The usual procedure is to mix together carefully in a suitable lead-lined tank equal parts of sodium bichromate and concen trated sulphuric acid after dissolving the bichromate in a small quantity of water. The reducing agent is then very cautiously added until the colour of the solution is changed from the original orange yellow colour to a bright bottle-green shade.
The following quantities produce a liquor which is considered of a satisfactory chemical composition :-200 lbs. Bichromate; 200 lbs. Sulphuric acid (95%) ; 5o lbs. Glucose; Too gal. Water. The tannage is carried out on lines more or less identical with those of vegetable leathers; the processing being done either in the drum or paddle-wheel for the lighter classes of leather, and by suspension in vats for chrome sole leather, hydraulic and similar leathers. Starting with a weak solution, the strength is increased until complete penetration has been effected.
Chrome tanned leather possesses properties not associated with leather produced by vegetable tanning. For example, properly tanned chrome leather will stand high temperature without any detrimental effect. Chrome leather is also more resistant to atmospheric influences, and to chemical fumes and moisture.
On account of the comparative quickness of the tanning process, and the good wearing properties of the leather, this process is largely replacing the vegetable tanning process in the case of boot and shoe uppers, and for special purposes where the above men tioned properties are advantageous ; also where extreme tensile strength is demanded. The disadvantage of this leather in com parison with vegetable leather for sole leather is that unless im pregnated with waxes it tends to slip in wear and it is not prac ticable to obtain the desired thickness.
Oil Tannage.—One of the very earliest processes for converting pelt into leather was the application of oil and fatty substances. The modern method of manufacturing chamois leather (made from the flesh split of a sheepskin), deer, antelope, and similar skins, consists in the impregnation with an oxidisable fish oil of the goods when in a weakly alkaline condition, and subsequently processing to effect further oxidation of the oil on the fibres.
The goods as they leave the splitting machine are re-limed to effect a further swelling of the fibres and decomposition of the fat cells with a view to subsequent removal of the natural grease. The goods having been re-limed sufficiently are then frized on the split side. This operation is somewhat similar to that of hand fleshing but consists of the removal of the coarse fibred surface or "ground" from the split side nearest to the grain, by cutting this away with a sharp two handled knife on a frizing beam.
The goods are now delimed either by employing a fermented in fusion of bran or other flour offal, or by the use of a suitable acid. The object to be attained by oil tanning is the substitution for the moisture in the fibres of a suitable fish oil. The goods are first drained and then pressed in some hydraulic or other press to express from them as much moisture as possible. They are then hung up to become partially dry. This requires very careful super vision to prevent the goods becoming too dry, and the fibres in consequence becoming repellant to the subsequent absorption of the oil and becoming stuck to each other. After as much moisture has been eliminated as practicable without causing the fibres to adhere to one another, the goods are then placed in a machine called the "stocks." This consists of two heavy hammers work ing in a semi-cylindrical vessel in which the goods are placed and subjected to a hammering or beating action; at the same time the goods are continuously turned over by reason of the shape of the vessel. They are sprinkled with a fish oil, usually cod liver oil, during the process of stocking or kneading and are processed until they become sufficiently saturated. They are then further dried and re-sprinkled with oil and re-stocked.
These operations are repeated until the goods are fully saturated and practically all moisture has been replaced by fish oil. The oil is then decomposed on the fibres by the application of heat.
Drying.—The old method is to hang the goods up in a loft which is ventilated as required by louvres, the goods usually hung up from tenterhooks in rails placed parallel to the current of air.