Leather

Leather

Skins are tanned to make leather in order to:

• get rid of smaller molecules which will degrade readily, within the skin;

• stop biological degradation—that is, rotting; and

• produce a product that is, flexible, strong and resistant to deterioration.

Skins which are to be tanned go through a number of pre-tanning processes. The skins are:

• cured—dried quickly to achieve a temporary preservation so that they can be transported without rotting;

• soaped—this returns the moisture to the dried skins and removes water-soluble materials;

• unhaired—this loosens hair and fats so that they can be scraped and pulled away. This process produces a plump hide;

• cleaned and the flesh side levelled. During this stage, hair, dirt, grease and remnants of chemicals from previous processes are removed;

• delimed—the unhairing process uses lime which makes the skins very alkaline. This stage of the processing reduces the alkalinity, in preparation for the next treatment stages;

• bated—this process makes the skins soft and flexible. It also cleans the fibre network of the skins and removes some of the smaller and weaker fibres. Traditionally, dung was used in this process;

• drenched—this is similar to bating, but uses weak organic acids. Traditionally, fermented grains were used; and

• pickled—this is the final stage that conditions the skins for tanning.

The next stage in making leather is tanning. The tanning process draws collagen fibres together and creates crosslinks between them. This crosslinking on a molecular and fibre level makes the skin much more resistant to deterioration. Generally, leathers are either vegetable-tanned or mineral-tanned.

Vegetable tanning uses tannins which are present in the barks, woods, leaves and fruits of certain plants. The colour of the leather prior to finishing ranges from pale-brown to a reddish brown, depending on the particular tanning agents used. Vegetable-tanned leathers are particularly suitable for bookbinding.

Mineral processing uses mineral salts to chemically stabilise the skin. In the 1880s, chrome salts were used to make leathers which were hard-wearing, stable and water-resistant. The resilience and open texture of chrome-tanned leathers meant that they could not be embossed. These leathers were unsuitable for some kinds of work, particularly bookbinding.

During investigations into the improved durability of leathers, a number of experiments have been carried out using combined tanning techniques.

Another product which is thought of as a leather is alum-tawed leather. These white leathers were produced using a solution of alum and salt. Leather produced by this process is not a true leather, because it does not have the same chemical stability and resistance to water that fully tanned leather has. Zirconium salts are used to produce a white leather that is washable.