The changes which organic substances undergo are various; thus, while leather becomes hard, papyrus becomes brittle. Like all other organic material they may undergo those destructive processes which are due to the growth of moulds or to the agency of various bacteria. They are also liable to be attacked by maggots, moths, and other insects. It is unnecessary here to describe in detail these numerous and varied changes; a few special cases only need be mentioned.
Acid peat, in which iron objects perish, is found to have a good preservative action upon wool and horn, whilst vegetable fibres are destroyed. On the other hand, in pile-dwellings wool and horn substances have disappeared. Olshausen[80] thinks that animal fibre is destroyed by simple decay brought about by the oxygen in solution in ordinary water, whilst in peat the immense quantity of vegetable matter takes up the oxygen which can therefore no longer serve for the oxidation of wool and similar material.
Under certain circumstances woollen textures are found to be remarkably well preserved in oak coffins, as may be seen in the Museum at Copenhagen.
Bones, horn, and ivory show great variety in their behaviour, which depends of course on the nature of their surroundings. Thus for instance in acid peat sometimes the animal matter only is preserved[81], while in graves, beyond a few remains of tooth enamel, there is often nothing to show that they have enclosed bodies. Burned bones are generally found to resist decay, for the destruction of the animal matter leaves them no longer liable to further decomposition[82].
Amber objects are well preserved in water or in peat, but if they have lain in earth, they are darkened and often friable.
If organic substances, such as wood, etc., have lain in the immediate neighbourhood of oxidized bronze, and are thereby saturated with copper compounds, they show a very good state of preservation, which continues after they have been placed in a collection. Similarly the remains of fabrics upon iron objects, which are permeated with rust, are sometimes found in good condition.
Objects imbedded in salt (sodium chloride) are in certain circumstances found in a good state of preservation and continue so, as is shown by the skins, leather and wooden articles which are exhibited in the Salzburg Museum.