The purpose of retanning is to produce a further stabilization of the collagen network. This involves further processing of the stabilised collagen network and may comprise a further tannage (e.g. with combinations of chrome, vegetable, glutaraldehyde or syntan agent) when special characteristics such as perspiration resistance are required. Conditioning, softening, dyeing or bleaching may also be carried out.

3.1.- Leather Dying

Almost all leather is dyed. With few exceptions, such as vegetable tanned leathers with the natural look, leather is artificially colored and this visual aspect is an essential part of its aesthetic properties.

Dying of leather is the application of soluble organic dyestuffs in aqueous floats to wet leather. This leads to fixation of the dye molecules not only on the surface of the tanned fiber network but inside as well. The type of coloration of leather is completely different from the finish operations performed on crust leather manufacture, where insoluble dyestuffs and/or pigments are applied together with polymeric binder substances on the surface of the dry leather.

The application of water soluble dyestuffs to leather results in a colored leather which can be used as it is to produce an "aniline leather" without pigmented finishing, providing the grain quality is appropriate.

Dyestuffs are generally synthesized organic chemical molecules of an aromatic or sometimes heterocyclic nature. The main types of dyestuffs are:

• Azo-dyestuffs. About 50 % of all dyestuffs used in the leather industry are azo-dyes.
  
• Metal-complex dyestuffs. Organic molecule complexed with metals in 1:1 or 2:1 proportions.
  
• Nitro- and nitroso- dyestuffs
  
• Carbonyl-dyestuffs
  
• Polymethane dyestuffs
  
• Oxidation-dyestuffs (obtained by oxidation of aromatic bases)
  
• Sulfur-dyestuffs
  
• Phtalocyanine dyestuffs

3.2.- Fatliquoring

Fat is next to tanstuff the most important component of leather. With the exception of sole leather any kind of softer leather contains remarkable amounts (5 - 20 %) of fat. Fat is the basis of the flexibility and it is said that the flexibility is due to the separation of the leather fiber of each other.

The fiber elements dehydrated by tanning are coated with a fat layer to give leather the desirable softness and handle by a sort of lubrication. At the same time, fatliquoring influences the physical properties of the leather, such as extensibility, tensile strength, wetting properties, waterproofness and permeability to air and water vapor.
Fat prevents the sticking, glueing or adhesion of fibers, which occurs though a certain fiber separation is effected by tanning.

Fatliquoring is a very complicated process depending on many factors. One further factor is the temperature of the liquor bath. With increasing temperatures the small particles are more stable against magnification. On the other side the fixation of fat by binding the charged groups is more prominent. High temperature is 60 °C, lowest possible temperature is about 35 °C. As lower the temperature as lower the uptake of fat but more even the fat distribution.

A further component of the fatliquoring system is the pH of the leather and the liquor float. Chrome leather as well as retanned chrome leather needs to have a pH high enough the charged groups to stay in the salt condition of all the components. Sulfates, sulfonates, phosphates can act as surface active agents only in the neutralized state. Leather to be retanned, dyed and fatliquored need to be neutralized in advance to an pH of about 5,5. After fatliquoring the float must be acidified to pH of about 4,5 or a bit lower to fix everything at the fiber.