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Dangers of Abrasive Cleaning
to Historic Buildings
Anne E. Grimmer
"Chemical or physical treatments, such as sandblasting, that cause damage
to historic materials shall not be used. The surface cleaning of structures,
if appropriate, shall be undertaken using the gentlest means possible."
The Secretary of the Interior's Standards for Rehabilitation.
Abrasive cleaning methods are responsible for causing 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 precede general
cleaning to assure that the method selected will not have an adverse effect
on the building materials. A historic building is irreplaceable, and should
be cleaned using only the "gentlest means 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,
ground 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 important 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 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 all 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
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. 1 "The Cleaning
and Waterproof Coating of Masonry Buildings," by Robert C. Mack, AIA.
Problems of Abrasive Cleaning
The crux of the problem is that abrasive cleaning is just that--abrasive.
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 between 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 considerable 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.
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 may accept 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.
Nonhistoric/Industrial: A representative of the building cleaning industry
might consider "high" pressure water cleaning to be 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 1,000 and 2,500 psi.
Historic: By contrast, conscientious dry or wet abrasive cleaning of
a historic structure would be conducted within the range of 20 to 100
psi at a range of 3 to 12 inches. Cleaning at this low pressure requires
the use of a very fine 00 or 0 mesh grit forced through a nozzle with
a 1/4-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 facade. 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 controlled conditions can most historic building material be abrasively
cleaned of soil or paint without measurable damage to the surface or profile
of the substrate.
Yet some professional cleaning companies which specialize 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 (200600 psi), or even a high pressure
(600-1800 psi) water rinse. However, 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 degree. Such techniques quite logically
cause greater damage to softer and more porous materials, such as brick
or architectural terra-cotta. 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 architectural
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 opposed to kiln-dried masonry materials such as brick
and architectural terra-cotta, building stones are generally homogeneous
in character at the time of a building's construction. However, as the
stone is exposed to weathering 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 easily 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 technique requires
very careful supervision in order to prevent damage to the stone. Polished
or honed marble or granite should never be treated abrasively, as the
abrasion would remove the finish in much the way glass would be etched
or "frosted" by such a process. It is generally 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 "Gentlest Means Possible.")
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 facade 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
possibility 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 (75125 micron) glass beads are
used at a low pressure of 60 to 80 psi. Because these glass beads are
completely spherical, there 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, which is naturally somewhat uneven,
will not be noticeably damaged by controlled abrasion. Such treatment
will, however, result in a small amount of pitting. But this slight abrasion
creates a good surface for paint, since the iron must he 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 tops of cornices or column capitals, or other detailed
areas of the facade.
This is still an abrasive technique, and without proper caution in handling,
it can be just as harmful to the building surface as any other 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 it is very time 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 he 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" 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 (including 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 surface, 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 commercially
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 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 stains from the surface of
the selected areas of a building facade. 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 sanding. 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 he explosive.
Furthermore, the hot air from heat guns can start fires in the building
cavity. Thus, adequate ventilation 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 he used.
Preparations for Cleaning: It cannot be overemphasized 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 he adversely affected by wet or freezing
weather or any byproducts 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 usually preferable to paint the brick rather than to apply a transparent
sealer since 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.
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 commercial 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
uncertainties 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."
Selected Reading List
Ashurst, John. Cleaning Stone and Brick. Technical Pamphlet 4. London:
Society for the Protection of Ancient Buildings. 1977.
Asmus, John F. "Light Cleaning: Laser Technology for Surface Preparation
in the Arts." Technology and Conservation. 3:3 (Fall 1978), pp. 14-18.
"The BareBrick Mistake." The Old House Journal. I:2 (November 1973).
p 2.
Brick Institute of America. Colorless Coatings for Brick Masonry. Technical
Notes on Brick Construction. Number 7E ( September/October 1976).
Gilder, Cornelia Brooke. Property Owner's Guide to the Maintenance and
Repair of Stone Buildings. Technical Series/ No. 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 House Journal, III:2 (February 1975). pp. 6-7.
----------. "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 Material in Monuments: A Review of the
Literature. Second enlarged edition. Rome: International Centre for Conservation,
1976.
Weiss, Norman R. "Cleaning of Building Exteriors: Problems and Procedures
of Dirt Removal." Technology and Conservation, 2/76 (Fall 1976), pp. 8-13.
----------. Exterior Cleaning of Historic Masonry Buildings. Draft.
Washington, D.C.: Office of Archeology and Historic Preservation, Heritage
Conservation and Recreation Service, U.S. Department of the Interior,
1976.
This Preservation Brief was written by Anne E. Grimmer, Architectural
Historian, Technical Preservation Services Division. Valuable suggestions
and comments were made by Hugh C. Miller, AIA, Washington, D.C.; Martin
E. Weaver, Ottawa, Ontario, Canada; Terry Bryant, Downers Grove, Illinois;
Daniel C. Cammer, McLean, Virginia; and the professional staff of Technical
Preservation Services Division. Deborah Cooney edited the final manuscript.
The illustrations for this brief not specifically credited are from
the files of the Technical Preservation Services Division. Washington,
D.C. June, 1979
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