6 PRESERV ATION 

BRIEFS 

Dangers of Abrasive Cleaning 

to Historic Buildings 

Anne E. Grimmer 

U.S. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

"The surface cleaning of structures shall be unde rtaken with the gentlest means possible . Sandblasting and other 

cleaning methods that will damage the historic building materials shall not be undertaken ."-The Secretary of the 

Interior's "Standards for Historic Preservation Projects." 

Abrasive cleaning methods are responsible for ca using a great 

deal of damage to historic building materials. To prevent 

indiscriminate use of these potentially harmful techniques. 

this brief has been prepared to explain abrasive cleaning 

methods, how they can be physically and aesthetically destructive to historic building materials, and why they generally 

are not acceptable preservation treatments for historic structures. There are alternative, less harsh means of cleaning and 

removing paint and stains from historic buildings. However. 

careful testing should preceed general cleaning to assure that 

the method selected will not have an adverse effect on th e 

building materials. A historic building is irreplaceable. and 

should be cleaned using only the "gen tlest me ans possible" 

to best preserve it. 

What is Abrasive Cleaning? 

Abrasive cleaning methods include all techniques that physically abrade the building surface to remove soils, discolorations or coatings. Such techniques involve the use of certain 

materials which impact or abrade the surface under pressure , 

or abrasive tools and equipment. Sand, because it is readily 

available, is probably the most commonly used type of grit 

material. However, any of the following materials may be 

substituted for sand, and all can be classified as abrasive 

substances: ground slag or volcanic ash, crushed (pulverized) 

walnut or almond shells. rice husks, gwund corncobs, ground 

coconut shells, crushed eggshells, silica flour , synthetic particles, glass beads and micro-balloons. Even water under pressure can be an abrasive substance. Tools and equipment that 

are abrasive to historic building materials include wire 

brushes, rotary wheels, power sanding disks and belt sanders. 

The use of water in combination with grit may also be 

classified as an abrasive cleaning method. Depending on the 

manner in which it is applied, water may soften the impact 

of the grit, but water that is too highly pressurized can be 

very abrasive . There are basically two different methods 

which can be referred to as "wet grit," and it is importa nt to 

differentiate between the two. One technique involves the 

addition of a stream of water to a regular sandblasting nozzle. 

This is done primarily to cut down dust. and has very little, 

if any , effect on reducing the aggressiveness, or cutting action 

of the grit particles. With the second technique, a very small 

amount of grit is added to a pressurized water stream. This 

method may be controlled by regulating the amount of grit 

fed into the water stream, as well as the pressure of the water. 

Why Are Abrasive Cleaning Methods Used? 

Usually, an abrasive cleaning method is selected as an expeditious means of quickly removing years of dirt accumulation, unsightly stains, or deteriorating building fabric or 

finishes, such as stucco or paint. The fact that sandblasting 

is one of the best known and most readily available building 

cleaning treatments is probably the major reason for its frequent use . 

Many mid-19th century brick buildings were painted immediately or soon after completion to protect poor quality 

brick or to imitate another material. such as stone. Sometimes 

brick buildings were painted in an effort to produce what was 

considered a more harmonious relationship between a building and its natural surroundings. By the 1870s, brick buildings 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
Abrasively Cleaned vs. Untouched Brick. Two brick rowhouses with 

a common far;ade provide an excellent point of comparison when only 

one of the houses has been sandblasted. It is clear that abrasive blasting. 

by removing the outer surface. has left the brickwork on the left rough 

and pitted. while that on the right still exhibits an undamaged and 

relatively smooth surface. Note that the abrasive cleaning has also 

removed a considerable portion of the mortar from the joints of the 

brick on the left side. which will require repointing. 

were often left unpainted as mechanization in the brick industry brought a cheaper pressed brick and fashion decreed 

a sudden preference for dark colors. However. it was still 

customary to paint brick of poorer quality for the additional 

protection the paint afforded. 

It is a common 20th-century misconception that a ll historic 

masonry buildings were initially unpainted. If the intent of 

a modern restoration is to return a building to its original 

appearance. removal of the paint not only may be historically 

inaccurate, but also harmful. Many older buildings were 

painted or stuccoed at some point to correct recurring maintenance problems caused by faulty construction techniques. 

to hide alterations, or in an attempt to solve moisture problems. If this is the case. removal of paint or stucco may cause 

these problems to reoccur. 

Another reason for paint removal. particularly in rehabilitation projects. is to give the building a " new image" in 

response to contemporary design trends and to attract investors or tenants. Thus. it is necessary to consider the purpose 

of the intended cleaning. While it is clearly important to 

remove unsightly stains. heavy encrustations of dirt. peeling 

paint or other surface coatings. it may not be equally desirable 

to remove paint from a building which originally was painted. 

Many historic buildings which show only a slight amount of 

soil or discoloration are much better left as they are. A thin 

layer of soil is more often protective of the building fabric 

than it is harmful. and seldom detracts from the building's 

2 

Abrading the Surface without Removing the Paint. Even though the 

entire outer surface layer of the brick has been sandblasted off. spots 

of paint still cling to the masonry. Sandblasting or other similarly 

abrasive methods are not always a successful means of removing paint. 

architectural and/or historic character. Too thorough cleaning 

of a historic building may not only sacrifice some of the building's character, but also. misguided cleaning efforts can cause 

a great deal of damage to historic building fabric. Unless 

there are stains, graffiti or dirt and pollution deposits which 

are destroying the building fabric. it is generally preferable 

to do as little cleaning as possible. or to repaint where necessary. It is important to remember that a historic building 

does not have to look as if it were newly constructed to be 

an attractive or successful restoration or rehabilitation project. For a more thorough explanation of the philosophy of 

cleaning historic buildings see Preservation Briefs: No. I 

"The Cleaning and Waterproof Coating of Masonry Buildings," by Robert C. Mack . AlA. 

Problems of Abrasive Cleaning 

The crux of the problem is that abrasive cleaning is just thatabrasive. An abrasively cleaned historic structure may be 

physically as well as aesthetically damaged. Abrasive methods 

"clean" by eroding dirt or paint, but at the same time they 

also tend to erode the surface of the building material. In this 

way, abrasive cleaning is destructive and causes irreversible 

harm to the historic building fabric. If the fabric is brick, 

abrasive methods remove the hard, outer protective surface, 

and therefore make the brick more susceptible to rapid weathering and deterioration . Grit blasting may also increase the 

water permeability of a brick wall. The impact of the grit 

particles tends to erode the bond beiween the mortar and the 

brick, leaving cracks or enlarging existing cracks where water 

can enter. Some types of stone develop a protective patina 

or " quarry crust" parallel to the worked surface (created by 

the movement of moisture towards the outer edge). which 

also may be damaged by abrasive cleaning. The rate at which 

the material subsequently weathers depends on the quality 

of the inner surface that is exposed. 

Abrasive cleaning can destroy. or substantially diminish. 

decorative detailing on buildings such as a molded brickwork 

or architectural terra-cotta. ornamental carving on wood or 

stone. and evidence of historic craft techniques. such as tool 

marks and other surface textures. In addition. perfectly sound 

and/or "tooled" mortar joints can be worn away by abrasive 

techniques. This not only results in the loss of historic craft 

detailing but also requires repointing. a step involving con-
slderable time, skill and expense, and which might not have 

been necessary had a gentler method been chosen. Erosion 

and pitting of the building material by abrasive cleaning creates a greater surface area on which dirt and pollutants collect. In this sense, the building fabric "attracts" more dirt, 

and will require more frequent cleaning in the future. 

In addition to causing physical and aesthetic harm to the 

historic fabric, there are several adverse environmental effects of dry abrasive cleaning methods. Because of the friction 

caused by the abrasive medium hitting the building fabric, 

these techniques usually create a considerable amount of 

dust, which is unhealthy, particularly to the operators of the 

abrasive equipment. It further pollutes the environment 

around the job site, and deposits dust on neighboring buildings, parked vehicles and nearby trees and shrubbery. Some 

adjacent materials not intended for abrasive treatment such 

as wood or glass, may also be damaged because the equipment 

may be difficult to regulate. 

Wet grit methods, while eliminating dust, deposit a messy 

slurry on the ground or other objects surrounding the base 

of the building. In colder climates where there is the threat 

of frost , any wet cleaning process applied to historic masonry 

structures must be done in warm weather, allowing ample 

time for the wall to dry out thoroughly before cold weather 

sets in. Water which remains and freezes in cracks and openings of the masonry surface eventually may lead to spalling. 

High-pressure wet cleaning may force an inordinate amount 

of water into the walls, affecting interior materials such as 

plaster or joist ends, as well as metal building components 

within the walls. 

Variable Factors 

The greatest problem in developing practical guidelines for 

cleaning any historic building is the large number of variable 

and unpredictable factors involved. Because these variables 

make each cleaning project unique, it is difficult to establish 

specific standards at this time. This is particularly true of 

abrasive cleaning methods because their inherent potential 

for causing damage is multiplied by the following factors: 

- the type and condition of the material being cleaned; 

- the size and sharpness of the grit particles or the mechanical equipment; 

- the pressure with which the abrasive grit or equipment is 

applied to the building surface; 

- the skill and care of the operator; and 

- the constancy of the pressure on all surfaces during the 

cleaning process. 

Micro-Abrasive Cleaning. This small. pencil-sized micro-abrasive unit 

is used by some museum conservators to clean small objects. This 

particular micro-abrasive unit is operated within the confines of a box 

(approximately 2 cubic feet of space). but a similar and slightly larger 

unit may be used for cleaning larger pieces of sculpture. or areas of 

architectural detailing on a building. Even a pressure cleaning unit this 

small is capable of eroding a surface. and must be carefully controlled. 

"Line Drop." Even though the operator afthe sandblasting equipment 

is standing on a ladder to reach the higher sections of the wall. it is still 

almost impossible to have total control over the pressure. The pressure 

of the sand hitting the lower portion of the wall will still be greater 

than that above. because of the "line drop" in the distance from the 

pressure source to the nozzle. (Hugh Miller) 

Pressure: The damaging effects of most of the variable factors 

involved in abrasive cleaning are self evident. However, the 

matter of pressure requires further explanation. In cleaning 

specifications, pressure is generally abbreviated as " psi " 

(pounds per square inch), which technically refers to the "tip" 

pressure, or the amount of pressure at the nozzle of the blasting apparatus. Sometimes "psig," or pressure at the gauge 

(which may be many feet away, at the other end of the hose), 

is used in place of "psi." These terms are often incorrectly 

used interchangeably. 

Despite the apparent care taken by most architects and 

building cleaning contractors to prepare specifications for 

pressure cleaning which will not cause harm to the delicate 

fabric of a historic building, it is very difficult to ensure that 

the same amount of pressure is applied to all parts of the 

building. For example, if the operator of the pressure equipment stands on the ground while cleaning a two-story structure, the amount of force reaching the first story will be 

greater than that hitting the second story, even if the operator 

stands on scaffolding or in a cherry picker, because of the 

"line drop" in the distance from the pressure source to the 

nozzle. Although technically it may be possible to prepare 

cleaning specifications with tight controls that would eliminate all but a small margin of error, it may not be easy to 

find professional cleaning firms willing to work under such 

restrictive conditions. The fact is that many professional 

building cleaning firms do not really understand the extreme 

delicacy of historic building fabric, and how it differs from 

modern construction materials. Consequently, they mily ac3 
cept building cleaning projects for which they have no experience. 

The amount of pressure used in any kind of cleaning treatment which involves pressure, whether it is dry or wet grit, 

chemicals or just plain water, is crucial to the outcome of the 

cleaning project. Unfortunately, no standards have been established for determining the correct pressure for cleaning 

each of the many historic building materials which would not 

cause harm. The considerable discrepancy between the way 

the building cleaning industry and architectural conservators 

define " high" and "low" pressure cleaning plays a significant 

role in the difficulty of creating standards. 

Nonhistoricllndustria/: A representative of the building cleaning industry might consider " high" pressure water cleaning 

to he anything over 5,000 psi, or even as high as 10,000 to 

15 ,000 psi' Water under this much pressure may be necessary 

to clean industrial structures or machinery, but would destroy 

most historic building materials. Industrial chemical cleaning 

commonly utilizes pressures between I ,000 and 2,500 psi. 

Spalling Brick, This soft. earlv 19th-cell/llry hrick was sandblasted ill 

the 1960s; consequentlv. ~el'ere spallillg has resulted. Some hricks hal'l' 

almo.1'I towlly disintegrated. and will el'enll/allv hUl'e to he replaced. 

(Rohert S. Gamhle) 

Historic: By contrast. conscientious dry or wet abrasive cleaning of a historic structure wo uld be conducted within the 

range of 20 to 100 psi at a range of 3 to 12 inches. Cleaning 

al this low pressure requires the use of a very fine 00 or 0 

mesh grit forced through a nozzle with a Y4 inch opening. A 

similar. even more delicate method being adopted by architectural conservators uses a micro-abrasive grit on small, 

hard-to-clean areas of carved, cut or molded ornament on a 

building fac;ade. Originally developed by museum conservators for cleaning sculpture, this technique may employ glass 

beads, micro-balloons, or another type of micro-abrasive 

gently powered at approximately 40 psi by a very small, almost pencil-like pressure instrument. Although a slightly 

larger pressure instrument may be used on historic buildings, 

this technique still has limited practical applicability on a large 

scale building cleaning project because of the cost and the 

relatively few technicians competent to handle the task. In 

general. architectural conservators have determined that only 

through very com rolled conditions can most historic building 

material be abrasivl:ly cleaned of soil or paint without measurable damage to the surface or profile of the substrate. 

Yet some professional cleaning companies which sepcialize 

in cleaning historic masonry buildings use chemicals and water 

at a pressure of approximately 1,500 psi, while other cleaning 

firms recommend lower pressures ranging from 200 to 800 psi 

for a similar project. An architectural conservator might decide, after testing. that some historic structures could be 

cleaned properly using a moderate pressure (200-600 psi), or 

even a high pressure (600-1800 psi) water rinse . However, 

4 

cleaning historic buildings under such high pressure should 

be considered an exception rather than the rule, and would 

require very careful testing and supervision to assure that the 

historic surface materials could withstand the pressure without gouging, pitting or loosening. 

These differences in the amount of pressure used by commercial or industrial building cleaners and architectural conservators point to one of the main problems in using abrasive 

means to clean historic buildings: misunderstanding of the 

potentially fragile nature of historic building materials. There 

is no one cleaning formula or pressure suitable for all situations. Decisions regarding the proper cleaning process for 

historic structures can be made only after careful analysis of 

the building fabric, and testing. 

How Building Materials React to Abrasive Cleaning 

Methods 

Brick and Architectural Terra-Cotta: Abrasive blasting does 

not affect all building materials to the same degrec. Such 

techniques quite logically cause greater damage to softer and 

more porous materials, such as brick or architectural terracotta. When these materials are cleaned abrasively, the hard , 

outer layer (closest to the heat of the kiln) is eroded, leaving 

the soft. inner core exposed and susceptible to accelerated 

weathering. Glazed architectural terra-cotta and ceramic veneer have a baked-on glaze which is also easily damaged by 

abrasive cleaning. Glazed architectual terra-cotta was designed for easy maintenance, and generally can be cleaned 

using detergent and water; but chemicals or steam may be 

needed to remove more persistent stains. Large areas of brick 

or architectural terra-cotta which have been painted are best 

left painted. or repainted if necessary . 

Plaster and Stucco: Plaster and stucco are types of masonry 

finish materials that are softer than brick or terra-cotta: if 

treated abrasively these materials will simply disintegrate. 

Indeed. when plaster or stucco is treated abrasively it is usually with the intention of removing the plaster or stucco from 

whatever base material or substrate it is covering. Obviously. 

such abrasive techniques should not be applied to clean sound 

plaster or stuccoed walls, or decorative plaster wall surfaces. 

Building Stones: Building stones are cut from the three main 

categories of natural rock: dense, igneous rock such as granite; sandy. sedimentary rock such as limestone or sandstone: 

and crystalline, metamorphic rock such as marble. As opAbrasive Cleaning of Tooled Granite, Even this carefully colllrolled 

"wet grit" blasting has erased verticallOoling marks in the CIII granite 

blocks on the left. Not only has the lOoling been destroyed, bill the 

damaged stone surface is now more susceptible 10 accelerated weathering. 
posed to kiln-dried masonry materials such as brick and architectural terra-cotta , building stones are generally 

homogeneous in character at the time of a bui lding's construction. However, as the stone is exposed to weatheri ng 

and environmental pollutants, the surface may become friable, or may develop a protective skin or patina. These outer 

surfaces are very susceptible to damage by abrasive or improper chemical cleaning. 

Building stones are frequently cut into ashlar blocks or 

"dressed" with tool marks that give the building surface a 

specific texture and contribute to its historic character as 

much as ornately carved decorative stonework. Such detailing 

is easi ly damaged by abrasive cleaning techniques: the pattern 

of tooling or cutting is erased, and the crisp lines of moldings 

or carving are worn or pitted . 

Occasionally, it may be possible to clean small areas of 

rough-cut granite , limestone or sandstone having a heavy dirt 

encrustation by using the "wet grit" method . whereby a small 

amount of abrasive material is injected into a controlled. 

pressurized water stream. However, this techn ique requires 

very ca reful supervision in order to prevent damage to the 

stone. Polished or honed marble or granite shou ld never be 

treated abrasively, as the abrasion would remove the fin ish 

in much the way glass would be etched or "frosted" by such 

a process. It is genera ll y preferable to underclean . as too 

strong a cleaning procedure will erode the stone, exposing 

a new and increased surface area to collect atmospheric moisture and dirt. Removing paint, stains or graffiti from most 

types of stone may be accomplished by a chemical treatment 

carefully selected to best handle the removal of the particular 

type of paint or stain without damaging the stone. (See section 

on the "Gentl est Means Possible") 

Abrasive Cleaning of Wood. This wooden windowsill. molding and 

paneling have been sandblasted to remove layers oj paint in the rehabilitation oj this commercial building. Not only is some paint still 

embedded in cracks and crevices oj the woodwork. but more importantly. grit blasting has actually eroded the summer wood. in eJJect 

raising the grain. and resulting in a rough surJace. 

Wood: Most types of wood used for buildings are soft. fibrous 

and porous, and are particularly susceptible to damage by 

abrasive cleaning. Because the summer wood between the 

lines of the grain is softer than the grain itself, it will be worn 

away by abrasive blasting or power tools, leaving an uneven 

surface with the grain raised and often frayed or "fuzzy," 

Once this has occurred, it is almost impossible to achieve a 

smooth surface again except by extensive hand sanding, which 

is expensive and will quickly negate any costs saved earlier 

by sandblasting. Such harsh cleaning treatment also obliterates historic tool marks, fine carving and detailing, which 

precludes its use on any interior or exterior woodwork which 

has been hand planed , milled or carved. 

Metals: Like stone, metals are another group of building 

materials which vary considerably in hardness and durability. 

Softer metals which are used architecturally. such as tin, zinc , 

lead , copper or aluminum, generally should not be cleaned 

abrasively as the process deforms and destroys the original 

surface texture and appearance, as well as the acquired patina . Much applied architectural metal work used on historic 

buildings-tin , zinc , lead and copper-is often quite thin and 

soft, and therefore susceptible to denting and pitting. Galvanized sheet metal is especially vulnerable, as abrasive treatment would wear away the protective galvanized layer. 

In the late 19th and early 20th centuries, these metals were 

often cut, pressed or otherwise shaped from sheets of metal 

into a wide variety of practical uses such as roofs, gutters and 

flashing , and fac;ade ornamentation such as cornices. friezes. 

dormers, panels, cupolas. oriel windows. etc. The architecture of the 1920s and 1930s made use of metals such as 

chrome, nickel alloys, aluminum and stainless steel in decorative exterior panels, window frames. and doorways. Harsh 

abrasive blasting would destroy the original surface finish of 

most of these metals, and would increase the possiblity of 

corrosion. 

However, conservation specialists are now employing a 

sensitive technique of glass bead peening to clean some of 

the harder metals, in particular large bronze outdoor sculpture . Very fine (75-125 micron) glass beads are used at a low 

pressure of 60 to 80 psi. Because these glass beads are completely spherical, ther are no sharp edges to cut the surface 

of the metal. After cleaning, these statues undergo a lengthy 

process of polishing. Coatings are applied which protect the 

surface from corrosion. but they must be renewed every 3 to 

5 years. A similarly delicate cleaning technique employing 

glass beads has been used in Europe to clean historic masonry 

structures without causing damage. But at this time the process has not been tested sufficiently in the United States to 

recommend it as a building conservation measure. 

Sometimes a very fine smooth sand is used at a low pressure 

to clean or remove paint and corrosion from copper flashing 

and other metal building components. Restoration architects 

recently found that a mixture of crushed walnut shells and 

copper slag at a pressure of approximately 200 psi was the 

only way to remove corrosion successfully from a mid-19th 

century terne-coated iron roof. Metal cleaned in this manner 

must be painted immediately to prevent rapid recurrence of 

corrosion. It is thought that these methods "work harden " 

the surface by compressing the outer layer. and actually may 

be good for the surface of the metal. But the extremely complex nature and the time required by such processes make it 

very expensive and impractical for large-scale use at this time. 

Cast and wrought iron architectural elements may be gently 

sandblasted or abrasively cleaned using a wire brush to remove layers of paint, rust and corrosion. Sandblasting was. 

in fact , developed originally as an efficient maintenance procedure for engineering and industrial structures and heavy 

machinery-iron and steel bridges, machine tool frames. engine frames, and railroad rolling stock-in order to clean and 

prepare them for repainting. Because iron is hard , its surface. 

5 
which is naturally somewhat uneven, will not be noticeably 

damaged by controlled abrasion. Such treatment will. how'-

ever, re~ult in a small amount of pitting. But this slight abrasion creates a good surface for paint, since the iron must bc 

repainted immediately to prevent corrosion. Any abrasive 

cleaning of metal building components will also remove the 

caulking from joints and around other openings. Such areas 

must be recaulked quickly to prevent moisture from entering 

and rusting the metal, or causing deterioration of other building fabric inside the structure. 

When is Abrasive Cleaning Permissible? 

For the most part, abrasive cleaning is destructive to historic 

building materials. A limited number of special cases have 

been explained when it may be appropriate, if supervised by 

a skilled conservator, to use a delicate abrasive technique on 

some historic building materials. The type of "wet grit" cleaning which involves a small amount of grit injected into a 

stream of low pressure water may be used on small areas of 

stone masonry (i.e., rough cut limestone, sandstone or unpolished granite), where milder cleaning methods have not 

been totally successful in removing harmful deposits of dirt 

and pollutants. Such areas may include stone window sills, 

the wps of cornices or column capitals, or other detailed areas 

of the fa<;ade. 

This is still an abrasive technique, and without proper caution in handling, it can be jus I as harmful 10 Ihe building 

surface as any olher abrasive cleaning method. Thus, the decision to use this type of "wet grit" process should be made 

only after consultation with an experienced building conservator. Remember that il is very lime consuming and expensive to use any abrasive technique on a historic building 

in such a manner that it does not cause harm to the often fragile 

and friable building materials. 

At this time , and only under certain circumstances, abrasive 

cleaning methods may be used in the rehabilitation of interior 

spaces of warehouse or industrial buildings for contemporary 

uses. 

Interior spaces of factories or warehouse structures in which 

the masonry or plaster surfaces do not have significant design, 

detailing, tooling or finish, and in which wooden architectural 

features are not finished, molded, beaded or worked by hand, 

may be cleaned abrasively in order to remove layers of paint 

and industrial discolorations such as smoke, soot, etc. It is 

expected after such treatment that brick surfaces will be rough 

and pitted, and wood will be somewhat frayed or "fuzzy" 

Permissible Abrasive Cleaning, In accordance with the Secretary of 

the Interior's Guidelines for Rehabilitation Projects, it may be acceptable to use abrasive techniques to clean an industrial interior space 

such as that il/ustraled here, because the masonry surfaces do not have 

significant design, detailing, tooling or finish, and the wooden architectural features are not finished, molded, beaded or worked by hand. 

6 

with raised wood grain. These nonsignificant surfaces will be 

damaged and have a roughened texture, but because they are 

interior elements, they will not be subject to further deterioration caused by weathering. 

Historic Interiors that Should Not Be Cleaned Abrasively 

Those instances (generally industrial and some commercial properties), when it may be acceptable to use an abrasive treatment 

on the interior of historic structures have been described. But for 

the majority of historic buildings, the Secretary of the Interior's 

Guidelines for Rehabilitation do not recommend "changing the 

texture of exposed wooden architectural features (induding structural members) and masonry surfaces through sandblasting or use 

of other abrasive techniques to remove paint, discolorations and 

plaster. ... " 

Thus, it is not acceptable to clean abrasively interiors of 

historic residential and commercial properties which have finished interior spaces featuring milled woodwork such as 

doors, window and door moldings, wainscoting, stair balustrades and mantelpieces. Even the most modest historic house 

interior, although it may not feature elaborate detailing, contains plaster and woodwork that is architecturally significant 

to the original design and function of the house. Abrasive 

cleaning of such an interior would be destructive to the historic integrity of the building. 

Abrasive cleaning is also impractical. Rough surfaces of 

abrasively cleaned wooden elements are hard to keep clean. 

It is also difficult to seaL paint or maintain these surfaces 

which can be splintery and a problem to the building's occupants. The force of abrasive blasting may cause grit particles to lodge in cracks of wooden elements, which will be 

a nuisance as the grit is loosened by vibrations and gradually 

sifts out. Removal of plaster will reduce the thermal and 

insulating value of the walls. Interior brick is usually softer 

than exterior brick, and generally of a poorer quality. Removing surface plaster from such brick by abrasive means 

often exposes gaping mortar joints and mismatched or repaired brickwork which was never intended to show. The 

resulting bare brick wall may require repointing, often difficult to match. It also may be necessary to apply a transparent 

surface coating (or sealer) in order to prevent the mortar and 

brick from "dusting." However, a sealer may not only change 

the color of the brick, but may also compound any existing 

moisture problems by restricting the normal evaporation of 

water vapor from the masonry surface. 

"Gentlest Means Possible" 

There are alternative means of removing dirt, stains and paint 

from historic building surfaces that can be recommended as 

more efficient and less destructive than abrasive techniques. 

The "gentlest means possible" of removing dirt from a building surface can be achieved by using a low-pressure water 

wash, scrubbing areas of more persistent grime with a natural 

bristle (never metal) brush. Steam cleaning can also be used 

effectively to clean some historic building fabric. Low-pressure water or steam will soften the dirt and cause the deposits 

to rise to the su rface , where they can be washed away. 

A third cleaning technique which may be recommended to 

remove dirt, as well as stains, graffiti or paint, involves the 

use of commerically available chemical cleaners or paint removers, which, when applied to masonry, loosen or dissolve 

the dirt or stains. These cleaning agents may be used in combination with water or steam, followed by a clear water wash 

to remove the residue of dirt and the chemical cleaners from 

the masonry. A natural bristle brush may also facilitate this 

type of chemically assisted cleaning, particularly in areas of 

heavy dirt deposits or stains, and a wooden scraper can be 
Do not Abrasively Clean these Interiors. Most historic residential and 

some commercial interior spaces contain finished plaster and wooden 

elements such as this stair balustrade and paneling which cOlltribwe 

to the historic and architectural character of the structure. Such interiors 

should not be subjected to abrasive techniques for the purpose of 

removing paint, dirt, discoloration or plaster. 

useful in removing thick encrustations of soot. A limewash 

or absorbent talc, whiting or clay poultice with a solvent can 

be used effectively to draw out salts or stai ns from the surface 

of the selected areas of a building fa<;ade. It is almost impossible to remove paint from masonry surfaces without causing some damage to the masonry, and it is best to leave the 

surfaces as they are or repaint them if necessary. 

Some physicists are experimenting with the use of pulsed 

laser beams and xenon flash lamps for cleaning historic masonry surfaces. At this time it is a slow, expensive cleaning 

method, but its initial success indicates that it may have an 

increasingly important role in the future. 

There are many chemical paint removers which. when applied to painted wood, soften and dissolve the paint so that 

it can be scraped off by hand . Peeling paint can be removed 

from wood by hand scraping and sandin g. Particularly thick 

layers of paint may be softened with a heat gun or heat plate. 

providing appropriate precautions are taken. and the paint 

film scraped off by hand . Too much heat applied to the same 

spot can burn the wood, and the fumes caused by burning 

paint are dangerous to inhale, and can be explosive. Furthermore. the hot air from heat guns can start fires in the 

building cavity. Thus. adequate venti lation is important when 

using a heat gun or heat plate. as well as when using a chemical stripper. A torch or open flame should never be used . 

Preparations for Cleaning: It cannot be overemph asized that 

all of these cleaning methods must be approached with caution . When using any of these procedures which involve water 

or other liquid cleaning agents on masonry , it is imperative 

that all openings be tightly covered. and all cracks or joints 

be well pointed in order to avoid the danger of water penetrating the building's facade, a circumstance which might 

result in serious moisture related problems such as efflorescence and/or subflorescence. Any time water is used on masonry as a cleaning agent, either in its pure state or in 

combination with chemical cleaners. it is very important that 

the work be done in warm weather when there is no danger 

of frost for several months. Otherwise water which has penetrated the masonry may freeze, eventually causing the surface of the building to crack and spall, which may create 

another conservation problem more serious to the health of 

the building than dirt. 

Each kind of masonry has a unique composition and reacts 

differently with various chemical cleaning substances. Water 

and/or chemicals may interact with minerals in stone and 

cause new types of stains to leach out to the surface immediately, or more gradually in a delayed reaction. What may 

be a safe and effective cleaner for certain stain on one type 

of stone, may leave unattractive discolorations on another 

stone, or totally dissolve a third type. 

Testing: Cleaning historic building materials. particularly 

masonry , is a technically complex subject. and thus. should 

never be done without expert consultation and testing. No 

cleaning project should be undertaken without first applying 

the intended cleaning agent to a representative test patch 

area in an inconspicuous location on the building surface. 

The test patch or patches should be allowed to weather for 

a period of time , preferably through a complete seasonal 

cycle, in order to determine that the cleaned area will not be 

adversely affected by wet or freezing weather or any by-products of the cleaning process. 

Mitigating the Effects of Abrasive Cleaning 

There are certain restoration measures which can be adopted 

to help preserve a historic building exterior which has been 

damaged by abrasive methods. Wood that has been sandblasted will exhibit a frayed or " fuzzed" surface, or a harder 

wood will have an exaggerated raised grain. The only way to 

remove this rough surface or to smooth the grain is by laborious sanding. Sandblasted wood, unless it has been extensively sanded, serves as a dustcatcher, will weather faster , 

and will present a continuing and ever worsening maintenance 

problem. Such wood, after sanding. should be painted or 

given a clear surface coating to protect the wood , and allow 

for somewhat easier maintenance. 

There are few successful preservative treatments that may 

be applied to grit-blasted exterior masonry . Harder, denser 

stone may have suffered only a loss of crisp edges or tool 

marks, or other indications of craft technique. If the stone 

has a compact and uniform composition, it should continue 

to weather with little additional deterioration. But some types 

of sandstone, marble and limestone will weather at an accelerated rate once their protective "quarry crust" or patina 

has been removed. 

Softer types of masonry , particularly brick and architectural 

terra-cotta, are the most likely to require some remedial treatment if they have been abrasively cleaned . Old brick. being 

essentially a soft, baked clay product. is greatly susceptible 

to Increased deterioration when its hard . outer skin is removed through abrasive techniques. This problem can be 

minimized by painting the brick. An alternative is to treat it 

with a clear sealer or surface coating but this will give the 

masonry a glossy or shiny look. It is usuafly preferable to 

paint the brick rather than to apply a transparent sealer since 

7 
Hazards of Sandblasting and Surface Coating. In order to "protect" 

this heavily sandblasted brick, a clear surface coating or sealer was 

applied. Because the air temperature was too cold at the time of ap· 

plication, the sealer failed to dry properly, dripping in places. and 

giving the brick surface a cloudy appearance. 

sealers reduce the transpiration of moisture, allowing salts to 

crystallize as subflorescence that eventually spalls the brick. 

If a brick surface has been so extensively damaged by abrasive 

cleaning and weathering that spalling has already begun. it 

may be necessary to cover the walls with stucco. if it will 

adhere. 

Of course. the application of paint. a clear surface coating 

(sealer). or stucco to deteriorating masonry means that the 

historical appearance will be sacrificed in an attempt to conserve the historic building materials. However, the original 

color and texture will have been changed already by the abrasive treatment . At this point it is more important to try to 

preserve the brick. and there is little choice but to protect it 

from "dusting" or spalling too rapidly. As a last resort. in 

the case of severely spalling brick. there may be no option 

but to replace the brick-a difficult. expensive (particularly 

if custom-made reproduction brick is used), and lengthy process. As described earlier. sandblasted interior brick work. 

while not subject to change of weather. may require the application of a transparent surface coating or painting as a 

maintenance procedure to contain loose mortar and brick 

dust. (See Preservation Briefs: No. 1 for a more thorough 

discussion of coatings.) 

Metals, other than cast or wrought iron, that have been 

pitted and dented by harsh abrasive blasting usually cannot 

be smoothed out. Although fillers may be satisfactory for 

smoothing a painted surface, exposed metal that has been 

damaged usually will have to be replaced . 

Selected Reading List 

Ashurst. John. Cleaning Stone and Brick . Technical Pamphlet 4. 

London: Society for the Protection of Ancient Buildings. IY77. 

Asmus. John F. "Light Cleaning: Laser Technology for Surface Preparation in the Arts." Technology and Conservation. 3: 3 (Fall 

1978). pp. 14-18. 

"The Bare-Brick Mistake. " The Old House Journal. I: 2 (November 

1973). p. 2 

Brick Institute of America. Colorless Coatings for Brick Masonrl'. 

Technical Notes on Brick Construction. Number 7E (September/ 

October 1976). 

Gilder. Cornelia Brooke . Property OWller's Guide to the Maintenance 

and Repair of Stone Buildings. Technical Series/ Nn. 5. Albany. 

New York: The Preservation League of New York State. 1977. 

Prudon . Theodore H.M . "The Case Against Removing Paint from 

Brick Masonry ." The Old HouseJournal. III : 2 (February 1975). 

pp . fr-7 

--- "Removing Stains from Masonry. " The Old House Journal. 

V: 5 (May 1977). pp. 58-59. 

Stambolov. T.. and J.R.J. Van Asperen de Boer. The Deterioration 

and Conservation of Porous Building Materials in Monumellts: 

A Review of the Literature. Second enlarged edition. Rome: 

International Centre for Conservation. 1976. 

8 

Summary 

Sandblasting or other abrasive methods of cleaning or paint 

removal are by their nature destructive to historic building 

materials and should not be used on historic buildings except 

in a few well-monitored instances. There are exceptions when 

certain types of abrasive cleaning may be permissible. but 

only if conducted by a trained conservator, and if cleaning 

is necessary for the preservation of the historic structure. 

There is no one formula that will be suitable for cleaning 

all historic building surfaces. Although there are many commerical cleaning products and methods available. it is impossible to state definitively which of these will be the most 

effective without causing harm to the building fabric . It is 

often difficult to identify ingredients or their proportions contained in cleaning products; consequently it is hard to predict 

how a product will react to the building materials to be 

cleaned. Similar uncertanities affect the outcome of other 

cleaning methods as they are applied to historic building 

materials. Further advances in understanding the complex 

nature of the many variables of the cleaning techniques may 

someday provide a better and simpler solution to the problems. But until that time. the process of cleaning historic 

buildings must be approached with caution through trial and 

error. 

It is important to remember that historic building materials 

are neither indestructible. nor are they renewable. They must 

be treated in a responsible manner. which may mean little 

or no cleaning at all if they are to be preserved for future 

generations to enjoy. If it is in the best interest of the building 

to clean it , then it should be done "using the gentlest means 

possible." 

Weiss. Norman R. "Cleani ng of Building Exteriors: Problems anu 

Procedures of Dirt Removal." Technolo?.\' and COllSen'atioll. 

2176 (Fall 1976). pp. 8- 13. 

---Exterior Cleaning of Historic Masollr.\' Buildings. Draft. Washington. D.C.: Office of Archeology and Historic Preservation. 

Heritage Conservation and Recreation Service. U.S. Depart· 

ment of the Interior. 1976. 

This Preservation Brief was writlen hy Anne E. Grimmer. Architectura l Hi,· 

torian. Technical Preservation Services Division. Valuahle suggestions and 

comments were made hy Hugh C. Miller. AlA. Washington. D.C.: Marlin E. 

Weaver. Otlawa. Ontario. Canada: Terry Bryant. Downers Grove. IIlinoi" 

Daniel C. Cammer. McLean. Virginia: and the professional staff of Technical 

Preservation Services Division. Dehorah Cooney edited the final manuscript. 

The illustrations for this brief not sRf'cifically credited are from the fi les of the 

Technical Preservation Services Division. 

This publication was prepared pursuant to Executive Order 11593. "Protection 

and Enhancement of the Cultural Environment." which directs the Secretary 

of the Interior to "develop and make available to Federal agencies and State 

and local governments information concerning professional methods and techniques for preserving, improving, restoring and maintaining historic properties." The Brief has been developed under the technical editorship of Lee H. 

Nelson, AlA, Chief, Preservation Assistance Division, National Park Service, 

U.S. Department of the Interior, Washington, D.C. 20240. Comments on the 

usefulness of this information are welcome and can be sent to Mr. Nelson at 

the above address. This publication is not copyrighted and can be reproduced 

without penalty. Normal procedures for credit to the author and the National 

Park Service are appreciated. June 1979.