August 10, 2009
The process of manufacturing leather is divided into three fundamental sub-processes: preparatory stages, tanning stages and crusting stage. All true leathers will go through these sub-processes. A further sub-process called surface coating can be added into the leather process sequence but not all leathers will receive a treatment. Seeing how so many types of leather exist, it has become difficult to compose a list of operations that all leathers must go through.
The preparatory stages are when the hide or skin is prepared for tanning. These preparatory stages could include: soaking , preservation, bating , liming, unhairing, reliming, deliming, fleshing, splitting, degreasing, bleaching, frizzing, pickling and depickling.
Tanning is the process in which the conversion of the protein of the raw hide or skin into a stable material which will not putrefy and is then suitable for a wide scope of final applications. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard flexible material so that when re- wetted (or wetted back) putrefy, while tanned material dries out to be a flexible form that does not become putrid when wetted back. There are a large number of different materials and methods for tanning that can be used, the choice is ultimately dependent on the final application of the leather. The most commonly know agent used for tanning materials is chromium, which leaves the leather once tanned a pale blue color due to the (chromium)= a very hard, metallic chemical element with a high resistance to corrosion. This product is commonly known as “wet blue”.
The hides once they have finished pickling will typically be between pH of 2.8-3.2. At this stage the hides would be loaded into a drum and immersed in a float containing the tanning liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the entire substance of the hide. Frequent checks are made to see the penetration by cutting the cross section of a hide and observing the degree of penetration. Once a good even degree of penetration has been obtained, the pH of the float is slowly raised in a process called basification. This basification process adheres the tanning to the leather and the more tanning material prepared the higher the hydrothermal stability and increased shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned would normally finish somewhere between 3.8-4.2.
Crusting is when the hide or skin is thinned, retanned and lubricated . Often, a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place.
The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations: wetting back, splitting, shaving, sammying, neutralization, rechroming, retanning, filling, stripping, stuffing, dyeing, fatliquoring, whitening, fixating, drying, setting, conditioning, milling, buffing and staking.
For certain leathers a surface coating is applied. Tanners refer to this as finishing. Finishing operations could include: polishing, oiling, brushing, padding, buffing, spraying, roller coating, curtain coating, embossing, ironing, ironing or combing (for hair-on), impregnation, tumbling and glazing.
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