aim of this type of finish is to produce leather, which retains its "natural"
look but at the same time achieves a degree of uniformity in color with
the minimum opacity necessary. This is achieved by using a mixture of
aniline dyestuffs and pigmented dyestuffs bound together using casein.
Pigments, unlike dyestuffs are insoluble in water or solvents and are
opaque. Casein, a protein that comes from milk exists as a water-soluble
colloid. It is not unlike albumen (known in the bookbinding world as
'glaire'), which is the protein present in egg white.
Binding takes place by the deposition of the binder around the pigment
due to the loss of water by evaporation and also hydration of the
leather fibers. Pigmented dyestuffs would not themselves adhere to
leather or form a "film" so, as in the case of ordinary
paints; other materials such as casein must be mixed into the finish.
For this reason caseins are known as "Binders".
The leather is first sprayed with a lighter shade than required of this
mixture, then a transparent 'top-off' coat with a slightly darker shade
is applied. The finish is then fixed using Cationic Casein. Cationic
Casein is positively charged whereas most caseins are Anionic and are
negatively charged. The reaction of the negative and positive charges
meeting binds the two together forming an impervious layer.
The finished leather is then polished. The combination of the darker
mixture and the burnishing effect on the tips of the grain enhances the
final color and gives a very attractive two-tone effect. This type of
finish has all the workable properties of aniline finished leathers.