The leather manufacturing process is divided into three sub-processes: preparatory stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating may be added into the sequence. The list of operations that leathers undergo vary with the type of leather.
The leather making process is in general restricted to batch processing, but if the surface coating sub-process is added then some continuous processing can be included. The operation flow has to follow the preparatory → tanning → crusting → surface coating sub-process order without deviation, but some of the sub-processes can be omitted to make certain leathers (or partially tanned/ untanned products).
The preparatory stages are when the hide/skin is prepared for tanning.During the preparatory stages many of the unwanted raw skin components are removed. Many options for pretreatment of the skin exist. Not all of the options may be performed. Preparatory stages may include:
· preservation- the hide/skin is treated with a method which renders it temporarily unputrescible.
· soaking - water for purposes of washing or rehydration is reintroduced.
· liming - unwanted proteins and "opening up" is achieved.
· unhairing - the majority of hair is removed.
· fleshing - subcutaneous material is removed.
· splitting - the hide/skin is cut into two or more horizontal layers.
· reliming - the hide/skin is further treated to achieve more "opening up" or more protein removal.
· deliming - liming and unhairing chemicals are removed from the pelt.
· bating - proteolytic proteins are introduced to the skin to remove further proteins and to assist with softening of the pelt.
· degreasing - natural fats/oils are stripped or as much as is possible from the hide/skin.
· bleaching - chemical modification of dark pigments to yield a lighter coloured pelt.
· pickling - lowering of the pH value to the acidic region. Must be done in the presence of salts. Pickling is normally done to help with the penetration of certain tanning agents, e.g., chromium (and other metals), aldehydic and some polymeric tanning agents
· depickling - raising of the pH out of the acidic region to assist with penetration of certain tanning agents
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