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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
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 gapalthough 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
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
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
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
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 \
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
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.
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.
