Builders in many parts of the world use plant stems, particularly trees, but also grasses and reeds, because they are relatively light and strong, tend to grow long and straight, and are often abundant. These plants have a structure built from a compound called cellulose. In wood this accounts for about 50% of the structure, the rest being hemicellulose and lignin. Cellulose and hemicellulose are polymers, chemicals formed by long chains of smaller base units. Cellulose is made from chains of glucose in much the same way that the plastic PVC (polyvinyl chloride) is made from chains of vinyl chloride molecules.
Cellulose is tough stuff, which is why we find wood so useful, and luckily it is also tough stuff to eat. For humans it would be pure dietary fibre, but we lack the appropriate bacteria in our gut to break it down. These are found in ruminants, like cows, and in certain insects, and it is the latter that are most likely to try and eat your house.
Termites can have a significant impact on traditional material culture. For example, parts of India do not have the same emphasis on wooden furniture and interiors that is typical in northern Europe. There is plenty of wood, and a tradition of woodworking that later became adept at producing furniture for western taste, but the presence of termites in the environment contributed to a greater emphasis on ceramics and stone in Indian building interiors. In Southeast Asia, timber, particularly for structural use, has to be chosen carefully, not just for its strength, but also for its ability to resist termites. Woods like teak and eucalyptus, with hard resinous heartwood, are preferred. Termites affect not only the used of wood in a culture, but also its preservation for archaeologists.
The tomb turned out to be a cache of material associated with mummification and other aspects of burial; clearly it was thought inappropriate to throw such materials away. The dryness of the tomb had preserved rare organic finds such as pillows and mats, but initial interest focused on seven wooden coffins, some with beautiful painted faces. On closer inspection, termites had attacked all but two, so that in places it was just the decoration that was holding the object together. This is a conservation nightmare, and stabilising these objects so that they can be removed from the tomb to lab, usually in pieces, is a slow and painstaking task. It has been suggested that the termites may have arrived from workmen’s huts constructed by later tomb builders, the same huts that hid this, and Tutankhamun’s tomb, for over 3000 years.
It is the dryness of the Valley of the Kings that preserves so much fragile organic material, including mummies. However, much damage has been done to the tombs by flash flooding, and many poorly situated tombs have been virtually destroyed by these rare climatic events. The ancient builders may have included deep pits and shafts in the entrances to tombs to protect their occupants from floodwater, rather than simply to deter robbers.
Organic materials tend to be preserved by very dry or very wet environments. In Northern Europe, mummies are found in bogs, and buildings and other timber structures are preserved in waterlogged conditions. Mention should also be made of preservation by extreme cold: bodies have been found preserved by glaciers and in remarkable frozen graves in northern Asia.
It is no coincidence that very dry, very cold, or waterlogged conditions are protection against fungi, often known as ‘dry rot’, that would naturally be reprocessing dead trees, but will happily imbibe timber in buildings if it becomes damp enough. Dry rot infections arise at about 30% moisture, and the fungi will eat the cellulose from the wood, leaving a brittle matrix of lignin. Ingress of water into a building is the commonest cause, and since prehistoric buildings contained significantly more wood than most modern buildings, they are vulnerable if not completely weatherproof.
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