Leather auxiliaries

Overview of the tanning process

The leather tanning process can be split down into five sections: processing of raw hides; cleaning of hides; tanning; dyeing, fat liqouring and drying and, finishing. You can find out more information about each of these stages below:

Processing of hides:

Hides must be temporarily preserved before being tanned in order to allow for storage and shipment. The most common method is to use salt to dehydrate the hides.

Cleaning of hides:

This contains a number of steps to remove undesirable components of the hide and allow further processing. Hides are soaked to rehydrate them and assist with the next stage of processing - tanning. Sodium sulphide and hydrated lime are used to remove hair from the hide and also to destroy keratin proteins. Weak acids are then used to delime the hides and to reduce the swelling that liming causes. Proteases are then used in the bating process to clean the grain and make the pelt smoother. These bated pelts are treated with sulphuric or formic acid to reach the optimum pH for the penetration of the tanning agents.

Tanning:

The tanning process can be split over a number of different steps and is the process that makes the hide resistant to microbial attack. Chromium, vegetable tannins and glutaraldehyde are the most common chemicals used in this process. Following treatment, the pH is raised allowing the tanning agents to bind to the collagen, a process known as basification.

Dyeing fat liqouring and drying: 

Chromium tanned leather is a bluish colour and must be dyed to achieve the desired colour. Dispersant chemistry helps deliver dye effectively and evenly to the leather. Chromium tanned leather will dry out to a hard and unusable material. Emulsified oils or fat liquors must be added to improve the softness, flexibility and feel of the leather. The pH of tanned hides is then lowered, fixing the treatments within the structure of the leather and once dried is ready for the final finishing process.

Finishing:

A wide range of complex chemical and physical processes can be employed in leather finishing. Chemical additives include but are not exclusive to resins, pigments, dyes and fillers. These can be added in order to improve the surface appearance or to achieve a specific effect. Pressing can provide smooth or patterned leathers before the leathers can then be converted into a wide variety of articles.