Mahmoud Mohammed

‘Changes in the Chemical Composition of Archaeological Wood Caused by Exposure to Different Environments and Its Relation with the Other Properties’ (PhD in cultural materials & conservation, 2018).

This thesis aims to characterise the deterioration mechanisms of dry cultural heritage wood by applying a multi-analytical technique that is comparable to that used for waterlogged archaeological wood. The deterioration and degradation resulting from the aging of wood, especially waterlogged wood, have been studied extensively compared to wood in dry environments. A similar scenario is repeated with the less information available about the chemical modifications of dry wood during aging, in particular on the molecular level. Previous studies (Crestini, El Hadidi, & Palleschi, 2009; Nilsson & Daniel, 1990) have noted this lack of research on dry wood and its chemical transformations, but this field still requires research into the relation between wood’s chemical composition and how it impacts wood’s other anatomical, physical and mechanical characteristics. This thesis explores also the effect of some other materials associated with wood in art objects on the chemical composition/characteristics of wood. It also seeks to characterise changes and modifications in the chemical composition of archaeological wood, and explain the degradation mechanisms of archaeological wood.

This thesis is divided into three sections. The first section examines the history of wood in cultural heritage (Chapter 1), the properties and structure of wood (Chapter 2) and wood deterioration (Chapter 3). The second section examines the materials and methods involved in the scientific investigation of the deteriorated wooden cultural heritage (Chapter 4). The third and final section analyses the results, discusses them and provides the conclusions of the research (Chapters 6–7). The thesis also includes a case study of the conservation analysis and treatment of a polychrome ancient Egyptian statuette (Chapter 8). The study employed analytical methods to identify the materials used in the object and their deterioration in order to implement a treatment and conservation plan to restore the object to as close to its original condition as possible and to preserve it for a longer period.

Different analytical techniques and instruments were employed to research these degradation mechanisms. Several samples were collected and examined, principally from various places in Egypt as well as from Italy and Australia. Reference samples for the identified taxa were also examined using the same techniques (Chapter 4). Analysis began with visual observation and microscopic studies using the light microscope (LM), a polarising light microscope (PLM) and a scanning electron microscope (SEM) to identify the wood species and to assess the anatomical and morphological changes of the wooden samples. The physical properties of the samples such as density were also examined and the moisture content of each sample was measured.

A multi-analytical chemical procedure was followed in order to describe the condition of the samples and to track their chemical changes and transformations. Wet chemical analysis (WCA), including Klason lignin and acid-insoluble lignin and ash content measurement was used to determine the holocellulose to lignin ratio (H/L), water soluble substances and the organic soluble substances and the ash minerals content. This was useful in order to compare this study to the previous studies using this technique for analysing the chemistry of wood with cultural heritage significance and also it was used as a reference for the following chemical analytical techniques.

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) was applied on thin slices of the same samples and also applied on extractive-free powder. These results were useful in the interpretation of the high level of extraneous substances extracted, especially by hot water. Very small samples were cut and examined by pyrolysis gas chromatography with mass spectrometer (Py(HMDS)-GC/MS), which was very helpful in understanding the molecular changes in the chemical composition of samples tested (Chapter 4).

Samples were able to be identified by species or family (Chapter 5). For the ancient Egyptian samples, Ficus sycomorus (sycamore fig) was the main wood identified and other species included Faidherbia albida, and Taxus baccata. Other species identified from the samples included: Pinus sylvestris, Tamarix sp., Picea abies Karst., Pinus halepensis, Ostrya carpinifolia, Cupressaceae family and Quercus sp. Microscopic assessment showed different preservation states for the samples studied; however, the trend was that the most ancient samples were the more deteriorated ones. Fungi and bacteria were evident in some samples, especially those from ancient Egypt, but the identification of the biological agent was complicated. Physical measurement (Chapter 6) confirms the trend of decay revealed by the microscopic study as the ancient Egyptian samples had the least density compared to the other cultural heritage samples and reference samples.

The results showed the significant differences between the degradation of waterlogged archaeological wood and the dry wood, both in the chemical modification and in the other properties that occurred subsequently.

Supervisors: Professor Robyn Sloggett, Dr Petronella Nel, and Professor Uta Wille (Chemistry).