1 INTRODUCTION

Wood deteriorates rapidly from a variety of different biotic processes when exposed to moderate environmental conditions. In situations of environmental extremes, however, such as a dry tomb chamber buried beneath rock or soil, wood decomposition is impeded. Archaeological wood that has survived exceedingly long periods of time may have been protected from the most aggressive wood-destroying fungi, but inevitably some abiotic or biotic deterioration takes place. While the deterioration and conservation of waterlogged wood have been the focus of much study, until recently far less attention has been given to archaeological wood from dry environments. Understanding the mechanisms of deterioration as well as the condition of the artifact at the ultrastructural level makes it possible to tailor treatment, physical supports, and environmental conditions most appropriate for the long-term preservation of archaeological woods. In addition, information on the decay present can provide insights into past microbiological events as well as past physicochemical and environmental conditions of the site.

Different abiotic and biotic forms of deterioration result in distinct changes within wood cells (Blanchette et al. 1990; Fengel 1991; Kirk and Cowling 1984; Liese 1970; Singh and Butcher 1990). These chemical and morphological degradation patterns can be used to determine the type of decay and the causal agent. Information obtained from previous studies of microbial degradation have provided a useful classification system for fungal and bacterial degradation of wood. Soft, brown, and white rot are broad categories of different types of decay caused by fungi. Within each group, further differentiation can be made depending on the degradative mode of action and cell wall decay patterns (Blanchette 1991; Eriksson et al. 1990; Singh and Butcher 1990). Bacteria degrade wood differently from fungi, and ultrastructural studies have revealed distinct forms of attack (Nilsson and Daniel 1983; Daniel and Nilsson 1986; Nilsson and Singh 1983; Singh and Butcher 1990). Tunneling and erosion bacteria degrade wood, causing minute tunnels or eroded zones within cell walls, whereas other species of bacteria may attack only pit membranes. As new investigations focus on the different forms of bacterial attack, more specific classification systems will undoubtedly be utilized. Recent reviews of microbial degradation have been published (Blanchette et al. 1990; Eriksson et al. 1990; Highley and Illman 1991), and this information can be used as criteria to evaluate decay in archaeological wood and other wood of historic value (Blanchette et al. 1990; Blanchette et al. 1991b).

Abiotic factors are also responsible for degrading wood, but our knowledge of these processes is meager due not only to the long time period required for significant degradation to become apparent but to the masking effects of the ever-present microbial population. In a study of wood from the high Arctic that had been buried or frozen for 20–60 million years, microorganisms were found to be excluded but deterioration was still evident (Blanchette et al. 1991a; Obst et al. 1991). A slow acid hydrolysis has been postulated that gradually degraded cell wall carbohydrates and modified residual lignin. In situations where wood is in contact with high concentrations of acid or alkaline compounds, deterioration may be substantial even after a few decades (Blanchette et al. 1991b; Parameswaran 1981). Although the presence of metal ions in wood is typically considered to have a preservation effect, degradation of wood can also occur from products of metal corrosion. Iron or other metal corrosion products can weaken wood and cause significant cell wall alterations (Baker 1974; Marian and Wissing 1960; Parameswaran and Borgin 1980). Metal ions are active catalysts promoting chemical reactions that initiate a nonbiological type of cell wall degradation (Baker 1974; Blanchette and Simpson 1992; Keepax 1975). Moisture and soluble chlorides accelerate the corrosion process and the deterioration of wood. In certain circumstances, metal corrosion not only causes extensive cell wall degradation but produces iron or other metal pseudomorphs that display a replica of the woody cell wall (Blanchette and Simpson 1992).

Many wooden cultural properties from ancient Egypt have been preserved for thousands of years in extraordinarily good condition. Wood fragments from various objects have been used in past research to identify wood species and to detect changes that may have occurred in the wood. In these studies, the macroscopic structure of the wood samples selected for study was sound and appeared intact (Borgin et al. 1975; Nilsson and Daniel 1990). No microbial degradation was found by Borgin et al. (1975), but mechanical damage to the wood cell walls was evident. Separations within the secondary walls and fractures in middle lamellae were observed. In the study by Nilsson and Daniel (1990) of wood samples from ancient tombs, a weakening of the wood revealed by cracks and delaminations within cell walls was observed in one sample; however, the source of this deterioration was not identified. In another sample, a soft rot form of fungal degradation was evident within the secondary wall layers of the wood cells.

Observations of wooden artifacts in museum collections of ancient Egyptian art clearly demonstrate that not all wooden objects are in good condition. Many cultural properties have substantial deterioration. In a preliminary study, objects that appeared to display different stages of decay were selected for ultrastructural examination (Blanchette et al. 1991b). Advanced stages of decomposition were apparent in all of the selected woods.

In our study we examined a large number of different wooden objects from the collections of ancient Egyptian art in the Museum of Fine Arts, Boston, and the Metropolitan Museum of Art, New York, that displayed visual evidence of deterioration. Ultrastructural observations were used to determine the type of decay that was present, evaluate the current condition of the wood, and provide general knowledge of deterioration common to woods found in dry archaeological sites. The results presented here should serve as a guide for types of decomposition that can be expected in woods from similar environments.