North American Hides And Skins
We Supply Raw Materials To The Leather Industry
The Process

Hides and Skins


    Excess flesh and fatty substances are removed by a mechanical process.


    The raw materials used by the leather industry originate largely as by-products of the meat-
    packing industry. Before entering the tanning process, the raw skins are "cured" by salting or
    drying them promptly after being removed from the slaughtered animal. The more common
    methods used in curing require the use of salt (sodium chloride) in one of two ways: wet-
    salting or brine-curing. In wet-salting, the skins are liberally salted and piled on top of one
    another until they form a pack. They are left in the pack for about 30 days to allow the salt to
    thoroughly penetrate the skin. Brine-curing is a much quicker method. In agitated brine-curing,
    the method most commonly used, skins are placed in large vats called raceways that contain a
    disinfectant and brine maintained close to full salt saturation. After about 16 hours in the
    raceway, the skins are completely penetrated by the salt.


    The cured skins are soaked in pure water to eliminate salt, blood, and dirt, and also to replace
    moisture lost in the curing process. After the skins have soaked for a period varying from two
    hours to seven days, the flesh is removed mechanically from the inner surface. To loosen the
    hair, the skins are then immersed for one to nine days in a solution of lime and water
    containing a small amount of sodium sulfide. Following this operation the hair is easily
    removed by a dehairing machine, and the distinctive pattern known as the grain can be
    distinguished on the outer surface of the skin. To ensure clear, clean surfaces, any remaining
    flesh and hair is scraped off, usually by hand with a dull knife, by a process called scudding.


    The next operation involves deliming the skins by soaking them in a weak solution of acid,
    which reduces the swelling caused by the lime. Simultaneously, most types of skins are treated
    with a "bating" material consisting of enzymes to give a smoother grain and render the skin soft
    and flexible. The amount of bating varies greatly, from none at all for sole leather to a
    concentrated treatment for leather to be used in kidskin gloves. After the deliming and bating
    operations, the stock can be tanned.
    Each type of skin may be treated by several tanning processes. The process is chosen
    according to the use for which the leather is intended. The two principal tanning processes are
    mineral, or chrome, tanning, and vegetable tanning. Chrome tanning often can be completed in
    a single day, whereas vegetable tanning requires many weeks or months. Vegetable tanning
    results in a firmer leather with greater water and stretch resistance. Chrome tanning shrinks
    the stock and produces a longer-wearing leather with greater resistance to heat. The
    processes are sometimes combined to derive some of the advantages of each.


    In this process the tanning agent, which renders the skin immune to decay and prevents
    shrinkage, is a substance known as tannin. Tannin is extracted from the bark, wood, fruit, and
    leaves of trees. Chestnut wood, oak bark, and hemlock bark are the major domestic sources of
    the tannin used by the United States leather industry. Foreign sources, which provide more than
    80 percent of the tannin supply, include the wood of the quebracho tree of South America,
    mangrove bark from the island of Borneo, wattle bark from South Africa, and myrobalan fruit
    from India.
    In vegetable tanning the hides are suspended from rocking frames in a series of vats
    containing increasingly stronger tannin solutions, called liquors. After several weeks the hides
    are transferred to a "layaway" section, which consists of larger vats containing still stronger
    liquors. Each week more tannin is added to the liquor, until the hides have absorbed enough
    tannin to complete the process. The last stages of the process may be accelerated by the use
    of warm liquors. Flexible vegetable-tanned leathers to be used for belting, luggage, upholstery,
    or harnesses are less heavily tanned than the leather intended for shoe soles.


    The mineral tanning process is known as chrome tanning because the tanning agent used
    most frequently is a salt compound of chromium. Chrome-tanned leathers, which stretch more
    than vegetable-tanned leathers, are suitable for handbags, shoe uppers, gloves, and garments.
    To prepare the stock for chrome tanning, the bated skins are pickled in a solution of salt and
    acid. The skins are then immersed in a basic chromium-sulfate solution within a large
    revolving drum that tumbles the skins. This type of liquor penetrates the skins so rapidly that
    tannage is accomplished in less than a day. The chrome process originally involved the use of
    two different liquors, both solutions of compounds of chromium, and required substantially
    more time. Known as the two-bath process, it is still used for some varieties of leather.
    Aluminum or zirconium compounds may be used in place of chromium in the production of
    white leather. Alum, formaldehyde, gluteraldehyde, and synthetic tannins (Syntans) are also
    used to impart special characteristics.
    In the production of combination-tanned leather, the skin is first chrome-tanned and then
    retanned with vegetable tannins. The modified applications of both processes produce leather
    with some of the advantages of each type.


    After tanning, all types of leather undergo various operations that differ according to the use of
    the desired product. Vegetable-tanned leather for shoe soles is first bleached a lighter color.
    Next, it is infused with such materials as epsom salts, oils, and glucose, and then lubricated
    with hot emulsions of soap, greases, and sometimes wax. Finally, the stock is run through
    rolling machines to give the leather a desired degree of firmness and a high gloss. Chrome-
    tanned leather intended for shoe uppers is split and shaved to the desired uniform thickness. It
    is then placed in a rotating drum for the dyeing process, which usually involves the use of
    several types of coloring materials to achieve color fastness and durability. Before or after
    dyeing, the leather is rolled in a "fat liquor," which contains emulsified oils and greases. More
    than 100 leather colors exist, ranging from traditional tans and browns to such exotic shades
    as fuchsia and turquoise.
    After dyeing and fat-liquoring, the stock is stretched for drying. Workers paste the stock on
    frames made of glass or ceramics or "toggle" it on perforated metal frames. The frames are
    then conveyed through drying tunnels with controlled heat and humidity.


    Heavy leathers are finished by coating the grain surface with a finishing compound, and finally
    by brushing it under a revolving, brush-covered cylinder. The grain surface of light leathers is
    buffed, or sandpapered, to correct imperfections in the skin. Buffing the flesh side of leather
    raises the nap and produces the popular leather known as suede. For smooth finishes, most
    light leather is seasoned, or treated with a mixture of such materials as waxes, shellac or
    emulsified synthetic resins, dyes, and pigments. Pigments are used sparingly to avoid a
    painted look. Glazing gives the grain a highly polished surface. Several coats of thick, oily
    varnish are required to give patent leather its characteristic high gloss.


    Today, many artificial substances are produced and sold as "leather goods." These modern
    synthetics include such plastics as polyvinyl chloride and nonwoven fibers impregnated with
    binders. These materials lack leather's porous quality, pliable nature, and resilience. However,
    the artificial materials cost less to produce than leather and have come to command a large
    share of the leather market, particularly in shoe soles.

    Contributed by: Tanners Council of America, Inc.
Northwoods International