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More on mummies
Stopping the rot - how mummification works, and where to learn more.
Bacteria can degrade dead bodies in an amazingly short time, eating them to reproduce. (Of course, before bacteria can do their job the digestive enzymes in the gut and other organs start eating the body itself.)
 The Egyptians defeated the process by drying, as bacteria need moisture. The system evolved over centuries, but they classically removed the soft bits inside the body (including brains, intestines and stomach, although generally not the heart) and dried the body cavity with natron, the hydrated sodium carbonate deposits (Na2CO3.10H2O) found in two main salt lakes in the country.
The natron compounds also contained sodium bicarbonate (our baking, soda, NaHCO3) and table salt (sodium chloride, NaCl). These compounds dried flesh through osmosis: water moves through the cells of the body to create a brine concentrate with the salt.
Because the Egyptians were skilled chemists (they used natron in glassmaking to lower the boiling point of the liquid), it is possible that the mummification compounds were mixed specially, to achieve specific effects.
Once dry, the empty body was packed out with bulky materials such as linen or sawdust to keep its shape, before the elaborate process of wrapping, sealing, painting and adornment began. The organs were also washed, dried and wrapped.
Freeze-dried or deep-frozen mummies don't decay because the bacteria die or are immobilised by the cold: it's why the food in your freezer doesn't go off.
Several things seem to happen when bodies fall into bogs. First of all, the main body of water, peat and sphagnum moss is protected by a thin top layer of moss, and can remain undisturbed for centuries. Sphagnum moss has anti-bacterial properties (it was used to bind wounds in World War I) and the low level of bacterial activity in bogs may be down to a chemical (sphagnol) it releases. On the other hand, bacteria may be discouraged by the acidity of the water and its low oxygen level. Another theory says that the sphagnum may neutralise surrounding bacteria thanks to the action of a pectin-like sugar (a polysaccharide with reactive carbonyl groups attached) called sphagnan in its cells. The bodies themselves are also tanned in the water, like leather: the bog acids dissolve the fatty layer of skin and its non-fibrous proteins, drive out water, and make the fibres of the remaining protein, or collagen, stick together.
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