Investigation of deterioration in archaeological wood used in architectural elements: Microscopic study

S.A.M. Hamed

Cairo University, Faculty of Archaeology, Conservation Department, 12613 Orman, Giza, Egypt

This paper focuses on investigating deterioration of archaeological wood which were used as architectural elements such as ceilings, doors, floors, domes, mashrabias. Nine samples were collected from different architectural elements, then a scanning electron microscope (SEM) study was undertaken, to monitor the significant structural changes in the wood samples according to their function. SEM data, however, show that loads and stresses affect the anatomical structure of wood badly, in addition to other deterioration factors, especially in the large wooden objects and structural wooden elements. The main problem is the separation and fractures resulting in the wood structure which might cause the structure to collapse. So, developing new materials and techniques that could be used in the treatment and conservation of archaeological wood subjected to this type of deterioration is restricted at understanding condition of this wood.

Keywords: Archaeological wood; deterioration; architectural elements; and SEM.

1. Introduction

Timber has a long history of use as a structural building material and finds its way into many varied applications, from shipbuilding and marine architecture to civil and structural engineering [1]. In Egypt, wood used as architectural elements in historic buildings such as ceilings, doors, floors, domes, and mashrabias. All wood structures, if ill maintained, are liable to decay and deterioration. The behavior of wood as a construction material is complex. Wood is an anisotropic material; its strength properties vary with the direction of loading to grain direction, it shrinks and swells in response to changes in atmospheric moisture, characteristically to varying extents in different directions, it generally requires seasoning before use in construction and is liable to decay. The risk of decay in timber depends on the durability of the species. Fluctuating moisture content causes dimensional movements in wood, increasing stresses in structural connections and causing splits and fissures. Furthermore, moisture contents above 18% increase the risk of infestations from organisms that feed on timber, destroying its structural properties [2]. Borgin used SEM to study the mechanism of fracture and cohesive failure of the structure of wood and found that the weakest part of the ultrastructure of wood which exposed to shear along the grain and tension across the grain was the bonds between parallel strands of microfibrils [3]. This study aims to monitor the anatomical changes that occur in archaeological wood used in different architectural elements in order to allow appropriate conservation schemes to be planned and implemented.

2. Materials and Methods

2.1 Wood samples:

Nine samples were collected from different architectural elements and different places as follow:

Table 1 Samples of the study.

Sample No.

The site The architectural

element period

1 Bayt Al Razzaz Mashrabia’s turning wood

Ottoman era

2 Radwan baik’s maqaad Mashrabia’s turning wood

Ottoman era

3 Prince Aytmish El-Bagassi’s Mosque Ceiling Mamluk era 4 Taqi al-Din Bustami’s Hospice Ceiling Mamluk era 5 Mohamed Ali’s palace Ceiling Mohamed Ali’s era 6 Prince Omar Tousson’s palace

(Shubra) Ceiling Mohamed Ali’s era

7 Taqi al-Din Bustami’s Hospice Door lintel Mamluk era 8 Al-Salih Tala’I Mosque Door Fatimid-era 9 Mary Gerges’ church Door Coptic era

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2.2 Scanning electron microscopy (SEM):

A scanning electron microscope (SEM) study was undertaken, to monitor the significant structural changes in the wood samples according to their function. The samples were mounted on aluminum stubs with double- sided cellophane tape. After coating with gold (Polaron sputter coater), samples were examined by SEM (JEOL scanning electron microscope JXA-840A).

3. Results and Discussion

Architectural wooden elements are subjected to various deterioration factors according to their location and their function. Weathering and mechanical deterioration are the most important factors. Wood used for structural purposes in many historic buildings suffers often from decay or mechanical failure. In the past few decades large amounts of wooden beams were completely replaced, although in some cases the beam suffered only partial decay [4]. SEM results show clearly that the structure of wood exposed to mechanical deterioration, due to loads and stresses, undergoes distinct anatomical changes, especially in the large wooden objects and structural wooden elements. However, the extent of deterioration depends on the function of the structural wooden element. SEM micrographs of sample [no.1] which was taken of Mashrabia’s turning wood from Bayt Al Razzaz showed missing parts all over the wood tissue (Fig. 1a), Cracks and separations along the cell walls, as well as some regions in the wood structure are collapsed (Fig. 1b, c). Mechanical damage to the cell walls was evident in Mashrabia’s turning wood from Radwan baik maqaad [sample no. 2]. The SEM micrographs illustrate the presence of numerous sustained cracks and fractures across the cells appeared along the wood grain (Fig.2a). Besides, separation and delamination within the cell walls, losing some parts of the wood cell walls, as well as loosening of cell wall layers can be seen (Fig. 2b). These results emphasize that the wood is exposed to heavy loads. According to Umney and Rivers this can be related to their shape and design. As such curved structures are formed by cutting from straight grained timber and there is a section of so-called short grain at the curve which may be exposed under load to high shearing forces resulting in mechanical failure [5].

Fig. 1 SEM micrographs of longitudinal view of turning wood taken from Bayt Al Razzaz (bar: (a) 500μm, (b) 50 μm, (c) 100 μm).

a b

c

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Fig. 2 SEM micrographs in longitudinal view of turning wood taken from Radwan baik maqaad (bar: (a) 200μm, (b) 200 μm).

The wood sample taken from the ceiling of Prince Aytmish El-Bagassi’s mosque [sample no. 3] showed separation along the cell walls which led to large voids between cells and detaching the adjacent cells as a result of losing the middle lamellae region completely. Deep fracture within the cell wall can be observed. It appears that the wood tissue suffered from deformation due to mechanical deterioration (Fig. 3a, b, c). A general weakening of the wood structure was apparent. The results of sample no.4 which was taken from Hospice Taqi al-Din Bustami’ ceiling show clearly that there are some missing layers in the cell walls (Fig.4 a). Also, separation between adjacent cells can be seen. Checks and fractures occurred within cell walls. It appears that the pit boundaries remain largely intact except in some of them (Fig 4b). Weakening of the wood structure revealed by deep cracks, checks, separation between cells, losing some parts of cells, and collapsing some areas of the wood tissue, appeared most clearly in SEM micrographs of sample no.5 which was taken of the ceiling from Mohamed Ali’ palace (Suez) (Fig. 5a, b). SEM micrographs of sample no. 6 taken of a ceiling from Prince Omar Tousson’s palace (Shubra) showed that mechanical deterioration caused deformation of the wood structure due to compression force. Separations, cracks, fractures and losing parts as well as collapsing areas of the wood structure (Fig. 6a, b). Generally wooden beams and boards of the ceiling are exposed to loads of ceiling layers lies above them, as well as rain water which resulting in loads caused by its accumulation over the ceiling and internal stresses caused by expansion or contraction of wood due to gain or loss of moisture. This explains the curvature of cells in sample no. 6. Also, losing middle lamellae region due to loads and stresses and weathering caused extensive separations between adjacent cells as Blanchette et al. pointed out [6].

b a

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Fig. 3 SEM micrographs of sample no.3 taken from the ceiling of Prince Aytmish El-Bagassi’s mosque (bar: (a) 100μm, (b) 100 μm, (c) 100 μm).

Fig. 4 SEM micrographs of sample no.4 taken from the ceiling of Hospice Taqi al-Din Bustami (bar: (a) 100μm, (b) 100 μm).

Fig. 5 SEM micrographs of wood sample no.5 taken of the ceiling from Mohamed Ali’ palace (Suez) (bar: (a) 200μm, (b) 100 μm).

b a

c

b a

b a

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Fig. 6 SEM micrographs of wood sample no.6 taken of of a ceiling from Prince Omar Tousson’s palace (Shubra) (bar: (a) 100μm, (b) 50μm).

The wood sample no.7, taken from Door lintel of Hospice Taqi al-Din Bustami, exhibited numerous separations between adjacent cells showing evidence of deterioration of the middle lamellae regions. Also, fractures across the cell walls and fissures can be observed (Fig. 7a, b). Losing many parts of the wood cells, due to insect attack, was apparent in sample no. 8 which was taken of a door from Al-Salih Tala’I Mosque. In addition, data showed the existence of extended cracks and fractures especially around the pit borders. Separation between cells was also clear (Fig. 8). Examination of sample no. 9 from a door in Mary Gerges’ church showed that the wood surface was covered with foreign deposits from old wood finishes and varnishes. There are checks and fissures all over the structure. Besides, the structural damage of the pits (Fig. 9). As doors were exposed to outdoor weathering, SEM micrographs showed that wood exhibited surface deterioration, loss of middle lamella, separation of cells, and damage of pit structures as Hon and Feist mentioned [7]. In addition, mechanical deterioration affected the integrity of the wood structure and caused cracks and fractures.

Fig. 7 SEM micrographs of sample no.7 taken from the Door lintel of Hospice Taqi al-Din Bustami (bar: (a) 100μm, (b) 100 μm).

Fig. 8 SEM micrograph of wood sample no.8 taken of a door from Al-Salih Tala’I Mosque (bar: 100μm).

a b

b a

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Fig. 9 SEM micrograph of wood sample no.9 taken of a door in Mary Gerges church (bar: 10μm).

4. Conclusion

Architecture wooden elements respond in different ways to an externally applied load and/or change in environmental conditions. Comprising deterioration factors such as weathering condition, biological deterioration and mechanical deterioration, is considerably more extensive than exposure to one parameter. The main problem is the separation and fractures resulting in the wood structure which might cause the structure to collapse. This justifies the total replacement of damaged wood by similar elements of new wood. So, developing new materials and techniques that could be used in the treatment and conservation of archaeological wood subjected to this type of deterioration is restricted at understanding mechanisms of deterioration as well as the condition of wood used in structural purposes.

References

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(2):169-183.

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