Leather, at the time of completion of the tannage does not contain sufficiant lubricants to prevent it from drying into a hard mass.Almost all light leathers need a greater softness and flexibility than is imparted by tannage. This is attained in the fatliquoring process by introducing oil into the leather, so that the individual fibres are uniformly coated. The percentage of oil on the weight of leather is quite small, from 3-10 %. The precise manner in which this small quantity of oil is distributed throughout the leather materially affects the subsequent finishing operations and the character of the leather. Proper lubrication or fatliquoring greatly affects the physical properties of break,stretch, stitch tear, tensile strength, and comfort of leather. Over lubrication will result in excessive softness and raggy leather in the bellies and flanks. Under lubrication, or improper penetration, results in hard bony leather that may crack in use.
To allow a small amount of oil to be spread uniformly over a very large surface of the leather fibres it is necessary to dilute the oil. Although this could be done with a true solvent such as benzene, it is cheaper, safer and more convenient to use the method of emulsification. In an emulsion with water, the oil is dispersed in microscopically small droplets, giving it a white, milky appearance. It is important that the oil drops in water should remain as an emulsion until they penetrate the leather, and should not separate out as large drops or as a layer of oil, which could not penetrate the leather fibre and would only give a greasy surface layer.
The properties of the finished leather can be varied by controlling the degree to which the emulsion penetrates the leather before it “breaks” depositing the oil on the fibres. By such a technique, in the case of chrome-tanned leather, it is possible to concentrate the bulk of the fatliquor in the surface layers, leaving the middle containing relatively little oil. This yields a leather which is soft but resilient, with a tight break. In contrast if the fatliquor is allowed to penetrate uniformly, the leather will be soft and stretchy, with any natural grain looseness accentuated.
The commonest material used as a surfactant is soap. However in the presence of hard water, calcium or mineral salts or acid, the hydrophilic nature of he soap is reduced and it loses its surfactant powers.Most leather is acid; sulphated or sulphited, alcohols or oils, have much better resistance to these conditions and thus much better wetting action, and emulsions formed with their aid are much more stable to these conditions.
They are all classed as colloids, anionic or cationic surfactans, depending on the charge of the ionic group carried.Anionic surfactants are more effective at high pHs and on anionic materials, eg.vegetable tanned leather.Cationic surfactants are more effective at lower pHs and on cationic materials,eg. Chrome leather. Non Ionic surfactants in which the hydrophilic group does not ionize(consists of several hydroxyl groups) are used as auxiliaries in parafin degreasing, as wetting agents, and to stabilize fatliquors to obtain emulsion penetration into the leather.
Location of the oil:  If we consider a cross section of the hide upon bending, we see that on the outside of the bend the fibers must stretch, and on the inside of the bend must compress.In the center of the skin there is very little motion of the fibers over one another during bending. Therefore both the grain and the flesh surfaces must be lubricated(to prevent break or grain wrinkle), but less lubrication is necessary in the center.