Making Leather – Tanning Process
Making leather is a complicated process. The easiest way to understand the process of leather preparation is to understand the layer separation in which the leather is split across it’s thickness to produce a top layer (hair side) and the under layer. The top layer is called the grain and is the side exposed to the elements giving it the durability and malleability it needs to be considered fine leather. The underside is much stiffer and less durable than the top layer. This is practical for use after a coating or treatment is applied.
The process of tanning involves five distinct stages. Tanning, Selecting, Splitting/Shaving, Re-tanning and Finishing. Each one of these processes is complicated and requires many steps. The following is an outline of some of the more important points of each step.
Step 1: Tanning
Soaking – When the leather arrives at the tannery it may be soaked to extrude salts used in preserving leather. This is done in revolving drums which can hold up to 200 hides.
Liming – Hair and epidermis are removed and a solution of lime (calcium hydroxide) and sodium sulphide are applied to soften and enhance hide for softness and flexibility needed for upholstery leather.
This is the process which converts pre-tanned hide into leather.
Mineral tanning is normally done with alkaline chrome-3 salts. It penetrates the hide fairly quickly (24-48 hours). This results in a pale duck-egg blue, which after processing yields a fine, soft, modern finish. When there is absence of chromium tanning, other methods combine vegetable with polymers and syntans (synthetic tannins) as an alternative.
Other tanning methods are:
- Pure Vegetable Tanning
- Synthetic Tanning
- Oil Tanning
- Combination Tanning
Step 2: Selecting
After tanning excess water is removed from hide. Hides are then graded according to the quantity and locations of natural features and faults. Aniline and Nubuck leathers demand the best quality hides. Leather where hides that are heavily coated or embossed can utilize a lesser quality of hides.
Step 3: Splitting/Shaving
Splitting – The hide is split into layers. The top or grain layer will produce a fine, smooth grain leather. The bottom is used for suede or split leather for other uses.
Shaving – Given a uniform thickness.
Step 4: Re-Tanning
Dressing the hides involves the following:
- Dyeing – Dyes are added to color leather.
- Re-tanning – Additional tanning substances are sometimes added to modify the physical characteristics of the leather to suit its final use.
- Setting – A process which mechanically removes creases and excess water.
- Drying – The hides are stretched dried on large frames or vacuum dried.
- Trimming- The rough and ragged edges are removed.
Step 5: Finishing
The purpose of finishing are:
- To minimize the appearance grain blemishes without losing the natural beauty of the leather product.
- To give the required degree of gloss.
- To ensure the leather is soft, malleable and moldable.
- To give a more protective surface.
- To provide a surface which is easily cleaned.
- To give special effect like antique or special grain effects.
The finishing process uses a combination of surface coatings techniques such as padding, spraying or roller coating. Then there are mechanical processes such as buffing, staking and embossing.