William S. Ginell

The role of sepiolite-palygorskite in the decay of ancient Egyptian limestone sculptures

An ancient Egyptian limestone sculpture was found to be undergoing major structural decay when stored in a museum environment. Mineralogical and petrographic analysis of the limestone showed a high proportion of clay (≥- 10% by weight) that was concentrated along bedding planes. The clay fraction consisted mostly of sepiolite (>90%) and palygorskite (<10%). Minor quantities (≤l%) of soluble salts (NaCl and NaNO3) were also found. Wetting/drying with distilled water and relative humidity cycling resulted in the same delamination cracking damage as that observed in the museum environment. Thermomechanical analyses (TMA) confirmed that the damage was due to expansion (>4.5%) parallel to bedding planes when the limestone was immersed in water. The expansion due to swelling of the clays was directly observed at high magnification in an environmental scanning electron microscope (ESEM) when wetting/drying cycles were performed. X-ray diffraction (XRD) analysis showed that crystalline swelling of sepiolite occurred. This was determined by a shift of (110) reflection (from 12.07 to 12.20 Å) and a decrease of (060) reflection (4.47 Å, to 4.44 and 4.41 Å), when in contact with ethylene glycol (EG) and dimethyl sulfoxide (DMSO), respectively. Swelling also occurred due to hydration of the clay surfaces and to electrostatic forces between clay particles, which, it was assumed, was promoted by the presence of Na counterions in water solution. Possible treatments for the conservation of these artistic objects are proposed and discussed.

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

SEISMIC STABILIZATION OF HISTORIC ADOBE STRUCTURES

Abstract Many historic adobe structures in the southwestern United States and throughout the world are vulnerable to destruction or damage by earthquakes. In the recent Northridge earthquake in Los Angeles, a significant number of the remaining, unretrofitted adobe structures dating to the 18th and early 19th century were extensively damaged. Yet it has been observed that many other adobes have performed well during large seismic events. Responses differ because low-strength adobe, when used in thick walls, develops cracks during seismic events, but these cracks do not necessarily lead to catastrophic building collapse. The concept of seismic retrofitting based on improving the stability of the adobe buildings, rather than the more conventional criterion of improving the strength, seems to be a valid approach. The objective of such a concept is to prevent walls from overturning by restricting the relative displacement of blocks formed following cracking, thereby enabling the structure to dissipate energy by friction and rocking without catastrophic collapse. Shaking table tests of model adobe buildings have been carried out. Various types of simple retrofitting measures that were evaluated were designed to be relatively noninvasive and respectful of the historic fabric of the building. These measures consisted of thin, flexible horizontal or vertical straps applied to both sides of walls and small diameter steel rods inserted within the walls used in conjunction with a thin wood bond beam. The intent of these measures was to provide overall structural continuity rather than strength improvement. The results of the tests involving nine 1:5 scale model buildings clearly demonstrated the effectiveness of the stability-based techniques. Wall height-to-thickness ratios were varied from 5 to 11, and maximum table displacements of ±38 cm (prototype domain) were used. Some retrofitted models were able to withstand twice the displacement that resulted in collapse of an unretrofitted model. Additional tests carried out on two instrumented 1:2 scale model adobe buildings further demonstrated the effectiveness of the retrofitting measures.

A NEW TECHNIQUE FOR DETERMINING THE DEPTH OF PENETRATION OF CONSOLIDANTS INTO LIMESTONE USING IODINE VAPOR

AbstractIn evaluating the effectiveness of consolidants for limestone, difficulty is encountered in the determination of the depth of penetration of consolidants. A simple procedure using iodine vapor for visualizing penetration depth is described. The technique is compared with other methods, such as fluorescent dye indicators, elemental analysis, scanning electron microscopy (SEM) observation, charring in an inert atmosphere, and acid etching, for effectiveness, applicability, and suitability.

Recent Developments in the use of Epoxy Resins for Stone Consolidation

Amine groups in epoxy resin curing agents may be responsible for the effectiveness of this consolidant in limestone. Changes in stone appearance caused by the use of epoxy consolidants can be minimized by proper resin selection, application procedures, and by modest outdoor exposure. Vacuum impregnation of deteriorated stone with epoxy resins results in deep penetration and is a process now in commercial use. -- AATA

Limestone Stabilization Studies at a Maya Site in Belize

Stone used in the construction of the 8th–11th century Maya structures at Xunantunich in Belize is a low strength, porous limestone that is nearly pure calcium carbonate. Degradation of archaeologically excavated stone structures in the humid, tropical environment of Belize is caused mainly by wind and water erosion and the wide cyclic variations of humidity and temperature. However, damage to the limestone is accelerated to varying extents by the chemical and mechanical effects of lichens, mosses, algae, fungi, and bacteria that are endemic to the region. To evaluate the effectiveness of possible stabilization treatments, tests were conducted in which stone-penetrating consolidant solutions were applied to limestone samples, which were then exposed to both sunny and shaded environments over a period of four years. The results of these treatments were evaluated visually and by particle and water erosion resistance measurements on the aged samples. The effectiveness of several biocides in controlling the establishment and growth of microflora on the exposed samples and on in situ, ancient stonewalls was also studied. Some tests were conducted to determine if the organic polymer consolidants would support, or even accelerate, the growth of microflora on the stone and how the concurrent use of biocides would affect the results. Several consolidant solutions were found that could penetrate and stabilize the usually moist limestone and, in combination with biocides, would minimize the growth of the local microflora.