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Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 6 Dangers Of Abrasive Cleaning To Historic Buildings

Procedure code:

0451005S

Source:

Preservation Briefs 6, National Park Service, Pad

Division:

Masonry

Section:

Masonry Cleaning

Last Modified:

08/30/2012

Details:

Preservation Briefs: 6 Dangers Of Abrasive Cleaning To Historic Buildings



PRESERVATION BRIEFS: 6

DANGERS OF ABRASIVE CLEANING TO HISTORIC BUILDINGS

 

The link immediately below connects to the NPS website and the latest version of PB #6:

 

http://www.nps.gov/history/hps/tps/briefs/brief06.htm

Anne E. Grimmer

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documents
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01026-2
              Available ONLY in packaged sets
              Briefs 1-14 - $13.00

Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


"THE SURFACE CLEANING OF STRUCTURES SHALL BE UNDERTAKEN 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"

***INTRODUCTION***

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 (04510-06-S).


***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 the 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 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.

Nonhistorical/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
1,00 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
the 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 (200-600 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 an 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 by 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 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 (75-125
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
extreme 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 be 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 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" 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

PREPARATIONS FOR CLEANING:  




 


masonry cleaning, abrasive cleaning of masonry, Preservation Brief 6