"Leather Soles Damaged by Heat The leather soles of boots and shoes are very easily damaged by heat - far more easily than most people realise. The danger is especially great when the leather is wet. Innumerable leather shoes are propped up in front of open fires or placed on radiators or hot pipes - and ruined as a consequence.
The damage done may not be immediately obvious on the surface of the soles but it will seriously affect their life. Wet shoes are more easily damaged but shoes worn under normal dry conditions contain enough moisture to suffer by such maltreatments.
Damp soles should be allowed to dry out slowly in a well-ventilated place which is not warmer than a normal living room. It is important that an airy and well ventilated place should be used. Heating should be avoided whether the shoes are wet or dry.
One safe way to assist the drying out of very wet shoes is to take up the excess moisture by stuffing them with dry newspaper for a few hours, or overnight - but keep them well away from open fires or hot pipes.
Almost all leather soles are made of vegetable tanned leather, which is particularly sensitive to heat. Heat generates water vapour or steam within the substance of the leather and this steam attacks the leather fibres., entirely destroying their character. The leather is thus turned into a soft tarry mess which becomes hard and brittle on cooling.
The damage is not always visible on the surface even when the inner substance of the leather has been badly burnt. This rather curious effect arises in the following way. The application of heat dries the outside of the leather quite rapidly because the vapour gets away quickly from the surface, but the steam which is generated inside the leather cannot escape so quickly and has time to cause internal damage by attacking the fibres of the leather. In subsequent wear, when the outer surface of the leather is worn away, the affected part is soon exposed and rapidly disintegrates. Where the burning is less severe the damage to the fibres may not be apparent to the naked eye and can only be seen with the aid of a microscope; nevertheless the wearing properties of the leather will have been seriously impaired.
Burnt soles are a frequent cause of unjustified wear complaints. In addition, it is certain that as a result of less obvious heat damage the life of many leather soles is shortened unnecessarily."