Eric F. Hansen

Lime Mortars for the Conservation of Historic Buildings

Abstract Awareness of the need for compatible materials for the preservation of the architectural heritage has resulted in the revival of lime-based mortar technology and applications. However, knowledge of the preparation process and procedure influencing the final quality of lime mortars is limited, and controversy persists in the conservation community regarding the most appropriate material for conservation treatments (for example, hydrated lime versus aged lime putty). This paper reviews current knowledge on lime mortar technology, including burning, slaking, aging and carbonation of lime. Special emphasis is given to the effects of aging on the morphological evolution of hydrated lime and on the carbonation process, since these aspects have not been discussed thoroughly in the technical and conservation literature. The improvements observed in the physical properties of hydrated lime after prolonged storage under water can be attributed to particle size reduction (<1μm) and morphology changes (from prism to plate-like crystals). Studies on the carbonation of non-aged commercial hydrated lime and traditionally aged slaked lime revealed higher carbonation rates in the case of aged lime. Some recommendations are given for the processing of lime and the preparation of lime mortar for conservation treatments. The use of aged lime putty is recommended because this material, with higher plasticity and water-retention capacity, results in mortars of higher strength that carbonate faster.

A review of selected inorganic consolidants and protective treatments for porous calcareous materials

Abstract Incorporating the results of a meeting held in London in December 2000, sponsored by English Heritage and The Getty Conservation Institute, this article reviews both consolidants and protective treatments for the conservation of deteriorated limestone and lime plaster. Carbonate deposition (including both inorganic solutions and biomineralization), barium hydroxide treatment, ammonium oxalate and tartaric acid treatments are covered. The article reviews selected literature, identifies open questions and promotes discussion of a range of issues, encompassing application techniques, performance, compatibility and retreatability. While many questions concerning these important systems have been addressed in published sources, there are significant opportunities for new research.

THE ROLE OF CLAYS IN THE DECAY OF ANCIENT EGYPTIAN LIMESTONE SCULPTURES

One type of Egyptian limestone from Naga el-Deir (Abydos/Thebes region) exhibits an ongoing problem of deterioration typified in the form of continued delamination of the surface in a stela from Naga el-Deir acquired from archaeological investigations carried out in the early 20th century. Previous testing of this limestone type indicated the presence of sodium chloride and sodium nitrate. The sculptures have been treated and desalinated either by immersion in water or by aqueous poulticing, but the decay process was not halted, and major loss of surface stone was still noticeable after storage for a period of years. Mineralogical and petrographic data (x-ray diffrac- tion (XRD), scanning-electron microscopy (SEM), and optical microscopy) indicate that this stone has a high proportion of clays (up to 10% by weight). Lab- oratory tests suggest that the clays, concentrated along bedding planes, are largely responsible for the type of deterioration noted. The role of clay minerals in the decay of this type of limestone was demonstrated by performing a series of experiments, including wet- ting/drying cycles and relative humidity changes, thermomechanical analysis, and accelerated decay tests using water and ethylene glycol. One conclusion of this study is that in some instances, desalination procedures can induce more deterioration than can rigid environmental control. Another conclusion is that attribution of deterioration to the presence of salts may be insufficient, and further petrographic analysis should be initiated prior to desalination of clay-rich limestones. Unconventional methods for possible stabilization of the clay structure (by means of

The Effects of Relative Humidity on Some Physical Properties of Modern Vellum: Implications for the Optimum Relative Humidity for the Display and Storage of Parchment

AbstractThe effects of different relative humidities on some physical properties of three types of calfskin parchment (vellum) were investigated. Standard samples were subjected to (1) tensile fracture, and (2) measurement of the force that developed when the restrained samples were subjected to step decreases in relative humidity in the region between 60% and 11%. The results indicate that although no particular level of relative humidity can be excluded in general from consideration as a storage or display cohdition on the basis of tensile testing data alone, at 11% RH there is an adverse effect on some individual tensile properties. Relative humidities above 40% increase gelation and opportunities for biological growth. About 25% RH is the lowest level that can be tolerated without inducing large stresses in the material. Onfull consideration of these results, the physical chemistry and chemical reactivity of collagen, and the results of a recent study of the biodeterioration of parchment, a relative h...

Consolidation of Porous Paint in a Vapor-Saturated Atmosphere a Technique for Minimizing Changes in the Appearance of Powdering, Matte Paint

AbstractWhen resins are applied for consolidation of a matte paint surfacet the colors may darken because of increased surface gloss, the result of the formation of a film over the surface to be protected. Darkening may also occur when resin fills the void space in the powdering paint. A technique is described for consolidating matte, low-gloss, powdery surfaces with minimal if not negligible darkening of the colors. The technique outlined involves the application of a stable thermoplastic resin solution in an atmosphere containing a very high concentration of the solvent used to dissolve the resin, thus slowing solvent evaporation and allowing the resin to penetrate the paint layer and surround the exposed pigment particles. In addition to retaining the matte appearance of the object, “tide lines” may be prevented and adhesion of the pigment may be improved.

Incipient Maya Burnt-Lime Technology: Characterization and Chronological Variations in Preclassic Plaster, Stucco and Mortar at Nakbe, Guatemala

Patterned technological behaviors in the production and use of burnt-lime products can be characterized using optical microscopy and image analysis of petrographie thin-sections to determine the texture (supplemented by X-ray diffraction analysis). Variations in technological styles have been identified in 21 samples from the ancient Maya site of Nakbe, Peten, Guatemala dating to the Middle Preclassic (1,000 B. C. - 300 B. C.) and Late Preclassic (300 B. C. - 150 A. D.) periods. Middle Preclassic floors exhibit a chaotic texture with a highly random aggregate particle size. Late Preclassic modeled stucco or painted fragments have a well-sorted texture with a low acid-insoluble content ( 15%), the major phases being quartz (-10%) and clays (∼5%). The variations correlate both with the time of occurrence and the function of the end-product. These issues of construction methods and levels of technology contribute data to aid our understanding of cultural development in this area at a critical time when the Maya were formulating power structures.

Lime Putties and Mortars

Abstract Based upon a scientific understanding of the reasons different properties develop, slaked lime putty, aged slaked lime putty (which undergoes time-dependent changes upon storage in water) and putty from hydrated lime (i.e., soaked dry hydrate) are discussed, along with mortars made with them. The results of recent research on lime mortars are reviewed and interpreted focusing on the implications for the appropriate or inappropriate use of high-calcium lime mortar types in architectural conservation. The nanostructure and irreversible colloid nature of calcium hydroxide crystals in slaked lime putty, which determine the desirable working properties of putties, such as plasticity, density and viscosity, are modified through crystal coarsening triggered by drying. These properties are not fully regained upon subsequent reintroduction of water (e.g., putty preparation with hydrated lime such as commercially available dry powder). The calcium hydroxide crystals in slaked lime putty may befurther modified upon storage in water for extended time periods, also ciffecting the working properties and density. The nature of the crystals also influences the carbonation, and development if porosity and compressive strength if the mortar. Additionally, the results if tri-axial compressive strength testing, as opposed to uni-axial compression, suggest that the limited strength of high-calcium lime mortar contributes to the durability if masonry.

THE EFFECTS OF SOLUTION APPLICATION ON SOME MECHANICAL AND PHYSICAL PROPERTIES OF THERMOPLASTIC AMORPHOUS POLYMERS USED IN CONSERVATION: Poly(vinyl acetate)s

AbstractThe tensile properties of solution-cast poly(vinyl acetate) (PVAC) films are determined by the thermodynamic quality of the solvents. Higher strength and lower elongation are evident in polymer films of poly(vinyl acetate) AYAT (Union Carbide) cast from a solution of acetone or an acetone/ ethanol/water mixture, while lower strength and greater elongation are evident in films cast from a solution of chloroform or toluene after drying for 180 days. Films of PVAC-AYAT cast from a toluene solution retained a significant amount (>4%) of solvent, while films cast from the other three solvents retained little solvent (>0.3%). For all polymer films, the glass transition temperature (Tg) was elevated greater than 18°C above the Tg of the bulk polymer. The differences in the tensile properties of the films with little retained solvent is explained on the basis of the thermodynamic quality of the solvents, 1.e., the physical properties of the solution-cast polymer film in the dry state may be affected by th...

The Effects of Solvent Quality on Some Properties of Thermoplastic Amorphous Polymers Used in Conservation

This paper reviews the effects of solvent quality on the physical “amorphous” polymers applied from solution, with a focupsh yosni craelc epnrto pteesrttiinegs ooff materials used in the conservation of art objects. Because the solvent composition or quality affects the shape and orientation of polymer molecules in solution, the nature of the partly crystalline dry film can also be affected. Thus, the physical and optical properties of a number of polymers have been shown to vary when deposited from solutions of different quality. This phenomenon is specifically related to the desired performance of a coating, adhesive or consolidant used for the conservation of an art object. The effects of “good” or “poor” quality solvents on the physical properties of solution-cast films of poly(vinylacetate) and Acryloid B-72 are discussed. In addition to polymer specific variations due to solvent quality on the glass transition temperature and the tensile properties (strength and elongation), the effects of quantitative amounts of retained solvent on these properties are considered.

Matte paint : its history and technology, analysis, properties, and conservation treatment with special emphasis on ethnographic objects

This supplement to Art and Archaeology Technical Abstracts has a primary emphasis on ethnographic objects, with special attention given to information on the deterioration and conservation of matte paint on immovable objects, including painted walls and architectural ornamentation, and on the conservation of unpainted matte areas. This topical bibliography contains more than 1500 abstracts of periodical, monographic, and unpublished literature from the fields of anthropology, conservation, ethnobotany, art history, and coatings science.