetting agents), that facilitate the chemical reaction that removes the dirt. Generally, the masonry is wet first for both types of cleaners, then the chemical cleaner is sprayed on at very low pressure or brushed onto the surface. The cleaner is left to dwell on the masonry for an amount of time recommended by the product manufacturer or, preferably, determined by testing, and rinsed off with a low- or moderate -pressure cold, or sometimes hot, water wash. More than one application of the cleaner may be necessary, and it is always a good practice to test the product manufacturer's recommendations concerning dilution rates and dwell times. Because each cleaning situation is unique, dilution rates and dwell times can vary considerably. The masonry surface may be scrubbed lightly with natural or synthetic bristle brushes prior to rinsing. After rinsing, pH strips should be applied to the surface to ensure that the masonry has been neutralized completely. Acidic Cleaners Acid -based cleaning products may be used on non-acid sensitive masonry, which generally includes: granite, most sandstones, slate, unglazed brick and unglazed architectural terra cotta, cast stone and concrete. Most commercial acidic cleaners are composed primarily of hydrofluoric acid, and often include some phosphoric acid to prevent rust -like stains from developing on the masonry after the cleaning. Acid cleaners are applied to the pre -wet masonry which should be kept wet while the acid is allowed to "work", and then removed with a water wash. Alkaline Cleaners Alkaline cleaners should be used on acid -sensitive masonry, including: limestone, polished and unpolished marble, calcareous sandstone, glazed brick and glazed architectural terra cotta, and polished granite. (Alkaline cleaners may also be used sometimes on masonry materials that are not acid sensitive --after testing, of course --but they may not be as effective as they are on acid -sensitive masonry.) Alkaline cleaning products consist primarily of two ingredients: a non-ionic detergent or surfactant; and an alkali, such as potassium hydroxide or ammonium hydroxide. Like acidic cleaners, alkaline products are usually applied to pre -wet masonry, allowed to dwell, and then rinsed off with water. (Longer dwell times may be necessary with alkaline cleaners than with acidic cleaners.) Two additional steps are required to remove alkaline cleaners after the initial rinse. First the masonry is given a slightly acidic wash --often with acetic acid --to neutralize it, and then it is rinsed again with water. Chemical Cleaners to Remove Paint and Other Coatings, Stains and Graffiti Removing paint and some other coatings, stains and graffiti can best be accomplished with alkaline paint removers, organic solvent paint removers, or other cleaning compounds. The removal of layers of paint from a masonry surface usually involves applying the remover either by brush, roller or spraying, followed by a thorough water wash. As with any chemical cleaning, the manufacturer's recommendations regarding application procedures should always be tested before beginning work. Alkaline Paint Removers These are usually of much the same composition as other alkaline cleaners, containing potassium or ammonium hydroxide, or trisodium phosphate. They are used to remove oil, latex and acrylic paints, and are effective for removing multiple layers of paint. Alkaline cleaners may also remove some acrylic water-repellent coatings. As with other alkaline cleaners, both an acidic neutralizing wash and a final water rinse are generally required following the use of alkaline paint removers. Organic Solvent Paint Removers The formulation of organic solvent paint removers varies and may include a combination of solvents, including methylene chloride, methanol, acetone, xylene and toluene. Other Paint Removers and Cleaners Other cleaning compounds that can be used to remove paint and some painted graffiti from historic masonry include paint removers based on N -methyl -2 -pyrrolidone (NMP), or on petroleum-based compounds. Removing stains, whether they are industrial (smoke, soot, grease or tar), metallic (iron or copper), or biological (plant and fungal) in origin, depends on carefully matching the type of remover to the type of stain. Successful removal of stains from historic masonry often requires the application of a number of different removers before the right one is found. The removal of layers of paint from a masonry surface is usually accomplished by applying the remover either by brush, roller or spraying, followed by a thorough water wash. Potential Hazards of Chemical Cleaning https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 5/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings Since most chemical cleaning methods involve water, they have many of the potential problems of plain water cleaning. Like water methods, they should not be used in cold weather because of the possibility of freezing. Chemical cleaning should never be undertaken in temperatures below 40 degrees F (4 degrees C), and generally not below 50 degrees F. In addition, many chemical cleaners simply do not work in cold temperatures. Both acidic and alkaline cleaners can be dangerous to cleaning operators, and clearly, there are environmental concerns associated with the use of chemical cleaners. If not carefully chosen, chemical cleaners can react adversely with many types of masonry. Obviously, acidic cleaners should not be used on acid -sensitive materials; however, it is not always clear exactly what the composition is of any stone or other masonry material. For, this reason, testing the cleaner on an inconspicuous spot on the building is always necessary. While certain acid -based cleaners may be appropriate if used as directed on a particular type of masonry, if left too long or if not adequately rinsed from the masonry they can have a negative effect. For example, hydrofluoric acid can etch masonry leaving a hazy residue (whitish deposits of silica or calcium fluoride salts) on the surface. While this efflorescence may usually be removed by a second cleaning—although it is likely to be expensive and time-consuming— hydrofluoric acid can also leave calcium fluoride salts or a colloidal silica deposit on masonry which may be impossible to remove. Other acids, particularly hydrochloric (muriatic) acid, which is very powerful, should not be used on historic masonry, because it can dissolve lime -based mortar, damage brick and some stones, and leave chloride deposits on the masonry. Alkaline cleaners can stain sandstones that contain a ferrous compound. Before using an alkaline cleaner on sandstone it is always important to test it, since it may be difficult to know whether a particular sandstone may contain a ferrous compound. Some alkaline cleaners, such as sodium hydroxide (caustic soda or lye) and ammonium bifluoride, can also damage or leave disfiguring brownish -yellow stains and, in most cases, should not be used on historic masonry. Although alkaline cleaners will not etch a masonry surface as acids can, they are caustic and can burn the surface. In addition, alkaline cleaners can deposit potentially damaging salts in the masonry which can be difficult to rinse thoroughly. Poulticing to Remove Stains and Graffiti Graffiti and stains, which have penetrated into the masonry, often are best removed by using a poultice. A poultice consists of an absorbent material or clay powder (such as kaolin or fulleris earth, or even shredded paper or paper towels), mixed with a liquid (a solvent or other remover) to form a paste which is applied to the stain. The poultice is kept moist and left on the stain as long as necessary for it to draw the stain out of the masonry. As it dries, the paste absorbs the staining material so that it is not redeposited on the masonry surface. Some commercial cleaning products and paint removers are specially formulated as a paste or gel that will cling to a vertical surface and remain moist for a longer period of time in order to prolong the action of the chemical on the stain. Pre -mixed poultices are also available as a paste or in powder form needing only the addition of the appropriate liquid. The masonry must be pre -wet before applying an alkaline cleaning agent, but not when using a solvent. Once the stain has been removed, the masonry must be rinsed thoroughly. Abrasive and Mechanical Cleaning Generally, abrasive cleaning methods are not appropriate for use on historic masonry buildings. Abrasive cleaning methods are just that—abrasive. Grit blasters, grinders, and sanding discs all operate by abrading the dirt or paint off the surface of the masonry, rather than reacting with the dirt and the masonry which is how water and chemical methods work. Since the abrasives do not differentiate between the dirt The iron stain on this granite post may be removed by applying a commercial rust - removal product in a poultice. Photo: NPS files and the masonry, they can also remove the outer surface of the masonry at the same time, and result in permanently damaging the masonry. Brick, architectural terra cotta, soft stone, detailed carvings, and polished surfaces, are especially susceptible to physical and aesthetic damage by abrasive methods. Brick and architectural terra cotta are fired products which have a smooth, glazed surface which can be removed by abrasive blasting or grinding. Abrasively -cleaned masonry is damaged aesthetically as well as physically, and it has a rough surface which tends to hold dirt and the roughness will make future cleaning more difficult. Abrasive cleaning processes can also increase the likelihood of subsurface cracking of the masonry. Abrasion of carved details causes a rounding of sharp corners and other loss of delicate features, while abrasion of polished surfaces removes the polished finish of stone. Mortar joints, especially those with lime mortar, also can be eroded by abrasive or mechanical cleaning. In some cases, the damage may be visual, such as loss of joint detail or increased joint shadows. As mortar joints constitute a significant portion of the masonry surface (up to 20 per cent in a brick wall), this can result in the loss of a considerable amount of the historic fabric. Erosion of the mortar joints may also permit increased water penetration, which will likely necessitate repointing. https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 6/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings Sandblasting has permanently damaged this walnut shells are commonly used for cleaning monumental bronze sculpture, and brick wall. Photo: NPS files skilled conservators clean delicate museum objects and finely detailed, carved stone features with very small, micro -abrasive units using aluminum oxide. A number of current approaches to abrasive blasting rely on materials that are not usually thought of as abrasive, and not as commonly associated with traditional abrasive grit cleaning. Some patented abrasive cleaning processes—one dry, one wet—use finely -ground glass powder intended to "erase" or remove dirt and surface soiling only, but not paint or stains. Cleaning with baking soda (sodium bicarbonate) is another patented process. Baking soda blasting is being used in some communities as a means of quick graffiti removal. However, it should not be used on historic masonry which it can easily abrade and can permanently "etch" the graffiti into the stone; it can also leave potentially damaging salts in the stone which cannot be removed. Most of these abrasive grits may be used either dry or wet, although dry grit tends to be used more frequently. Ice particles, or pelletized dry ice (carbon dioxide or CO2), are another medium used as an abrasive cleaner. This is also too abrasive to be used on most historic masonry, but it may have practical application for removing mastics or asphaltic coatings from some substrates. Some of these processes are promoted as being more environmentally safe and not damaging to historic masonry buildings. However, it must be remembered that they are abrasive and that they "clean" by removing a small portion of the masonry surface, even though it may be only a minuscule portion. The fact that they are essentially abrasive treatments must always be taken into consideration when planning a masonry cleaning project. In general, abrasive methods should not be used to clean historic masonry buildings. In some, very limited instances, highly -controlled, gentle abrasive cleaning may be appropriate on selected, hard -to -clean areas of a historic masonry building if carried out under the watchful supervision of a professional conservator. But, abrasive cleaning should never be used on an entire building. Grinders and Sanding Disks Grinding the masonry surface with mechanical grinders and sanding disks is another means of abrasive cleaning that should not be used on historic masonry. Like abrasive blasting, grinders and disks do not really clean masonry but instead grind away and abrasively remove and, thus, damage the masonry surface itself rather than remove just the soiling material. Planning a Cleaning Project Once the masonry and soiling material or paint have been identified, and the condition of the masonry has been evaluated, planning for the cleaning project can begin. Testing Cleaning Methods In order to determine the gent/est means possible, several cleaning methods or materials may have to be tested prior to selecting the best one to use on the building. Testing should always begin with the gentlest and least invasive method proceeding gradually, if necessary, to more complicated methods, or a combination of methods. All too often simple methods, such as a low-pressure water wash, are not even considered, yet they frequently are effective, safe, and not expensive. Water of slightly higher pressure or with a non-ionic detergent additive also may be effective. It is worth repeating that these methods should always be tested prior to considering harsher methods; they are safer for the building and the environment, often safer for the applicator, and relatively inexpensive. The level of cleanliness desired also should be determined prior to selection of a cleaning method. Obviously, the intent of cleaning is to remove most of the dirt, soiling material, stains, paint or other coating. A "brand new" appearance, however, may be inappropriate for an older building, and may require an overly harsh cleaning method to be achieved. When undertaking a cleaning project, it is important to be aware that some stains simply may not be removable. It may be wise, therefore, to agree upon a slightly lower level of cleanliness that will serve as the standard for the cleaning project. The precise amount of residual dirt considered acceptable may depend on the type of masonry, the type of soiling and difficulty of total removal, and local environmental conditions. Cleaning tests should be carried out in an area of sufficient size to give a true indication of their effectiveness. It is preferable to conduct the test in an inconspicuous location on the building so that it will not be obvious if the test is not https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 7/12 Abrasive Blasting Blasting with abrasive grit or another abrasive material is the most frequently used . ry abrasive method. Sandblasting is most commonly associated with abrasive cleaning. f• art P –"777771 Finely ground silica or glass powder, glass beads, ground garnet, powdered walnut and r• other ground nut shells, grain hulls, aluminum oxide, plastic particles and even tiny pieces of sponge, are just a few of the other materials that have also been used for abrasive cleaning. Although abrasive blasting is not an appropriate method of cleaning historic masonry, it can be safely used to clean some materials. Finely -powdered Sandblasting has permanently damaged this walnut shells are commonly used for cleaning monumental bronze sculpture, and brick wall. Photo: NPS files skilled conservators clean delicate museum objects and finely detailed, carved stone features with very small, micro -abrasive units using aluminum oxide. A number of current approaches to abrasive blasting rely on materials that are not usually thought of as abrasive, and not as commonly associated with traditional abrasive grit cleaning. Some patented abrasive cleaning processes—one dry, one wet—use finely -ground glass powder intended to "erase" or remove dirt and surface soiling only, but not paint or stains. Cleaning with baking soda (sodium bicarbonate) is another patented process. Baking soda blasting is being used in some communities as a means of quick graffiti removal. However, it should not be used on historic masonry which it can easily abrade and can permanently "etch" the graffiti into the stone; it can also leave potentially damaging salts in the stone which cannot be removed. Most of these abrasive grits may be used either dry or wet, although dry grit tends to be used more frequently. Ice particles, or pelletized dry ice (carbon dioxide or CO2), are another medium used as an abrasive cleaner. This is also too abrasive to be used on most historic masonry, but it may have practical application for removing mastics or asphaltic coatings from some substrates. Some of these processes are promoted as being more environmentally safe and not damaging to historic masonry buildings. However, it must be remembered that they are abrasive and that they "clean" by removing a small portion of the masonry surface, even though it may be only a minuscule portion. The fact that they are essentially abrasive treatments must always be taken into consideration when planning a masonry cleaning project. In general, abrasive methods should not be used to clean historic masonry buildings. In some, very limited instances, highly -controlled, gentle abrasive cleaning may be appropriate on selected, hard -to -clean areas of a historic masonry building if carried out under the watchful supervision of a professional conservator. But, abrasive cleaning should never be used on an entire building. Grinders and Sanding Disks Grinding the masonry surface with mechanical grinders and sanding disks is another means of abrasive cleaning that should not be used on historic masonry. Like abrasive blasting, grinders and disks do not really clean masonry but instead grind away and abrasively remove and, thus, damage the masonry surface itself rather than remove just the soiling material. Planning a Cleaning Project Once the masonry and soiling material or paint have been identified, and the condition of the masonry has been evaluated, planning for the cleaning project can begin. Testing Cleaning Methods In order to determine the gent/est means possible, several cleaning methods or materials may have to be tested prior to selecting the best one to use on the building. Testing should always begin with the gentlest and least invasive method proceeding gradually, if necessary, to more complicated methods, or a combination of methods. All too often simple methods, such as a low-pressure water wash, are not even considered, yet they frequently are effective, safe, and not expensive. Water of slightly higher pressure or with a non-ionic detergent additive also may be effective. It is worth repeating that these methods should always be tested prior to considering harsher methods; they are safer for the building and the environment, often safer for the applicator, and relatively inexpensive. The level of cleanliness desired also should be determined prior to selection of a cleaning method. Obviously, the intent of cleaning is to remove most of the dirt, soiling material, stains, paint or other coating. A "brand new" appearance, however, may be inappropriate for an older building, and may require an overly harsh cleaning method to be achieved. When undertaking a cleaning project, it is important to be aware that some stains simply may not be removable. It may be wise, therefore, to agree upon a slightly lower level of cleanliness that will serve as the standard for the cleaning project. The precise amount of residual dirt considered acceptable may depend on the type of masonry, the type of soiling and difficulty of total removal, and local environmental conditions. Cleaning tests should be carried out in an area of sufficient size to give a true indication of their effectiveness. It is preferable to conduct the test in an inconspicuous location on the building so that it will not be obvious if the test is not https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 7/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings successful. A test area may be quite small to begin, sometimes as small as six square inches, and gradually may be increased in size as the most appropriate methods and cleaning agents are determined. Eventually the test area may be expanded to a square yard or more, and it should include several masonry units and mortar joints. It should be remembered that a single building may have several types of masonry and that even similar materials may have different surface finishes. Each material and different finish should be tested separately. Cleaning tests should be evaluated only after the masonry has dried completely. The results of the tests may indicate that several methods of cleaning should be used on a single building. When feasible, test areas should be allowed to weather for an extended period of time prior to final evaluation. A waiting period of a full year would be ideal in order to expose the test patch to a full range of seasons. If this is not possible, the test patch should weather for at least a month or two. For any building which is considered historically important, the delay is insignificant compared to the potential damage and disfigurement which may result from using an incompletely tested method. The successfully cleaned test patch should be protected as it will serve as a standard against which the entire cleaning project will be measured. Environmental Considerations The potential effect of any method proposed for cleaning historic masonry should be evaluated carefully. Chemical cleaners and paint removers may damage trees, shrubs, grass, and plants. A plan must be provided for environmentally safe removal and disposal of the cleaning materials and the rinsing effluent before beginning the cleaning project. Authorities from the local regulatory agency --usually under the jurisdiction of the federal or state Environmental Protection Agency (EPA) --should be consulted prior to beginning a cleaning project, especially if it involves anything more than plain water washing. This advance planning will ensure that the cleaning effluent or run-off, which is the combination of the cleaning agent and the substance removed from the masonry, is handled and disposed of in an environmentally sound and legal manner. Some alkaline and acidic cleaners can be neutralized so that they can be safely discharged into storm sewers. However, most solvent -based cleaners cannot be neutralized and are categorized as pollutants, and must be disposed of by a licensed transport, storage and disposal facility. Thus, it is always advisable to consult with the appropriate agencies before starting to clean to ensure that the project progresses smoothly and is not interrupted by a stop -work order because a required permit was not obtained in advance. Vinyl guttering or polyethylene -lined troughs placed around the perimeter of the base of the building can serve to catch chemical cleaning waste as it is rinsed off the building. This will reduce the amount of chemicals entering and polluting the soil, and also will keep the cleaning waste contained until it can be removed safely. Some patented cleaning systems have developed special equipment to facilitate the containment and later disposal of cleaning waste. Concern over the release of volatile organic compounds (VOCs) into the air has resulted in the manufacture of new, more environmentally responsible cleaners and paint removers, while some materials traditionally used in cleaning may no longer be available for these same reasons. Other health and safety concerns have created additional cleaning challenges, such as lead paint removal, which is likely to require special removal and disposal techniques. Cleaning can also cause damage to non -masonry materials on a building, including glass, metal and wood. Thus, it is usually necessary to cover windows and doors, and other features that may be vulnerable to chemical cleaners. They should be covered with plastic or polyethylene, or a masking agent that is applied as a liquid which dries to form a thin protective film on glass, and is easily peeled off after the cleaning is finished. Wind drift, for example, can also damage other property by carrying cleaning chemicals onto nearby automobiles, resulting in etching of the glass or spotting of the paint finish. Similarly, airborne dust can enter surrounding buildings, and excess water can collect in nearby yards and basements. Safety Considerations Possible health dangers of each method selected for the cleaning project must be considered before selecting a cleaning method to avoid harm to the cleaning applicators, and the necessary precautions must be taken. The precautions listed in Material Safety Data Sheets (MSDS) that are provided with chemical products should always be followed. Protective clothing, respirators, hearing and face shields, and gloves must be provided to workers to be worn at all times. Acidic and alkaline chemical cleaners in both liquid and vapor forms can also cause serious injury to The lower floors of this historic brick and architectural terra- cotta building have been covered during chemical cleaning to protect pedestrians and vehicular traffic from potentially harmful overspray. Photo: NPS files. passers-by. It may be necessary to schedule cleaning at night or weekends if the building is located in a busy urban area to reduce the potential danger of chemical overspray to pedestrians. Cleaning during non- business hours will allow HVAC systems to be turned off and vents to be covered to prevent dangerous chemical fumes from entering the building which will also ensure the safety of the buildingis occupants. Abrasive and mechanical methods produce dust which can pose a serious health hazard, particularly if the abrasive or the masonry contains silica. https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 8/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings Water -Repellent Coatings and Waterproof Coatings To begin with, it is important to understand that waterproof coatings and water-repellent coatings are not the same. Although these terms are frequently interchanged and commonly confused with one another, they are completely different materials. Water-repellent coatings—often referred to incorrectly as "sealers", but which do not or should not "seal"—are intended to keep liquid water from penetrating the surface but to allow water vapor to enter and leave, or pass through, the surface of the masonry. Water-repellent coatings are generally transparent, or clear, although once applied some may darken or discolor certain types of masonry while others may give it a glossy or shiny appearance. Waterproof coatings seal the surface from liquid water and from water vapor. They are usually opaque, or pigmented, and include bituminous coatings and some elastomeric paints and coatings. Water -Repellent Coatings Water-repellent coatings are formulated to be vapor permeable, or "breathable". They do not seal the surface completely to water vapor so it can enter the masonry wall as well as leave the wall. While the first water-repellent coatings to be developed were primarily acrylic or silicone resins in organic solvents, now most water-repellent coatings are water-based and formulated from modified siloxanes, silanes and other alkoxysilanes, or metallic stearates. While some of these products are shipped from the factory ready to use, other water -borne water repellents must be diluted at the job site. Unlike earlier water-repellent coatings which tended to form a "film" on the masonry surface, modern water-repellent coatings actually penetrate into the masonry substrate slightly and, generally, are almost invisible if properly applied to the masonry. They are also more vapor permeable than the old coatings, yet they still reduce the vapor permeability of the masonry. Once inside the wall, water vapor can condense at cold spots producing liquid water which, unlike water vapor, cannot escape through a water-repellent coating. The liquid water within the wall, whether from condensation, leaking gutters, or other sources, can cause considerable damage. Water-repellent coatings are not consolidants. Although modern water -repellents may penetrate slightly beneath the masonry surface, instead of just "sitting" on top of it, they do not perform the same function as a consolidant which is to "consolidate" and replace lost binder to strengthen deteriorating masonry. Even after many years of laboratory study and testing, few_ consolidants have proven very effective. The composition of fired products such as brick and 0 architectural terra cotta, as well as many types of building stone, does not lend itself to consolidation. r $ Some modern water-repellent coatings which contain a binder intended to replace the natural binders in stone that have been lost through weathering and natural erosion are described in product literature as both a water repellent and a consolidant The fact that the newer water- repellent coatings penetrate beneath the masonry surface instead of just forming a layer on top This clear coating has failed of the surface may indeed convey at least some consolidating properties to certain stones. and is pulling off pieces of the stone as it peels. Photo: NPS However, a water-repellent coating cannot be considered a consolidant. In some instances, a files water-repellent or "preservative" coating, if applied to already damaged or spalling stone, may form a surface crust which, if it fails, may exacerbate the deterioration by pulling off even more of the stone. Is a Water -Repellent Treatment Necessary? Water-repellent coatings are frequently applied to historic masonry buildings for the wrong reason. They also are often applied without an understanding of what they are and what they are intended to do. And these coatings can be very difficult, if not impossible, to remove from the masonry if they fail or become discolored. Most importantly, the application of water-repellent coatings to historic masonry is usually unnecessary. Most historic masonry buildings, unless they are painted, have survived for decades without a water-repellent coating and, thus, probably do not need one now. Water penetration to the interior of a masonry building is seldom due to porous masonry, but results from poor or deferred maintenance. Leaking roofs, clogged or deteriorated gutters and downspouts, missing mortar, or cracks and open joints around door and window openings are almost always the cause of moisture - related problems in a historic masonry building. If historic masonry buildings are kept watertight and in good repair, water-repellent coatings should not be necessary. Rising damp (capillary moisture pulled up from the ground), or condensation can also be a source of excess moisture in masonry buildings. A water-repellent coating will not solve this problem either and, in fact, may be likely to exacerbate it. Furthermore, a water-repellent coating should never be applied to a damp wall. Moisture in the wall would reduce the ability of a coating to adhere to the masonry and to penetrate below the surface. But, if it did adhere, it would hold the moisture inside the masonry because, although a water-repellent coating is permeable to water vapor, liquid water cannot pass https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 9/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings through it. In the case of rising damp, a coating may force the moisture to go even higher in the wall because it can slow down evaporation, and thereby retain the moisture in the wall. Excessive moisture in masonry walls may carry waterborne soluble salts from the masonry units themselves or from the mortar through the walls. If the water is permitted to come to the surface, the salts may appear on the masonry surface as efflorescence (a whitish powder) upon evaporation. However, the salts can be potentially dangerous if they remain in the masonry and crystallize beneath the surface as subflorescence. Subflorescence eventually may cause the surface of the masonry to spall, particularly if a water-repellent coating has been applied which tends to reduce the flow of moisture out from the subsurface of the masonry. Although many of the newer water-repellent products are more breathable than their predecessors, they can be especially damaging if applied to masonry that contains salts, because they limit the flow of moisture through masonry. When a Water -Repellent Coating May be Appropriate There are some instances when a water-repellent coating may be considered appropriate to use on a historic masonry building. Soft, incompletely fired brick from the 18th -and early -19th centuries may have become so porous that paint or some type of coating is needed to protect it from further deterioration or dissolution. When a masonry building has been neglected for a long period of time, necessary repairs may be required in order to make it watertight. If, following a reasonable period of time after the building has been made watertight and has dried out completely, moisture appears actually to be penetrating through the repointed and repaired masonry walls, then the application of a water-repellent coating may be considered in selected areas only. This decision should be made in consultation with an architectural conservator. And, if such a treatment is undertaken, it should not be applied to the entire exterior of the building. Improper cleaning methods may have been responsible for the formation of efflorescence on this brick. Photo: NPS files. Anti -graffiti or barrier coatings are another type of clear coating—although barrier coatings can also be pigmented—that may be applied to exterior masonry, but they are not formulated primarily as water repellents. The purpose of these coatings is to make it harder for graffiti to stick to a masonry surface and, thus, easier to clean. But, like water- repellent coatings, in most cases the application of anti -graffiti coatings is generally not recommended for historic masonry buildings. These coatings are often quite shiny which can greatly alter the appearance of a historic masonry surface, and they are not always effective. Generally, other ways of discouraging graffiti, such as improved lighting, can be more effective than a coating. However, the application of anti -graffiti coatings may be appropriate in some instances on vulnerable areas of historic masonry buildings which are frequent targets of graffiti that are located in out-of-the-way places where constant surveillance is not possible. Some water-repellent coatings are recommended by product manufacturers as a means of keeping dirt and pollutants or biological growth from collecting on the surface of masonry buildings and, thus, reducing the need for frequent cleaning. While this at times may be true, in some cases a coating may actually retain dirt more than uncoated masonry. Generally, the application of a water-repellent coating is not recommended on a historic masonry building as a means of preventing biological growth. Some water-repellent coatings may actually encourage biological growth on a masonry wall. Biological growth on masonry buildings has traditionally been kept at bay through regularly -scheduled cleaning as part of a maintenance plan. Simple cleaning of the masonry with low-pressure water using a natural- or synthetic -bristled scrub brush can be very effective if done on a regular basis. Commercial products are also available which can be sprayed on masonry to remove biological growth. In most instances, a water-repellent coating is not necessary if a building is watertight. The application of a water-repellent coating is not a recommended treatment for historic masonry buildings unless there is a specific problem which it may help solve. If the problem occurs on only part of the building, it is best to treat only that area rather than an entire building. Extreme exposures such as parapets, for example, or portions of the building subject to driving rain can be treated more effectively and less expensively than the entire building. Water-repellent coatings are not permanent and must be reapplied periodically although, if they are truly invisible, it can be difficult to know when they are no longer providing the intended protection. Testing a water-repellent coating by applying it in one small area may not be helpful in determining its suitability for the building because a limited test area does not allow an adequate evaluation of a treatment. Since water may enter and leave through the surrounding untreated areas, there is no way to tell if the coated test area is "breathable." But trying a coating in a small area may help to determine whether the coating is visible on the surface or if it will otherwise change the appearance of the masonry. Waterproof Coatings https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 10/12 8/7/2019 Preservation Brief 1: Assessing Cleaning and Water -Repellent Treatments for Historic Masonry Buildings Grimmer, Anne E. Preservation Brief 6: Dangers of Abrasive Cleaning to Historic Buildings. Washington, DC: Preservation Assistance Division, National Park Service, U.S. Department of the Interior, 1979. Grimmer, Anne E. Keeping it Clean: Removing Exterior Dirt, Paint, Stains and Graffiti from Historic Masonry Buildings. Washington, DC: Preservation Assistance Division, National Park Service, U.S. Department of the Interior, 1988. Park, Sharon C., AIA. Preservation Brief 39: Holding the Line: Controlling Unwanted Moisture in Historic Buildings. Washington, DC: Heritage Preservation Services, National Park Service, U.S. Department of the Interior, 1996. Powers, Robert M. Preservation Tech Note, Masonry No. 3, "Water Soak Cleaning of Limestone". Washington, DC: Preservation Assistance Division, National Park Service, U.S. Department of the Interior, 1992. Sinvinski, Valerie. "Gentle Blasting. " Old -House Journal. Vol. XXIV, No. 4 (July -August 1996), pp. 46-49. Weaver, Martin E. Conserving Buildings: A Guide to Techniques and Materials. New York: John Wiley & Sons, Inc., 1993. Weaver, Martin E. Preservation Brief 38: Removing Graffiti from Historic Masonry. Washington, DC: Preservation Assistance Division, National Park Service, U.S. Department of the Interior, 1995. Winkler, E.M. Stone in Architecture: Properties, Durability. Third, completely revised and extended edition. Berlin, Germany: Springer -Verlag, 1997. nps.gov EXPERIENCE YOUR ANIERir-A"m https://www.nps.gov/tps/how-to-preserve/briefs/1-cleaning-water-repellent.htm 12/12