Treating vegetable leather after tanning

Bleaching, filling, fixation of vegetable tanned leathers

Bleaching

It is applied for leveling, lightning or changing the tan color, mostly after final tannage.

Mainly bleaching syntans, highly sulphited mimosa or Quebracho extracts, specially formulated synthetic bleaching tanning agents and less frequently bleaching oils are used for this purpose. Acid bleaching, which was often used in the past, can no longer be recommended because defects to the fiber were caused by the presence of strong free acid.

Filling

It is used for heavy leathers to improve firmness and toughness of the sections of skin which are of poor substance and also improve the yield of sole leathers.

Untreated liquid or powdery tanning extracts are used sulphited tanning agents should not be they incase the water absorption of the leathers. However, excessive filling is not advised due to the high content of washing-out substances. Deposits of water-soluble loading agents such as barium sulphite or magnesium sulphate should also be avoided. When the shoes come into contact with water they produce efflorescence of salt due to migration on the upper leather.
Fixation
The unbound tanning agent in the interfibrillar spaces of the leather, which is removable by washing, is precipitated and thus converted into insoluble form which cannot be removed by washing.
Skin glue, casein, magnesium sulphate, aluminum sulphate or urea formaldehyde condensation products and hexamethylenetetramine are used, whereby the latter two are preferable because they achieve better bonding. They should always be dosed and used carefully; otherwise fixation is only superficial and may result in a greasy grain surface and flesh side. Crackiness of grain may also occur.