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What is Tanning?
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A process that uses tanning agents to convert a raw hide into a stable, nonperishable
material. This is achieved by cleaning, drying, and preserving the hide. (See below
for a more detailed look at the tanning process.)
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Curing
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The leather hide is cured by immersion in a salt solution to protect it from deterioration
and to preserve it for future use.
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Soaking
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The leather hide is soaked in water to remove the salt solution and re-hydrate the
hide to its original flaccid condition.
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De-hairing
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The soaked hides are treated with a lime solution for the de-hairing process. The
lime solution not only removes the hair, but also fats and soluble proteins.
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Tanning
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Tanning involves drumming the hides in a mixture of chemicals. Two of the most common
types of tanning include chromium tanning, which results in a wet blue appearance,
and vegetable tanning.
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Splitting
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The average thickness of a cowhide is 5mm before splitting. The hides are split
in a splitting machine which activates a fast running wirecutter. This operation
divides the hide into two pieces: top grain leather and split leather. The top grain
hides are then shaved on the flesh side to create a uniform thickness. Standard
top grain upholstery leather thickness can be 0.9mm-1.4mm. Split leathers are typically
used on non-stress areas, such as outside backs and outside arms.
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Sorting
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The hides are then sorted or separated by differing levels of quality. This quality
is based on the extent of natural markings which could include tick bites, brands,
barbed wire scars, and/or stretch marks. This process is called selection. The highest
quality of leather hides are normally termed “A” selection. “B” and “C” hides possess
a higher number of natural markings, therefore reducing overall selection.
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Dyeing
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The leather hide is drum dyed. The dye concentration in the drum affects level of
penetration into the hide, richness of shade, colorfastness to light, ease of dry
cleaning, and resistance to rubbing. The dyed leather is then treated with fat-liquors
for softness and strength.
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Drying
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After the dyeing process, the leather hide can retain 45-60% water. Most upholstery
leather is tunnel dried where the temperature and humidity are controlled. After
the leather is dried, it dries to a paler shade as the oils spread uniformly and
the dyes penetrate deeply.
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Finishing
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The finishing of a leather hide involves both mechanical and chemical treatments.
The hide is first trimmed and conditioned with fat-liquors to obtain uniform moisture
content. The hide is then softened with a massage machine and stretched. Some leathers
are further processed by applying a top coat finish to the grain surface. The top
coat adheres to the leather’s surface while increasing the resistance to abrasion,
cracking, peeling, rubbing, light, and heat. A pigmented finish is sometimes applied
which contains a variety of colorations, resins, lacquers, oils and/or waxes. Afterwards,
the hide is milled in a dry drum to soften the leather. Note: leather can be finished
a variety of ways depending on quality of hide, type of leather, and level of pigmentation.
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