1 PRESERVATION 

BRIEFS 

Assessing Cleaning and Water-Repellent 

Treatments for Historic Masonry Buildings 

Robert C. Mack, AlA 

Anne Grimmer 

u.s. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

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 waterrepellent 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. 

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 (Fig. 1). Although specifically 

intended for historic buildings, the information is applicable 

to all masonry buildings. This publication updates and 

expands Preservation BriefI: 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. 

Figure 1. Low-to medium- pressure steam (hot-pressurized water was/ling), is being used to clean the exterior of the U.S. Tariff Commission Building, the first marble building constructed in Wa shington, D.C., in 1839. This method was selected by an architecural conservator as the "gentlest means possible" 

to clean the marble. Steam ca n soften heavy soiling deposits such as those on the cornice and column capitals, and facilitate easy removal. Note how these deposits have been removed from the right side oJ the cornice which has already been cleaned. 
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Figure 2. Biological growth as shown on this marble foundation 

can usually be removed using a low-press ure water wash, possibly witiz 

a non-ionic detergent added to it, and scrubbing with a natura l or 

syllthetic bristle brush. 

Preparing for a Cleaning Project 

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 nonhistoric paint from the masonry; retard deterioration by 

removing soiling materials that may be damaging the 

masonry; or provide a clean surface to accurately match 

rep ointing 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 (Fig. 2). 

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 (Fig. 3). Was it to cover bad 

rep ointing or unmatched repairs? Was the building 

painted to protect soft brick or to conceal deteriorating 

stone? Or, was painted masonry simply a fashionable 

Figu re 3. Th is small test area has revealed a red brick patch that does 11 0t 

match the original beige brick. Th is may explain why the building was 

painted, and may suggest to the owner that it may be preferable to keep 

it pa inted. 

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 (Fig. 4). 

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. Previous treatments of the 

building and its surroundings should be researched and 

building maintenance records should be obtained, if 

available. Sometimes if streaked or spotty areas do not 

seem to get cleaner following an initial cleaning, closer 

inspection and analysis may be 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 (Fig. 5). 

Successful removal may require testing several cleaning 

agents to find something that will dissolve and remove the 

coating. Complete removal may not always be possible. 

Repairs may have been stained to match a dirty building, 

and cleaning may make these differences apparent. Deicing salts used near the building that have dissolved can 

Figure 4. The foundation of this brick building is limestone, but the 

decorative trim above is architectural terra cotta intended to simuillte 

stone. 

Figure 5. Repeated wllter washing did rIOt remove the staining inside 

this limestone porte cochere. Upon closer examination, it was 

determined to be a water-repellent coating that had been applied many 

years earlier. An alkaline cleaner may be effective in removing it. 

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. 

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. 

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Figure 6. Timed water soaking can be very effective for cleaning 

limestone and marble as shown here at the Marble Collegiate Church 

in New York City. In this case, a twelve-hour water soak using a 

multi-nozzle manifold was followed by a final water rinse. Photo: Diane 

S. Kaese, Wiss, Janney, Elstner Associates, lnc., N. Y. , N. Y. 

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. 

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 hung from moveable 

scaffolding or a swing stage that continuously mists the 

surface of the masonry with a very fine spray (Fig. 6). 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 methodit 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 mediumpressure water is probably one of the most commonly 

used methods for removing dirt or other pollutant soiling 

from historic masonry buildings (Fig. 7). 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 nonionic detergents include Igepal by GAF, Tergitol by Union 

Carbide and Triton by Rohm & Haas.) Thus, the addition 

of a non-ionic detergent, or surfactant, to a low- or mediumpressure water wash can be a useful aid in the cleaning 

'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. 
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 (Figs. 8-9). 

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 essential of all, it is important 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 (Figs. 10-11). 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. 

Figure 7. Glazed architectural terra cotta often may be cleaned 

successfully with a low-pressure water wash and hand scrubbing 

supplemented, if necessary, with a non-ionic detergent. Pho to: Na tional 

Park Service Files. 

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, 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. 

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Figure 8. (Left) Low-press ure (under 100 psi) steam cleaning 

(hot-pressurized water washing), is part of the regular maintenance 

program at the Jefferson Memorial, Washington, D.C. The white marble 

interior of this open structure is subject to constant soiling by birds, 

insects and visitors. (Right) Th is portable steam cleaner enables prompt 

clea nup when necessary. Photos: Na tional Park Service Files. 

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 (Fig. 12). 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 acidsensitive 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. 

Figure 9. (Left) This small steam cleaner- the size of a vacuum cleaner- offers a very controlled and gentle means of cleaning limited, or hard-to-reach 

areas or carved stone details. (Right) It is particularly useful for interiors where it is important to keep moisture to a minumum, such as inside 

the Washington Monument, Washington, D.C., where it was used to clean the commemorative stones. Photos: Audrey T. Tepper. 
Figure 10. High-pressure water washing too close to the surface has 

abraded and, consequently, marred the limestone on this early-20th 

century building. 

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 (Fig. 13). 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 

(Fig. 14). 

Potential hazards of chemical cleaning. 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. 

Figure 11. Rinsing with high-pressure water following chemical 

cleaning has left a horizontal line of abrasion across the bricks on this 

late-19th century row house. 

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 (Fig. 15). 

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. 

Figure 12. A mild acidic clean ing agent is being used to clean this 

heavily soiled brick and granite building. Additional applications of the 

cleaner and hand-scrubbing, and even poulticing, may be necessary to 

remove the dark stains on the granite arches below. Photo: Sharon C. 

Park, FAlA. 

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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. 

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 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 (Figs. 18-19). 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. 

Figure 13. Sometimes it may be preferable to paint over a thick asphaltic 

coating rather than try to remove it, because it can be difficult to remove 

completely. However, in this case, many layers of asphaltic coating 

were removed through multiple applications of a heavy duty chemical 

cleaner. Each application of the cleaner was left to dwell following the 

manufacturer's reccommendations, and then rinsed thoroughly. 

(As much as possible of the asphalt was first removed with wooden 

scrapers.) Although not all the asphalt was removed, this was 

determined to be an acceptable level of cleanliness for the project. 

Figure 14. Chemical removal of paint from this brick building has 

revealed that the cornice and window hoods are metal rather than 

masonry. 

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 repainting. 

Figure 15. The whitish deposits left on the brick by a chemical paint 

remover may have resulted from inadequate rinsing or from the 

chemical being left on the surface too long and may be impossible to 

remove. 
Poulticing to Remove Stains and Graffiti 

a 

c 

d 

Figure 16. (a) The limestone base was heavily stained by runoff 

from the bronze statue above. (b) A poultice consisting of copper 

stain remover and ammonia mixed with fuller's earth was applied 

to the stone base and covered with plastic sheeting to keep it from 

drying out too quickly. (c) As the poultice dried, it pulled the stain 

out of the stone. (d) The poultice residue was removed carefully 

from the stone surface with wooden scrapers and the stone was 

rinsed with wa ter. Photos: John Dugger. 

b 

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 fuller 's earth, or even shredded paper or paper 

towels), mixed with a liquid (solvent or other remover) to 

form a paste which is applied to the stain (Figs. 16-17). 

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. 

Figure 17. A poultice is being used to remove salts from the brownstone 

statuary on the facade of this late-19th century stone church. Photo: 

National Park Service Files. 

9 
10 

Figure 18. The glazed bricks in the center of the pier were covered by a 

signboard that protected them being damaged by the sandblasting 

which removed the glaze from the surrounding bricks. 

Abrasive Blasting. Blasting with abrasive grit or another 

abrasive material is the most frequently used abrasive 

method. Sandblasting is most commonly associated with 

abrasive cleaning. Finely ground silica or glass powder, 

glass beads, ground garnet, powdered walnut and 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 walnut shells 

are commonly used for cleaning monumental bronze 

sculpture, and skilled conservators clean delicate museum 

objects and finely detailed, carved stone features with very 

small, micro-abrasive units using aluminum oxide. 

Figure 19. A comparison of undamaged bricks surroundng the electrical 

conduit with the rest of the brick facade emphasizes the severity of the 

erosion caused by sandblasting. 

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 (Fig. 20). Cleaning with 

baking soda (sodium bicarbonate) is another patented 

process. Bakmg 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. 

Figure 20. (Left) A comparison of the limestone surface of a 1920s office building before and after "cleaning" with a proprietary abrasive process using 

fine glass powder clearly shows the effectiveness of this method. But this is an abrasive technique and it has "cleaned" by removing part of the masonry 

surface with the dirt. Because it is abrasive, it is generally not recommended for large-scale cleaning of historic masonry, although it may be suitable to 

use in certain, very limited cases under controlled circumstances. (Right) A vacum chamber where the used glass powder is collected for environmentally 

safe disposal is a unique feature of this particular process. The specially-trained operators in the chamber wear protective clothing, masks and breathing 

equipment. Photos: Tom Keohan. 
Figure 21. Low-pressure blasting with ice pellets or ice crystals (left) is 

an abrasive cleaning method that is sometimes recommended for use 

on interior masonry because it does not involve large amounts of water. 

However, like other abrasive materials, ice crystals "clean" by removing 

a portion of the masonry surface with the dirt, and may not remove 

sOllie stains that have penetrated into the l1lasollry without causing 

further abrasion (right). Photos: Audrey T. Tepper. 

Ice particles, or pelletized dry ice (carbon dioxide or C02), 

are another medium used as an abrasive cleaner (Fig. 21). 

TItis 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, highlycontrolled, 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 

gentlest 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 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 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 

(Fig. 22). 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. 

11 
12 

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 

intermpted 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 

Figure 22. Cleaning test areas may be quite small at first and gradually 

increase in size as testing determines the "gentlest means possible". 

Photo: Frances Gale. 

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 passers-by (Fig. 23). 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 building's occupants. 

Abrasive and mechanical methods produce dust which 

can pose a serious health hazard, particularly if the abrasive 

or the masonry contains silica. 

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 --Dften 

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 (Fig. 

24). 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 waterrepellent coatings to be developed were primarily acrylic 

or silicone resins in organic solvents, now most waterrepellent 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 waterborne 

water repellents must be diluted at the job site. Unlike 

earlier water-repellent coatings which tended to form a 

"film" on the masonry surface, modem 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 

modem 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 architectural terra cotta, as well as many 

types of building stone, does not lend itself to consolidation. 

Some modem 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 newer water-repellent 

coatings penetrate beneath the masonry surface instead 

of just forming a layer on top of the surface may indeed 

convey at least some consolidating properties to certain 

stones. However, a water-repellent coating cannot be 

considered a consolidant. In some instances, a waterrepellent 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 (Fig. 25). 

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. 

Figure 23. A tarpaulin protects and shields pedestrians from potentially 

harmful spray while chemical cleaning is underway on the granite 

exterior of the U.S. Treasury Building, Washington, D.C. 

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 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 

13 
14 

Figure 24. Although the application of a water-repellent coating W IlS 

probably not needed on either of these buildings, the coating on the 

brick building (above), is not visible and has not changed tile character 

of the brick. But the coating on the brick colllmn (below), has a high 

gloss that is incompatible with the historic character of the masonry. 

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 wails, 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. 

Anti-graffiti or barrier coatings are another type of clear 

coating-although barrier coatings can also be pigmentedthat 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 (Fig. 

26). 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 waterrepellent 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 
Figure 25. The clear coating applied to this limestone molding has 

fa iled and is taking off some of the stone surface as it peels. Photo: 

Frances Ga le. 

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 

Figure 26. The anti-graffiti or barrier coating on this column is very 

shiny and wou ld not be appropriate to use on a historic masonry 

building. The coating has discolored as it has aged and whitish streaks 

reveal areas of bare concrete where the coating was incompletely 

applied . 

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 such 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 

In theory, waterproof coatings usually do not cause 

problems as long as they exclude all water from the 

masomy. If water does enter the wall from the ground or 

from the inside of a building, the coating can intensify the 

damage because the water will not be able to escape. 

During cold weather this water in the wall can freeze 

causing serious mechanical disruption, such as spalling. 

In addition, the water eventually will get out by the path 

of least resistance. If this path is toward the interior, 

damage to interior finishes can result; if it is toward the 

exterior, it can lead to damage to the masomy caused by 

built-up water pressure (Fig. 27). 

In most instances, waterproof coatings should not be 

applied to historic masonry. The possible exception to 

this might be the application of a waterproof coating to 

below-grade exterior foundation walls as a last resort to 

stop water infiltration on interior basement walls. 

Generally, however, waterproof coatings, which include 

elastomeric paints, should almost never be applied above 

grade to historic masonry buildings. 

Figure 27. Instead of correcting the roof drainage problems, an 

elastomeric coating was applied to the already saturated limeston e 

cornice. An elastomeric coating holds moisture in the masonry because 

it does not "breathe" and does not allow liquid moisture to escape. If 

the water pressure builds up sufficiently it can cause the coating to 

break and pop off as shown in this example, often pulling pieces of the 

masonry with it. Photo: National Park Service Files. 

15 
16 

Summary 

A well-planned cleaning project is an essential step in 

preserving, rehabilitating or restoring a historic masonry 

building. Proper cleaning methods and coating treatments, 

when determined necessary for the preservation of the 

masonry, can enhance the aesthetic character as well as the 

structural stability of a historic building. Removing years 

of accumulated dirt, pollutant crusts, stains, graffiti or 

paint, if done with appropriate caution, can extend the life 

and longevity of the historic resource. Cleaning that is 

carelessly or insensitively prescribed or carried out by 

inexperienced workers can have the opposite of the intended 

effect. It may scar the masonry permanently, and may 

actually result in hastening deterioration by introducing 

harmful residual chemicals and salts into the masonry or 

causing surface loss. Using the wrong cleaning method or 

using the right method incorrectly, applying the wrong 

kind of coating or applying a coating that is not needed 

can result in serious damage, both physically and 

aesthetically, to a historic masonry building. Cleaning a 

historic masonry building should always be done using 

the gentlest means possible that will clean, but not damage 

the building. It should always be taken into consideration 

before applying a water-repellent coating or a waterproof 

coating to a historic masonry building whether it is really 

necessary and whether it is in the best interest of preserving 

the building. 

Selected Reading 

Architectural Ceramics: Their History, Manufacture and 

Conservation. A Joint Symposium of English Heritage and the 

United Kingdom Institute for Conservation, September 22-25, 

1994. London: English Heritage, 1996. 

Ashurst, Nicola. Cleaning Historic Buildings. Volume One: 

Substrates, Soiling & Investigation. Volume Two: Cleaning 

Materials & Processes. London: Donhead Publishing Ltd., 1994. 

Association for Preservation Technology. Special Issue: 

Preservation of Historic Masonry. Papers from the Symposium 

on Preservation Treatments for Historic Masonry: Consolidants, 

Coatings, and Water Repellents, New York, New York, November 

11-12,1994. APT Bulletin. Vol. XXVI, No.4 (1995). 

Grimmer, Anne E. Preservation Brief 6: Dangers of Abrasive 

Cleaning to Historic Buildings. Washington, D.C: Preservation 

Assistance Division, National Park Service, U.s. Department 

of the Interior, 1979. 

Grimmer, Anne E. Ke~ping it Clean: Removing Exterior Dirt, Paint, Stains and Graffiti ]rom Historic Masonry Buildings. Washington, D.C.: Preservation Assistance Division, National 

Park Service, U.S. Department of the Interior, 1988. 

Park, Sharon c., AlA. Preservation Brief39: Holding the Line: 

Controlling Unwanted Moisture in Historic Buildings. 

Washington, D.C: 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, D.C: 

Preservation Assistance Division, National Park Service, U.s. 

Department of the Interior, 1992. 

Sinvinski, Valerie. "Gentle Blasting." Old-House Journal. Vol. 

XXIV, No.4 Ouly-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, D.C: 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. 

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. 

Anne Grimmer is a Senior Architectural Historian in the Technical 

Preservation Services Branch, Heritage Preservation Services 

Program, National Park Service, Washington, D.C 

The original version of Preservation Brief 1: The Cleaning and 

Waterproof Coating of Masonry Buildings was written by Robert C 

Mack, AlA. It inaugurated the Preservation Briefs series when 

it was published in 1975. 

The following historic preservation specialists provided technical 

review of this publication: Frances Gale, Training Director, 

National Center for Preservation Technology and Training, 

National Park Service, Natchitoches, LA; Judith M. Jacob, 

Architectural Conservator, Building Conservation Branch, 

Northeast Cultural Resources Center, National Park Service, N.Y., 

NY; Robert M. Powers, Architectural Conservator, Powers and 

Company, Inc., Philadelphia, PA; Antonio Aguilar, Kaaren Dodge, 

JoEllen Hensley, Gary Sachau, John Sandor and Audrey T. Tepper, 

Technical Preservation Services Branch, Heritage Preservation 

Services Program, National Park Service, Washington, D.C; and 

Kay D. Weeks, Heritage Preservation Services Program, National 

Park Service, Washington, D.C. 

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. Comments on the usefulness of 

this publication may be directed to: Sharon C. Park, FAIA, Chief, 

Technical Preservation Services Branch, Heritage Preservation 

Services Program, National Park Service, 1849 C Street, N.W., 

Suite NC200, Washington, D.C 20240 (www2.cr.nps.gov/tps). 

This publication is not copyrighted and can be reproduced without 

penalty. Normal procedures for credit to the authors and the 

National Park Service are appreciated. 

Front Cover: Chemical cleaning of the brick and architectural terra 

cotta frieze on the 1880s Pellsioll Buildillg, Washington, D.C. (now the 

National Building Museulll), is showll here in progress. Photo: 

Christina Henry. 

Photographs llsed to illustrate this Brief were taken by Anne Grimmer unless 

otherwise credited. 

ISSN:0885-7016 November 2000