Tannages

Vegetable Tannage

The conversion of raw hide into leather by use of vegetable tannins. This process produces leather with greater body and firmness than the more general method of chromium tanning. Vegetable tannage is used mainly for shoe soles and leathergoods

• Pyrogallol Tans usually impart a yellow-brown colour to leather.
• Chestnut extract is extensively used in Europe. It produces a leather with a pleasing medium brown color. Yields a very "full" leather,  which is firm in character with a nice tight grain.
• Sumac extract is produced from the leaves of the sumac shrub. Leather produced with sumac has a greenish-yellow color with good dyeing properties. The temper is soft and mellow. Of all of the vegetable tannins, this agent produces a leather which has the greatest resistance to aging due to iron fittings, fumes from open fires, industrial gases such as sulfur dioxide, etc. Bookbinding leathers are traditionally tanned with sumac because of its superior permanence.
• Oak Bark produces a grayish-brown leather. Leather produced with this tannin is firm, yet flexible.
• Catechol Tans usually impart a reddish brown color. These are astringent tanning agents which may cause problems such as surface shrinkage which gives a "pebbled" look to the grain of the leather.
• Mimosa (also referred to as Wattle) comes from the bark of South African mimosa trees. It produces a leather with a red-brown color which darkens on exposure to light. Commonly used in the U.S. combined with Quebracho in the production of vegetable tanned leathers.
• Quebracho is extracted from a South American tree. This substance imparts a red-brown color to the leather. The leather can be somewhat spongy, but is very "full". It darkens on exposure to light.
• Pine Bark is primarily used in Central Europe producing a red-brown color to the leather.
• Hemlock traditionally was used in North America for the production of sole leather. The extract comes from fir trees and produces a red-brown colored leather.

Mineral Tannage

The conversion of raw hide into leather by use of mineral substances, notably the salts of chromium, aluminum, and zirconium.

• Chrome (Chromium Sulfate) tanned leather is the most common tannage in the industrialized nations. Leather which has been tanned in this manner and has not been further processed is said to be "in the blue" (the color of the wet leather).
• Chrome Alum (Potassium Chrome Alum) produces a nearly white leather for special purposes such as furs, white "NuBuk", etc.
• Alum tanned leather is produced as the result of the action of a combination of aluminum and potassium sulfate on the hide or skin. Leather tanned with alum is white and produces a very strong leather. Alum has the disadvantage of being able to be washed out of the leather yielding an "untanned" skin unless the leather has been tanned with additional materials.
• Zirconium can be used as a tanning agent in the form of zirconium salts. This normally produces a firm, tight-grained leather which is white. Unlike alum tanned leather, zirconium is not washed out with water leaving "untanned" leather.

Combination Tannage

Leathers tanned with more than one tanning agent. For example, initially chrome tanned followed by a second tannage (called a Retan) with vegetable materials.

• Semi-Chrome tanned leather is leather which is chrome tanned after having first being vegetable tanned or syntanned. The chrome re-tannage makes the leather softer and more flexible. In the undeveloped nations leather is often vegetable tanned and exported as vegetable crust for chrome re-tannage.
• Semi-Alum tanned leather is vegetable tanned leather re-tanned with alum. This tannage was more popular many years ago. It has similar properties to semi-chrome leather.
• Chrome Re-Tan  is leather which is first chrome tanned and then tanned with vegetable extracts. The re-tannage produces a firmer and "rounder" piece of leather. Additionally, it tightens the grain and increases its "fullness". This is a common  tannage for work boot leathers and latigo.

Aldehyde Tannage

A tanning method using a formalin solution in the manufacture of white leathers and also washable glove leathers.

• Formaldehyde is dissolved in water to create formalin and used as the tanning solution primarily to produce leather which is white in color and washable; commonly used for tanning deerskins with the grain removed. Yellowing occurs upon aging. Grain leathers tanned with formaldehyde can be subject to "cracky" grain.
• Glutaraldehyde is superior to formaldehyde as a tanning agent with regards to yellowing with age, propensity for grain to crack and degree of  tannage. Leather tanned with glutaraldehyde can be re-tanned with vegetable extracts to produce a leather which is not as pale in color as leather pre-tanned with a syntan.
• Chrome tanned leather which has been pre-tanned with glutaraldehyde has superior resistance to perspiration.

Oil Tannage

Leathers tanned with certain fish oils. Produces a very soft, pliable, water absorbent leather such as chamois.

• Cod Liver Oil in the unoxidized form (raw cod liver oil, not exposed to air) is used primarily for producing chamois-type leather. In the process of oxidation tannage of the raw lamb or hair sheep splits takes place. Chamois skins have excellent water absorbency. They are used for cleaning, for garments, gloves and for filtering water from gasoline.

Syntan

A term covering a group of synthetic tanning materials, generally used in combination with vegetable, mineral or formaldehyde tannages. These materials are also often used for specialized purposes such as in bleaching, filling, etc. Generally, they are used the same way as vegetable tannins and produce leather with similar properties. The syntans  are faster tanning agents.

Liritan

A method where the leather is pre-tanned with Calgon (sodium hexametaphosphate) prior to tannage with vegetable tannins. Skirting was traditionally produced by  moving the hides from pits with weak tanning solutions to pits with successively stronger solutions to give a nice even  tannage.  The traditional pit method of vegetable tanning is slow. The Liritan Process allows the tanner to use a strong vegetable tanning solution  immediately after pretanning. Thus, the hides are tanned quicker and less capital is tied up in the process.

Source:Siegel of California
Email: siegel@siegelofca.com