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OCR for page 272
Cleaning and Surface Repair
JOHN ASHURST
This paper discusses the importance of cleaning as part of the maintenance
of historic building surfaces. Cleaning systems currently in use are reviewed
with special reference to recent developments. Methods of surface repair and
treatment of joints likely to be associated with the cleaning are described.
The Research and Technical Advisory Service of the Directorate of
Ancient Monuments and Historic Buildings (D~) iS involved win
the day-to-day maintenance and repair problems of historic stone build-
ings. It also prepares technical notes and checks cleaning and repair
programs that may be supported by government grants. This activity
involves contact not just with a wide range of buildings and surfaces,
but with an ever wider range of skills, abilities, and available resources.
Stemming from this involvement, a need has been established for more
practical guidance on techniques of cleaning and repair that relate
realistically to these frequently limited resources.
The work currently being carried out in conservation laboratories
is providing essential knowledge both on the behavior of building and
decorative stones and on the potential and limitations of various tech-
niques of maintenance. Much of this work relates, properly, to the
cleaning and conservation of stone surfaces that have high intrinsic
value, and the major practical benefit may therefore be to the con-
John Ashurst is Research Architect at the Directorate of Ancient Monuments and
Historic Buildings, Department of Environment, United Kingdom.
272
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Clearing and Surface Repair
273
servator with special skills concemed with relatively small-scare sur-
faces.
Some owners of stone buildings, some architects, and some masonry
contractors may decide that increasingly selective treatments and in-
creasing knowledge of the complexity of stone deterioration presented
and described in technical conservation papers are too far removed
from the practical business of dealing with large and possibly heavily
soiled surface areas in their ownership or care. Thus they may turn
instead to Unsuitable known methods or to new and untried services
and materials on the assurance of technical trade literature that may
seem refreshingly uncomplicated and does not "confuse with facts."
There is a gap—although happily it tends to be narrowing between
conservation and commercial specification This gap needs to be bridged.
Recognition of different standards leacis to good advice being given and
being heeded. For instance, the conservation of stone objects of value
will exclude much sandblasting, alkali and acid cleaning, and crude
washing as wholly inappropriate. Still, these tools of the repair and
cleaning industry are in everyday use and will continue in use. Such
cleaning systems need upgrading by relevant and sensible specification,
rather than by outlawing, which only serves to alienate site practice
from good recommendations altogether. In this way, techniques of
cleaning, such as clay poulticing, solvent packs and creams, ultrasonic
descaling, precision blasting, and consolidation techniques using poly-
ester and acrylic mortars and different silane systems, may be seen as
compatible with the proper care of, for instance, a medieval table tomb,
while washing, abrasive water jetting, controlled grit blasting, or mild
caustic or acid solvent systems will not necessarily be ruled out as
suitable means of removing the dirt from nineteenth-century cottages.
This is not to say that poor standards of repair and cleaning, or
damage to any building, should be acceptable. It may be wise to post-
pone any activity of this kind altogether in cases where available tech-
niques and labor are known to be inadequate; or it may be determined
that only a limited amount of cleaning should be carried out to enable
essential repairs to be completed.
Ways in which commercial cleaning, treatment, and repair tech-
niques may be improved and which require more dissemination are
suggested below.
WASElING
Most of the problems associated with washing limestone and brick-
work result from the large volume of water employed. This may cause
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274 CONSERVATION OF HISTORIC STONE BUILDINGS
staining, migration of salts with subsequent efflorescences, and damage
to internal fixings, timber, and plaster. Large quantities of water may
be avoided if a properly atomized system is used with mesh filters and
small orifices, but a wet-fog condition is difficult to maintain in prac-
tice, even with a closely sheeted scaffold. Of course, the finest available
sprays should be used, and the best control may be to regulate them
by a clock. Simple clock mechanisms may be preset to allow the jets
to operate for, say, 10 seconds, and then shut off for 4 minutes before
rewetting. The sprays should not be directed straight onto the soiled
surface, but aDowed to play across it. As progressive softening takes
place, the dirt should be eased away with small, nonferrous-wire or
fine bristle brushes. Clock-operated systems are easy to set up, and
the timing can be adjusted to the type of stone and the amounts and
types of dirt. To avoid risk of damage by freezing, washing should be
programmed for frost-free months.
MECHANICAL CLEANING
Most mechanical cleaning is drastic and may be seen almost as a
method of redressing the stone surface. Flexible Carborundum discs
and small Carborundum heads have made the technique more sophis-
ticated, but most surfaces cleaned in this way exhibit scour marks and
must be finished by hand rubbing, removing even more of the surface.
The method is best excluded except where disfiguration by oil, grease,
and paint, coupled with physical damage, requires a new surface to be
formed. To take back a surface in this way requires considerable skill
if it is to look good, and it should only be done as a last resort.
STEAM CLEANING
The use of the steam lance is much less common than it was in the
1930s. After that time it fell into disrepute, partly because of the
residual damage left by caustic soda used as a water softener and partly
because it was often no more effective than cold-water washing. Today
steam is useful as a support method where greasy deposits are en-
countered on stone surfaces or sticky substances need to be removed.
AIR ABRASIVE TECHNIQUES ("SAND/GRIT BLASTING",
Air-abrasive methods probably have caused more damage to masonry
surfaces in the past two decades than any others. The rapid manner
in which dramatic cleaning effects can be achieved has unfortunately
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Cleaning and Surface Repair
275
been a strong selling point. Since higher pressures and coarser abrasives
produce ever greater and more dramatic results, the commercial temp-
tation to misuse the system is obvious. Air-abrasive techniques are
used on a large scale to clean sandstone, limestone, brick, and even
terra-cotta and stucco. Despairing of control, some authorities have
banned the system in their areas of administration. However, many
contractors have demonstrated their ability to clean satisfactorily in
this way by using small units and fine abrasives at low pressures. Of
course, it would be nonsense to suggest that a building front should
be cleaned using an air-abrasive pencil and aluminum oxide abrasive,
but there is a wide range of equipment now available, both wet and
dry, that gives the operator full control and full visual command of
the cleaning operation.
Specification should prescribe the air-abrasive gun, the abrasive type
and size, and the acceptable pressures. Supervision should be regular,
including, if necessary, spot checks on air pressures with a hypodermic
needle and a gauge. Sensible cleaning times should be ascertained in
advance so that the temptation to speed up and save money is reduced.
An agreed -sample of cleaning should be on-site and constantly referred
to. Pressures over 40 psi should not be accepted, and in some cases a
lower limit must be set. Abrasives preferably should be nonsiliceous,
although it must be pointed out that large sections of the cleaning
industry use nothing but sands of various mesh sizes, and the dust
risk is certainly as great from a sandstone surface that is being cleaned
as from the abrasive. Scaffolds should be tightly sheeted to protect the
public, windows and other openings carefully protected and sealed,
and the operators fitted with filtered air-line helmets.
Air-abrasive techniques are primarily of interest in cleaning sand-
stone. They may also be used on heavily soiled limestone, but should
not be used on brick or terra-cotta. The work should be finished by
rinsing the building face with clean water, preferably using a low-
pressure water lance with a fan jet to remove adherent dust.
CHEMICAL CLEANING
Lee nonscientific building owner or specification writer is most vul-
nerable when beset by the multitude of chemical cleaning materials
and services that promise to solve all his problems. Indeed, two aspects
of chemical cleaning the absence of water saturation and abrasion
are undeniably attractive. The price that is sometimes paid, however,
is the damage resulting from residual soluble salts and disfigurement
from staining or formation of white silica "bloom." Most alkaline
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276
CONSERVATION OF HISTORIC STONE BUILDINGS
cleaners are based on sodium hydroxide, and most acid cleaners are
based on hydrofluoric acid. Additions of surfactants and rust inhibitors
and the use of complex or subtle trade names often confuse the po-
tential user further. Since it is unlikely that either cleaner will cease
to be used, anc} in some cases may genuinely be the best available
solution to a problem, sensible controls should be encouraged. In par-
ticular, those who are to use chemical cleaners must be fully aware
of the potential damage to themselves and the building and prepare
both themselves and the budding with appropriate protective clothing
and sheeting. Dilution of chemicals must not be permitted on site,
and appropriate first aid must be taught and understood. All work
must be to a previously agreed standard, set by a sample on-site. Proper
prewetting to reduce absorption must be carried out and the contact
between the chemical (preferably in the form of a thixotropic paste)
and the soiled surface kept to a minimum. Washing may be preceded
by lifting off the thixotropic paste with a suitable wooden scraper. The
washing itself, using a Tow-volume, fan-jet water lance, must be carried
out scrupulously and systematically, paying particular attention to
joints. The use of "neutralizing" solutions fi.e., the use of oxalic acid
after caustic alkiai cleaning) is sometimes recommended, but it is
difficult to see that this really contributes anything, and it is another
costly operation.
One of the common uses of a caustic alkali cleaner, combined with
a detergent, is to degrease before cleaning by another method. Caustic
alkali is used on limestone and brickwork, and dilute hydrofluoric acid
is used commonly on sandstone and granite, some brickwork, and
some terra-cotta.
If the risks to both personnel and buildings are properly understood
and rigid control and discipline relevant to the method are exercised,
satisfactory cleaning may be achieved and may be preferred to other
methods. Of course, there is a temptation to turn away from any
chemical cleaner, but since they are entrenched as systems there is
strong reason for trying to keep the techniques as useful and as safe
as possible.
POULTICE TECHNIQUES
In some instances poultice packs, familiar in the monument conser-
vation world, may be used simply and economically by the cleaning
contractor. There is no reason, for instance, why magnesium silicate
clay packs should not be applied to areas of detail to assist in softening
encrustation. However, caustic materials are sometimes included in a
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Cleaning and Surface Repair
~ 277
body of clay to break down of! or grease, and lime or whiting bodies
have been used traditionally with ammonium chloride to remove cop-
per staining, or with sodium citrate to remove iron stains. These mea-
sures may be alarming in the context of monument conservation, but
if carefully applied, they may be acceptable on less important struc-
tures. Once again, the specification must demand proper protection of
surrounding areas, careful prewetting, lifting off by spatula after min-
imum contact time, and thorough and careful rinsing. Clay packing
after the use of these poultices is sometimes useful.
SURFACE TREATMENTS
Water repellent materials such as silicone resins and metallic stearates
are commonly recommended and applied as dirt inhibitors after clean-
ing. While the retention of a cleaner appearance has been observed on
some surfaces, compared with cleaned but untreated areas, this is offset
by the patchy appearance that may develop after seven or eight years
and by the initial expense. These treatments should not, of course, be
applied to decaying surfaces or to areas likely to be subjected to major
moisture movements, and if they are to be used, sufficient time must
be allowed for the surface to become as dry as possible. This point is
often overlooked when a contract is running late.
Although material with shallow penetration into the surface must
not be thought of as a preservative, it may assist indirectly to extend
the life of stones or bricks. For example, the sound external face of
stone tracery may be treated back to the glass line to reduce the feed-
through of salt-contaminated water to an internal cIrying face that is
decaying. Brickwork may be treated with the appropriate class of sil-
icone to prevent staining and decay resulting from washing off lime-
stone dressings.
Lime-based treatments, although lately associated with the conser-
vation of external limestone sculpture and carved architectural detail,
may be usefully applied to more modest limestone facades that are
suffering from the effects of atmospheric pollution. The simplest treat-
ments, but the ones that most substantially change the appearance of
a surface, are the traditional tallow-based lime wash, a lime wash based
on hydraulic lime, or lime and pozzolanic cenospheres. These may be
economical to apply, but must be renewed at four- or five-year inter-
vals, depending on exposure. They have the disadvantage of increas-
ingly blurring and distorting the detail of the building. Sometimes, too,
lime washing is applied over dirty surfaces as a means of trying to
\
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278
i'
CONSERVATION OF HISTORIC STONE BUILDINGS
lighten and improve the appearance of a facade quickly and cheaply,
a practice that must be discouraged.
Better than lime washing is the system of either washing or cleaning
by hot lime poultice, and then rubbing in finely sieved lime putty and
sand bound with a small amount of casein (inhibiting organic growth
with forrnalin~the method developed by Baker at Wells. Such a treat-
ment closes up the texture of the stone without a drastic change in
appearance and can be worked in well with small repairs in- matching
mortar.
Although treatments of this kind have also been applied to sand-
stone, such practice should be generally discouraged because of its
tendency to encourage decay. There are some difficult exceptions;
when sandstone has been treated with lime over a period of many
years, the best course may be to continue lime washing. Id this situ-
ation it is essential that the lime coat be maintained intact and not
allowed to deteriorate.
A further treatment of interest is the so-called silicate paint system
developed in the 1890s by the- Keim family in Augsburg as a "per-
manent paint" system, primarily for stucco, but also applied exten-
sively to sandstone, limestone, and more recently to concrete. Surviv-
ing pigmented Keim treatments of 60 years are not uncommon. While
it is acknowledged that in theory such treatments, using sodium and
potassium silicate, carry a risk of forming soluble salts, the Keim track
record is good. The light consolidation achieved is such that the treat-
ment of large areas of low value may well be justified, especially when
the alternative is replacement.
The use of silanes is discussed at some length in other papers and
.s not included here for that reason alone. The direct labor force of
DAMHB and certain contractors have been trained in the use of the
Building Research Establishment's system, "Brethane," and have
achieved some excellent results. The results support the generally,
though not exclusively, held view that this class of materials is the
most promising of all the surface treatments, with a genuine claim to
be described as a "preservative."
SURFACE REPAIR
The most common surface repair is the refilling of weathered-out
joints. In the past this pointing operation has often been earned out
by using unsuitable materials with strong hydraulic cement binders,
frequently flush-filling over weathered and rounded arrises, thereby
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Cleaning and Surface Repair
279
establishing water traps. Retention of the original joint width and the
use of mortars no stronger than the stones in which they are to be
placed is accepted as good practice, but too rarely carried out. Joint
filling may be accompanied by local hand grouting or may be part of
a major grouting operation.
Liquid grouts and the flushing operation that must precede grouting
are notorious for introducing or moving concentrations of salty es-
pecially when cement grouts are used. Low sulfate cements, limes,
and fly ash are increasingly used, along with various additives, and are
better in this respect, but the flushing operation is stfl} a problem and
cannot be dispensed with.
Massive masonry walls con, however, often be consolidated satis-
factorily by gravity or Tow-pressure grouting in small sections. The
important point to cover in specifications is that valuable insertions
in the wall, such as monuments, should be isolated from grout flows
by an impervious barrier, which will involve some drilling or cutting
out and rebuilding. Such barriers may be epoxy mortars, pitch-extended
epoxy, or bitumen-coated lead {provided there is no staining risk).
Polyester mortar or acrylic resin and sand mortar may be used to
rebuild and protect monuments against salt contamination from the
surrounding structure.
Mortars for tamping and pointing historic masonry are normally
lime/aggregate mixtures in the proportions 1:3, but may be gauged
with small quantities of cement or finely ground pozzolanic material.
A hydraulic lime may be used if early strength and early resistance to
frost are required. Although the dangers of wet mixes based on hy-
draulic cements must be appreciated, a sensible compromise must be
reached whereby adequate performance is obtained from the mortar
without the risk that hamlfu} quantities of sodium salts will be trans-
ferred into the masonry. A cement/plasticizer mix may be used in
place of lime when pointing and filling sandstone joints, especially if
lime has contributed to peripheral decay of the stones. In this way
further contamination may be avoided.
Proper preparation and storage of mortar materials for weak lime
mortar should be encouraged. Evidence suggests that lime kept as putty
and mixed with the desired aggregates as Tong as possible before use
results in better performance than lime and aggregates mixed dry and
used immediately after adding water. Time, as well as thorough rnix-
ing, beating, and ramming, is needed to allow the aggregate particles
to become thoroughly coated with binder. Of course, cement and other
pozzolanic materials must only be added just before use.
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280
SUMMARY
CONSERVATION OF HISTORIC STONE BUILDINGS
The above notes are suggestions for specifications for contractors who
are to carry out work on historic buildings. The suggested measures
are compatible with conservation practice without being impracticable
or too demanding.
BIBLIO GRAPHY
[Prepared at the committee's request by Anne Grimmer, National Park Service.]
Anon. Cleaning external surfaces of buildings. Building Research Station Digest 113~1970~.
Ashurst, John. Cleaning Stone and Brick. Technical Pamphlet 4. Society for the Pro-
tection of Ancient Buildings: London, 1977.
Ashurst, John, and Francis G. Dimes. Stone in Building: Its Use and Potential Today.
The Architectural Press: London, 1977.
Clarke, B.L. Some recent research in cleaning external masonry in Great Britain. In The
Treatment of Stone. Centro per la Conservazione delle Sculture all'Aperto: Bologna,
1972.
Clayton, Ian. Special feature: Stone cleaning. Why buildings should be washed. Building
Conservahon 3~3~:20 {1981i.
Hempel, K. Notes on the conservation of sculpture: Stone, marble, and terra-cotta.
Studies in Conservahon 13: ( 1968~.
Lewin, S.Z., and Elizabeth J. Rock. Chemical considerations in the cleaning of stone
and masonry. In The Conservahon of Stone I Proceedings of the International Sym-
posium, Bologna, June 19-21, 1975), ed. R. Rossi-Manaresi. Centro per la Conserva-
zione delle Sculture all'Aperto: Bologna, 1976.
Mack, R.C. Preservation Briefs: 1. The Cleaning and Waterproof Coating of Masonry
Buildings. National Park Service: Washington, D.C., 1975.
Stambolov, T. Notes on the removal of iron stains from calcareous stone Studies in
Conservahon 13:45~7 { 1968~.
Stambolov, T., and J.R.J. Van Asperen de Boer. The Deterioration and Conservahon of
Porous Building Materials in Monuments. A Review of the Literate. International
Centre for Conservation: Rome, 1972.
Weiss, Norman R. Cleaning of Building Exteriors: Problems and procedures of dirt
removal. Technology and Conservahon 2~76~: 8~13;1967~.
Weiss, Norman R. Exterior Cleaning of Historic Masonry Buildings. draft report. Na-
tional Park Service: Washington, D.C., 1975.
Representative terms from entire chapter:
stone buildings
