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Report of the Committee on Conservation of Histonc Stone Buildings and Monuments Historic and artistic monuments are the most visible aspects of our cultural heritage. These monuments, often of stone construction, are universally threatened by pollution, urbanization, public access, weathering, and other man-made and natural phenomena. The factors that affect the deterioration of stone include its com- position, structure, and surface conditions; interaction of the surround- ing microclimate and environment with the stone; and the effective- ness of preservation methods. In some cases, actions intended to preserve, clean, or restore monuments have instead had deleterious effects. We need new approaches to the conservation of these monuments. The problem involves such disciplines as geology, chemistry, mete- orology, and civil and chemical engineering. Many scientists and en- gineers, however, are not aware of the details of the problem or of the various preservation treatments being employed. Greater progress could be made if more of the nation's scientific and technical skills could be applied to stone preservation and if greater effort were made to transfer existing technology to this field. This committee has surveyed the extent of the problem, including the physical, chemical, and biological processes involved and the meth- ods used to measure the rates of deterioration of stone structures. It has also reviewed the state of the art of preservation and restoration and the efficacies of the methods used. As the keystone of its work the committee convened a conference of scientists and preservationists to examine these and related matters. 1
2 CONSERVATION OF HISTORIC STONE BUILDINGS The conference was held February 2-4, 1981, in Washington, D.C. The activities included the presentation of a number of invited review papers and exploration of assigned topics by five independent discus- sion groups. Each group was chaired by a scientist or engineer and a specialist in historic preservation. The cochairmen summarized the discussions at a plenary session. The committee has analyzed the information presented at the con- ference, supplemented from other pertinent sources, and offers in this volume the review papers. In its report the committee gives conclu- sions and recommendations for research and development on materials and processes and their applications to the conservation of stone build- ings and monuments. The first part of the discussion that follows is organized around the topics assigned to the five discussion groups. This material is followed by consideration of general concerns that emerged in the deliberations of the several groups. CHARACTERIZATION OF MASONRY MATERIALS Dimension stone is among the most durable of materials, but the process of weathering alone assures that it is ultimately vulnerable. At present we have too little information about that vulnerability. Architects usually select a particular stone for its aesthetic qualities, with casual reference, at best, to basic data on porosity, pore size, moisture absorption, and other critical physical and chemical param- eters. We need more effective exchange of information on stone, rather than the development of new information. A good example of what is needed is the booklet The Stones of the Nations Capita], which was produced as on ad hoc activity at the U.S. Geological Survey. The booklet catalogs the building stones used in the monuments of the nation's capital. Cataloging the uses of stones in a particular region or in the nation as a whole would provide an accessible source of data useful in studying weathering. According to the U.S. Bureau of Mines, there were 263 companies producing dimension stone in 426 quarries in 39 states in 1980. How- ever, 10 leading operations accounted for 40 percent of the total value sold; only 53 quarries located in 15 states had sales of $500,000 or greater. Thus, compilation of data on the qualities of stone in active production could easily be made available to the architect or preser- vationist. The U.S. Bureau of Mines constantly studies the market for dimen- sion stone. The bureau maintains up-to-date inventories of producers
Report of the Committee 3 anti products. We have then the beginnings of a reference library that could collect samples and color photographs of stones and chronicle their use in buildings and monuments throughout the nation. Those interested in using a particular stone could go to a specific building or monument where it has been used and see what it looks like after years of exposure to the local environment. We need information on the variability of stone formations, the variability of stone within a quarry, and the variability within the rock formation itself. A sample taken from the middle of a granite batholith is quite different texturally, mineralogically, and chemically from a sample taken from the side of the same batholith. Because most rock formations are not particularly homogeneous, one cannot simply take a sample of rock from a quarry, test it, and extrapolate the results to the entire quarry or formation. Recommendation: The committee recommends the development of a national inventory of active quarries and available building stones. The inventory should include a reference library of stone samples. Sets of such samples should be kept by each state geological survey, state university, or similar organization, together with a data base of physical and chemical properties of each type and a listing of structures in which it has been used. Particular attention should be given to the variability of properties within each type and within each quarry. Ul- timately the inventory should be expanded to include quarries of his- torical interest. Its existence should be publicized and its use by ar- chitects, builders, and conservators should be encouraged. MECHANISMS OF DETERIORATION In addition to observing weathering history in the field, we must de- te~ine if weathering can be recreated in the laboratory. The com- mittee is not optimistic that this can be done. We note also that natural outcrops of building stones have an en- vironmental history quite different from that of stones placed on a building. In a historic structure in particular there is a humidity gra- dient from the outside in. Buildings that are air-conditioned in the summer or heated in the winter experience constantly changing hu- midity cycles. In situ outcrops are not supplied with water by capillary action and sprayed with salt to remove snow and ice. We must re- member that a building is more than a small sample of stone in a laboratory. It is a dynamic system, made of a variety of materials and reacting to a range of stresses. We must understand how these indi-
4 CONSERVATION OF HISTORIC STONE BUILDINGS vidual factors relate to the whole and then deal with weathering and decay as a systematic problem. Recommendation: The committee recommends the publication of a review of accelerated weathering tests and testing standards for di- mension stone with a clear assessment of the applicability and limi- tations of the tests in predicting weathering. Responsibility for this publication could be determined by the interactive group proposed in the concluding recommendation in this summary. Recommendation: The committee recommends the continued study of the National Bureau of Standards' test wall and the erection of other test walls in different climatic areas around the world where both stones and treatments could be tested. The purpose is to provide nat- ural-weathering data to complement data obtained in accelerated weathering tests. The committee has studied the papers in this volume that deal with mechanisms of decay and believes they indicate the state of the art fairly. Our discussions revealed many unresolved questions and the absence of a readily definable literature. There is literature on some aspects of the decay problem, but nearly all areas require further sus- tained study. Perhaps the most important matter is the effect of mois- ture as part of the freeze-thaw cycle or as the vehicle for migration of salts. A related problem concerns the structures of rocks. Stress-strain relationships are nonlinear, and microcracks not visible to the naked eye are extremely important. It is possible that microcracks might also provide access for movement of dilute solutions in rocks and so ac- celerate spelling phenomena associated with damage by salts. More information must be developed on wet and dry deposition of air pollutants and on the whole spectrum of pollutants, not only sul- fates and nitrates. Very little is known of air quality and microclimate in terms of the management of stone structures. Considerable difficulty is experienced in attempting to specify the range of monitoring required for buildings. It is not possible today to relate data accumulated at established air-monitoring stations to building sites that are often dis- t~nt from these stations. Techniques for site monitoring by nonscien- tific personnel are essential. Recommendation: The committee recommends the development of inexpensive methods and standards of measurement for monitoring air quality and meteorological events at buildings and other cultural
Report of the Committee 5 sites. Such methods should be designed to monitor the special con- ditions that affect these sites, as opposed, for example, to conditions that may affect human health. The roles of vibration caused by vehicles and of biological agents in the destruction of stone are other aspects of deterioration that need to be better understood. We need a systematic approach to the study of the decay of stone and the development of a general theory that en- compasses all known natural and anthropogenic causes of such decay. Recommendation: The committee recommends the development of a general theory of the decay of stone that includes all appropriate anthropogenic and natural phenomena. The model must include the effects of freeze-thaw cycles, wet and dry deposition of air pollutants, biological attack, and damage by salts. DIAGNOSIS OF DETERIORATION AND EVALUATION OF CONDITION In addition to their own weight, stone structures (including bridges) must often sustain such forces as geophysical Toads from wind, tem- perature, and earth tremors; geotechnical loads from lateral earth pres- sures, subsidence, and foundation rotation and translation; gravity Toads from walls, floors, and roofs; and vibrational loads from vehicular traffic, machinery, and blasting. Structural distress resulting from these Toads is usually manifested by fractures in the masonry itself, in the joints, or at structural inter- faces. This distress tends to accelerate deterioration, compromise the safety of the structure, and detract from its aesthetic qualities. Structural, diagnostic, and remedial techniques for stone structures are in a relatively crude state. Further, existing professional informa- tion is not well disseminated, and there is little significant structural research under way. Recommendation: The committee recommends that the American Society of Civil Engineers (ASCE) in cooperation with the American Institute of Architects (A~), the Association for Preservation Tech- nology (APT), the National Trust for Historic Preservation iNTHP~, the Society for Industrial Archaeology (S~A), and related professional and technical organizations develop and disseminate appropriate guides and standards for the diagnosis of structural distress in stone buildings, bridges, and monuments; for remedial actions to distressed structures; and for preventive actions to vulnerable structures.
6 CONSERVATION OF HISTORIC STONE BUILDINGS The dominant factor in deterioration of stone and masonry struc- tures is water and moisture. This point was stressed by many confer- ence participants. It is obvious that considerable work in related areas should be transferable to historic preservation. We encourage the pub- lication of papers on the subject in appropriate scientific and preser- vation journals. Diagnosis of the presence of moisture is done qualitatively with a simple portable instrument that measures electrical conductivity at the surface of stone or masonry. A more quantitative portable instru- ment is the neutron moisture probe used in soil science and civil engineering. It is commercially available and could be used for non- destructive diagnostic surveys of structures. An infrared scanner has been used successfully to identify moisture-laden areas of large flat roofs The moisture provides greater heat conduction, which is recog- nized by the infrared scant. The scanner costs about $10,000, but the savings effected by the diagnosis can make it cost-effective. In addition, technology used in the analysis and treatment of concrete highways is readily available for use on stone structures. A simple instrument used to measure vibration in roads could be used without modification to measure vibration in and movement of buildings. Both the oil industry and NASA have techniques to measure density, porosity, and water saturation of rocks, to make surface elemental analyses, and possibly to identify internal stresses and fractures. These techniques could be applied to dimension stone, but currently they are far too complex and expensive for use by preservationists. Making current instruments portable, safe, and easy to use by the nonscientist would require a significant engineering effort. Moreover, the readings must be readily interpretable in terms of helping the preservationist diagnose problems and devise treatments. The committee believes that the transfer of this technology~eveloped at great cost by industrial and governmental research laboratoriesto the preservation com- munity is a reasonable goal. Oil industry research laboratories are seeking to extract oil from rock by reducing surface tension. If this work succeeds, some of the resulting technology may be useful in reducing the surface tension in the pores of rock and other masonry materials to discourage capillary action and rising damp. Recommendation: The committee recommends that methods and equipment developed by NASA and the of] industry for characterizing and modifying rock and phenomena associated with it be adapted for use by preservationists. We urge preservationists to explore the uses
Report of the Committee of existing portable equipment employed by NASA, the oil industry, and bridge and highway engineers, and we invite research designed to adapt existing but overcomplex technology to the uses of preserva- tionists. TREATMENTS FOR PRESERVATION AND MAINTENANCE Preservation and maintenance cannot be separated, although the re- quirements of an initial treatment may differ significantly from the requirements for maintaining an existing condition. Many of the prob- lems associated with treatment involve the unavailability of existing information. This information includes adequate documentation of prior treatments and the development of records, documentation of materials and methods used, and listings of materials suppliers. Trade associations and other organizations that deal with potentially relevant information should be made aware of the problems and approaches of . . . . nlstonc preservation. Recommendation: The committee recommends continued support for the development of the Census of Treated Monuments initiated by the Division of Technical Preservation Services of the National Park Service. Development of this data base would do much to improve the level of documentation of treated monuments. Recommendation: The committee recommends that experimental treatments not be applied to registered landmarks and that adequate documentation be required as a condition for the treatment of regis- tered landmarks. Methods of cleaning stone structures require more study. There have been a number of successful treatments, but we are far from a rational, systematic approach. Criteria for coatings and vapor barrierstheir permeability, mechanical behavior, and compatibility with the sub- strate require considerable research. The question of consolidation of stone has implications beyond, for example, simply lining pores and hindering the migration of various constituents in the overall system. After an initial treatment is applied, should it be used again, or is the once-treated system a completely different system? Questions of compatibility that arise here are not well understood. Further, treatments themselves are not permanent, and the con- servator must be prepared to take continuing responsibility for the
8 CONSERVATION OF HISTORIC STONE BUILDINGS stone's condition. We recognize that those responsible for initial treat- ments are often not those responsible for subsequent maintenance; therefore, maintenance personnel must be made aware of the principles of historic preservation. It is important to document methods for the care and maintenance of stone to provide a basis for the design of new buildings. There is also a need to document the mechanisms of deterioration through the study of buildings that are being demolished. Standards for materials, quality control, recommended practices, testing laboratories, and performance are required. Organizations such as the International Union of Testing and Research Laboratories for Materials and Structures (EMBLEM), the International Standards Organi- zation (~so), and the American Society for Testing and Materials (ASTM) certainly could assume leadership roles in this respect. For example, the Committee on Stone or the newly established Subcommittee on Building Preservation and Rehabilitation Technology in ASTM might very well be expanded to handle some of these responsibilities. Recommendation: The committee recommends that ASTM, ISO, and RILEM take the lead in developing and adopting appropriate standards for performance, materials, laboratory and field testing, and practice in stone conservation. These bodies would work cooperatively with the Association for Preservation Technology jAPT), the American In- stitute of Architects jA~A), and other professional organizations con- cerned with historic preservation. EVALUATION OF TREATMENT FOR PRESERVATION AND MAINTENANCE The conference was marked by continuing skepticism over the ability of laboratory tests to evaluate treatments. A recurring theme was the importance of field data. Although we have good control in the labo- ratory, we have problems with scale, with reproduction of complicated ambient conditions, and with simulating 50 or more years of weath- ering in hours or days or weeks. The committee recognizes that a great deal of information on eval- uation of treatments already exists if it can be found. Almost all the treatments that have been proposed have, in fact, been tried some- where. But we need information on how well these treatments have worked. The earlier recommendation that the development of a Census of Treated Monuments in this country be supported on a continuing basis thus becomes even more important. Such a census would include
Report of the Committee 9 careful documentation of all treatments, including, where possible, those applied in the past. We must, however, remember that reports on treatments are much more likely to detail successes than failures, whereas information on failures is at least as important as information on successes. There is also a need for improved, nondestructive methods of eval- uating the progress of treatments in the field. Photography has been used successfully to evaluate treatments; the approach ranges from elaborate, rectified photographs to very simple photography. Another evaluation method is ground-penetration radar. In its normal form, such radar has a large range and not very good resolution. But by going to higher frequencies up to 900 MHz good resolution is possible up to 3 or 4 ft. which is probably adequate for the purpose. While it is rather difficult to interpret the results of ground-penetration radar, it may be used as a diagnostic tool to measure the effectiveness of treat- ment. Ultimately laboratory tests are required because so many more things can be tried in the laboratory than can be afforded in the field. Labo- ratory results must not be overinterpreted, yet we must put some reliance in them. The conference attendees agreed that if we under- stand the mechanisms of decay involved and can characterize the material we are working with, we can understand why it fails and can select means of eliminating that failure. RILEM has proposed a number of nondestructive tests that may be applicable. Volumes IV and V of the RILEM-UNESCO meeting in 1978 include a large number of tests that have been proposed for use in the field. Among them are several kinds of evaluations of strength and of various aspects of porosity, permeability, and conductivity in rock. There has been no attempt as yet to set limits on these tests, but a beginning has been made. This effort should lead to a period of stand- . . arc 1zatlon. Evaluation of treatment should be quantitative for a single discrete element or operation. However, there can and should be evaluations of entire systems that address elements such as costs, time, manage- ment, aesthetics, and acceptability to the clients. Perhaps we will see the development of a new generation of specialists in the administra- tion of cultural property who will oversee activities down to the level of builcting management and maintenance. These would be persons who have had thorough basic training in areas such as architecture, engineering, and history and who identify personally with the prop- erties entrusted to them. We might look to such specialists for sub- jective evaluation.
10 CONSERVATION OF HISTORIC STONE BUILDINGS Recommendation: The committee recommends the expansion of the RILEM effort to refine nondestructive tests for evaluating treatments in the field. Research is needed to develop a better understanding of mech- anisms of failure of treatments. EDUCATIONAL AND INFORMATION SERVICES Cooperative efforts between scientists and engineers on the one hand and historic preservationists and conservators on the other are limited by lack of a common vocabulary. This flaw could be remedied by the two groups' meeting together more frequently, considering each other's problems and possible solutions to them, and presenting technical concepts in tempts both groups can understand. Recommendation: The committee recommends that organizations such as the National Trust for Historic Preservation, the Association for Preservation Technology, and the U.S. National Committee of the International Council on Monuments and Sites organize scientific ad- visory committees within their structures to bring the scientific and conservation communities closer together. We also need greater awareness among graduate advisers and stu- dents of the interesting scientific problems posed by historic preser- vation. Recommendation: The committee recommends that universities with graduate programs in historic preservation develop strong curricula in the technology of historic buildings. FUNDING FOR RESEARCH The committee considers it inappropriate to suggest specific levels of funding required for the tasks outlined above. At the same time, how- ever, we strongly urge federal agencies to recognize that conservation and preservation of monuments is eminently important and a legiti- mate area for research funding. A major obstacle to funding is the interdisciplinary nature of the field. Senior investigators who readily obtain support for research in a single discipline commonly encounter great difficulty in obtaining funding for interdisciplinary work. To obtain funding a researcher must restrict his proposal to the narrowly defined interests of an existing funding program within a single agency. In a field so obviously dependent on a multidisciplinary approach,
Report of the Committee 11 where the review mechanisms of few agencies are prepared to respond favorably, it is virtually impossible to sustain a research effort. Recommendation: The committee recommends the establishment of an interagency task force with representatives from such funding agencies as the National Park Service, the Environmental Protection Agency, the National Science Foundation, and the National Endow- ment for the Humanities to address the problem of funding interdis- ciplinary research and to establish a coordinated policy for the review and funding of research on the conservation of buildings and monu- ments. To supplement funding from the public sector, those engaged in research will have to broaden their base of support to include corporate foundations and trade institutes. Where appropriate, the preservation community should explore sharing specialized equipment and support personnel. Recommendation: The committee recommends that the preserva- tion community seek research support in the private sector from such trade organizations as the Masonry Institute and the Brick Institute and from foundations such as the Mobil Foundation and the Alcoa Foundation. POLICY AND PLANNING There is at present no laboratory in the United States dedicated to the preservation of stone, nor is there a visible potential for the establish- ment of such a laboratory. fAs a historical note, the National Science Foundation operated a Prevention of Deterioration Center from 1942 to 1964.) The preceding recommendations call for a range of actions by gov- ernment agencies, industry, universities, professional organizations, standards organizations, and individuals concerned with historic pres- ervation. Taken together, these recommendations form a first outline for a coordinated national plan. We need some form of organizational mechanism to bring together the existing disparate groups so that they can work cohesively to pro- mote, coordinate, and implement activities in the conservation of his- toric stone buildings and monuments. No candidate for this role has emerged from the broad array of existing organizations.
2 CONSERVATION OF HISTORIC STONE BUILDINGS Recommendation: The committee recommends that representatives from scientific and engineering professional organizations meet with representatives of historic preservation and conservation organizations to develop an intersociety mechanism to coordinate preservation ac- tivities among their groups, to expand awareness of public policy, and to develop a coordinated national plan.