Notes on Leather Tanning, history and method
The treatment of animal skins to fit them for useful purposes is a practice which seems to exist wherever mankind has been found. In our Bible, Gen. 3 V 21, "Unto Adam also and his Wife did the Lord God make coats of skins and clothed them ". The Egyptians, who formed the first civilised nation of history, have left considerable evidence, both as relics in museums and drawings in the pyramids, of their knowledge of leather manufacture. The Roman World State made considerable progress in the various processes employed and the use of leather goods produced.
The last century has witnessed vast developments in both the scientific knowledge and the application of this knowledge to making leather manufacture a branch of chemical technology. Trace a map of the world and the following countries export hides, tanning materials etc. to England who import these materials.
- Hides (Cow) - South America, Africa, India, with a few imported from N. America. English cattle come under this class of hide.
- Hides (Buffalo) - East Indies
- Skins (Sheep) - Australia. English, Scotch and Welsh sheep come under this class of skin
Tanning materials come from leaves, twigs, and wood of trees :-
- Quebraco - S. America
- Wattle Bark - S. Africa
- Sumuch - Ground leaves and twigs from Sicily and Italy
- Myrabs - Fruit from India
- Chestnutwood - France and Italy
NATURE OF THE HIDE
All animals have an external covering for their flesh and fat which forms the raw materials for the manufacture of leather. The skin is composed of an intricate fibre structure, made up of fibris composing a fibre, and fibres making up a fibre bundle and interwoven with other fibre bundles.
PROCESS
English hides are sold by auction at public hide markets and are forwarded to the Tannery from the market. These hides have to be put into process at once, otherwise they begin to putrefy. Foreign hides are preserved by either drying or salting. The process of making leather can be divided into three stages :-
- 1. Preparation of the hide for tanning, which involves soaking the cured hide in water to wash away salt and rehydrate the hide. Immersion of the hide in lime liquors to loosen the hair and plump the hide. The hair is scraped off by a blunt knife machine and the hair sold to plasterers or felt manufacturers. The surplus flesh is cut away on a sharp knife machine and the hide is "rounded" into butt (or 2 bends), 2 Bellies and 1 Shoulder. Surplus lime is washed off by leaving it in a water pit overnight - i.e. deliming.
- 2. Tanning consists of putting the hides in weak solutions of tannic acids, gradually increasing in strength and the tanning penetrates through the hide slowly. The tanning operation varies in time from 3 weeks to 3 months, dependent on the type of hide being tanned and the thickness of it.
- 3. The finishing process consists of drying the leather in such a way as the tannin will not all diffuse into the "grain" (surface where the hair has been ) or the "flesh" (surface which was next to the skin). Oils are applied to the wet leather which form a film to help to arrest this diffusion. Whilst moist, the leather is "set out" (creases removed) and "rolled on" to help make the characteristic smooth surface of leather. The leather is thoroughly dried out and "rolled off" on very heavy pressure rollers which gives the polish to the finished leather. The leather is then brushed under a brushing machine and sent to the warehouse for sorting and dispatch to the shoe factories.
- 4. Uses of leather - Boots and shoes, belting, harness, gloves, upholstery, clothing, bags, cases, straps, bookbinding, washleathers, industrial purposes for weaving looms.