The tanning process converts the protein of the raw hide or skin into a stable material, which will not putrefy and is suitable for a wide variety of purposes. There is a vast array of tanning methods and materials.

Vegetable Tans: extracted from plant leaves, barks,fruits,roots of plantsor trees etc. , consist of large polyphenol molecules with some acidic groups and numerous secondary functions(dipole or hydrogen bond). The acidic groups may combine with the basic groups of the protein displacing the water of hydration. Vegatable tannage could be considered as replacing water molecules by vegetable tan molecules. Generally acid conditions (low pH) favor vegetable tan fixation in increasing the ionization of the protein basic groups.Relatively dense,firm or solid leather is produced.The color is pale brown and generally darkens in daylight. Removal of tan by water depends on finishing treatment, it washes out very slowly.Characteeristic uses are sole leathers, upholstery and bags, shoe lining leathers, book binding leathers, belts, straps.

Synthetic Tans: may be of various chemical structure, usually synthetic tanning materials. It is common for them to be made water soluble by the sulphonic acid group. This group is highly ionized and has strong attraction for the protein basic group with a consequent dehydrating effect.Usually low pH gives faster ander greater tan fixation.
Syntans with high secondary functions will have more pronounced effect and give fuller leather (replacement syntans), those with greater proportion of sulphonic groups give a thinner less flexible leather (auxiliary syntans). The color is paler than vegetable tans, may darken in daylight.Does not washout with water.Uses are white leathers and speciality leathers.

Minneral Tannages: the basic salts of chromium, zirconium and aluminum behave in a rather different way. Their initial fixation is on the acid groups of the protein where they displace some of the bound water, but they may form cross links between adjacent acid groups, which will stabilize the wet hydrated skin structure.
The dehydration effect of these tannages and the quantity fixed is less than with vegetable tannages and therefore the shrinkage and hardening on drying is more pronounced. Invariably some type of oil is applied to the wet fibres before drying. Its effect on softening the dried leather should be more pronounced on chrome leather than on vegetable tanned leather.
Very soft leather is produced.The color is white for aluminum and zirconium but pale green or blue for chromium tannage.Little change in daylight.Chromium tan does not wash out but aluminum can wash out.
Characteristic uses for chrome leathers areare shoe upper leathers, gloving and clothing leathers, some sole and belting leathers.Uses for alum tans are gloves, fur-skins.

Aldehyde Tannages: formaldehyde, gluteraldehyde or the aldehydes produced in chamois tannage, combine with basic groups of the protein and form cross links with basic groups on adjacent molecules in the wet protein. Quite small amounts of aldehyde are sufficient to produce a significant effect.Low pH reduces fixation.Soft leather is produced.Color after tannage is white, goes whiter in sunlight. It does not wash out.It is water absorbent. Uses are water washable gloves and clothing.

Oil Tannage:A very old way of imparting properties of finished leather to skins. Oil tanned leathers are light, soft, air-permeable, and resiatant to washing.Usually cod liver oil used.It is not sensitive to pH and has dull yellow color that bleaches in sunlight.Oil tan does not wash out and is very water absorbent.Uses are wash leathers and washable gloves.

Dehydration methods: a method of rendering the skin soft when dry; not real tanning, effect is lost if the skin is re-wetted.
Treatment of skin with high salt concentrations will dehydrate the skin protein.
Pickled and well dry salted skins dry out white, flexible,and have apparence of tannage. Solvent dehydration, ie.washing wet skin with acetone, will produce the same white leather. A white flexible dry product is produced by “freeze drying” whereby the wet skin is frozen and submitted to vacuum, the water volatilizes off in gas form without going through liquid phase.

General conditions that give rapid tan fixation(astringency) give a poor rate of tan penetration and vice versa. Rapid fixation of vegetable tan is favored by acid conditions (low pH), low non-tan content, whilst in the minneral tannages rapid fixation is caused by higher pHs (5-7) and lower pHs will give penetration.
It is common to give “combination tannages” using two or more types . Vegetable tans are often added to chrome leathers to improve fullness or firmness in flanks or grain, whilst the bulk of syntans are used in conjuction with other tannages to give a whiter leather or to speed up the tannage.

Note: “semi-chrome” refers to complete vegetable tannage followed by retannage with chrome; “chrome-retan” is a full chrome tannage followed by a vegetable or synthetic tannage.