CLEANING AND TANNING HIDES AND SKINS


DEFINITIONS:

LEATHER - any animal hide or skin that has been processed for use by
man.  Large animals are said to have HIDES, while small animals have
skins.  In either case, the hide or skin is composed of water and
proteins and unless preserved, decays quickly.

PELT -- a hide with its hair and flesh removed.

TANNIN or TANNIC ACID - an acid occurring naturally in the roots, wood,
bark, leaves and fruit of many plants, particularly in the bark of oak
species and in sumac and myrobalan; they also occur in galls,
pathological growths resulting from insect attacks.

CLEANING - removal of all undesirable matter like: hair, flesh, fat and
some interfibrillary matter - leaves a concentrated network of high
protein collagen fibers, greatly softened and interspaced with water.

THE SCIENCE OF TANNING - is the study of skin proteins and their
interactions with Acids, Bases, Salts, and Organic and Inorganic Tanning
Agents.


TECHNICAL INFORMATION:

1. Leather quality depends on how or why the animal was reared, whether
   it died of old age or was slaughtered, its age, sex and environment.

2. The following produces hide defects while the animal is still alive:
   Parasites, Diseases, Old Age, Malnutrition, Brand marks, Barbed wire
   scratches.  And on a dead animal: Poor stripping of the skin, handling
   and storage.

3. Mammalian Hides and Skins consist of three layers: Epidermis (a thin
   outer layer of epithelian cells), Corium or Dermis (the thickest layer),
   and Fleshy layer or Subcutaneous Adipose.  In the tanning process the
   epidermal and fleshy layers are removed; only the corium layers are
   tanned into leather.  The Corium layer consists of two layers.  The
   first layer is the grain membrane with the hyaline layer (arrangement of
   the hair pores).  It makes a distinctive surface pattern for each
   species of animal.  The second layer consists of large collagen fibre
   bundles interwoven at an angle in a three dimensional network.  The
   fibre structure varies in different parts of the skin and from one
   species to another.  Elastin fibres make leather that stretches.  Fatty
   skins yield spongy leather.

4. Fresh hides are (by weight) 60-70% water, 30-35% proteins (both
   fibrous and non-fibrous), 2% lipis, 0.5% carbohydrates, 1% mineral salts
   and other substances (ie pigments).  Collagen accounts for 85+% of the
   fibrous proteins in the corium layer; Elastin (or reticulin) are present
   in very small quantities.  The non-fibrous proteins are: Albumins
   (soluble in water), Globulins (soluble in salt solutions), and Mucins,
   Mucoids or Glycoproteins (soluble in dilute Alkalies).

5. The fibrous protein chains are held in position by chemical bonds
   giving a cohesive structure that is strengthened by the tanning process.


THE STEPS IN TANNING:

1. CURING is the process of dehydrating the hide without disturbing the
   skin structure.  This is necessary because within hours of slaughter,
   decay begins.  Hides need to be protected against excessive heat,
   humidity, rain and pests.
   A. Air Drying - proceed with caution, this produces a bony flint hide
   B. Salting
      1) Brined - treating the hide with a saturated solution of salt
      2) Wet Salted - rubbing the flesh side with salt
      3) Dry Salted - Air drying after doing B1 or B2
   C. Pickling (Salt and Alum)
      1) Brined - soaking the hide in an Alum/Salt Solution
         (formula at end of this paper)
      2) Wet Pickling - rubbing the flesh side with salt and alum
         (2:1 ratio)
   NB: Cold storage acts as a further preservative.

2. REHYDRATING is the process of soaking the Cured Hides in water to
   return them to their original flaccid condition, remove dirt, salt and
   some soluble proteins.  Rehydration is assisted by elevated temperature
   and agitation. (*Flint Hides need special attention, since the
   soluble proteins have cemented the fibres with the air drying.)

3. DE-HAIRING
   A. Sweating - wet hides are placed in a hot moist room and allowed to
      rot partially, the hair will pull out easily
   B. Fermenting - hides are placed in a fermented bath over-night
      (bath of fermented dog, hen or pigeon dung)
   C. Ashing or Liming - Soaked hides are treated with ash or lime for 1
      to 2 weeks. (addition of sodium sulfide reduces the de-hairing
      time to a few hours)
   D. Lime-Sulfide Paste - applied on the flesh side (saves the hair);
      hair can be easily pulled in 3 to 12 hours
   E. Ammonium Sulfate or Chloride - modern method
   NB: Liming not only removes hair, fats and soluble proteins, but
       also swells and conditions the fibre structures.
   (Wood Ash and Liming were used in the early medieval periods, later
   periods used Fermenting and Sweating; no dates available on the Lime
   and Sulfide Paste method)

4. DE-LIMING OR BATING is the removal of excess alkali from the pelt by
   using a mild acid (ie: boric, acetic, lactic) bath.

5. TANNING is the process of treating the hide with an agent, called
   tannin, that displaces the water and then combines with the coats and
   collagen fibres.  NB: By the early 11th C. AD, three basic tanning
   processes were used: oil, vegetable, alum
   A. Animal Brains and Urine
      1) Sub-Arctic People use animal brains or human urine to tan
         caribou and moose skins
      2) Early American Colonists learned from the Indians the art of
         tanning fine leather (deer and buck skins) to the consistency
         of fine chamois with the use of Animal Brains
      3) African tribes Air Dry, Scrape clean and Tan using a mixture
         of plant juices, bone marrow, urine and rotted brains.  The
         results is a fine kid having the softness of cloth.
   B. Oil
      1) Eskimos use Fish Oil
      2) Japanese and Ethiopians use rape and safflower oils
   C. Smoking - involves a reaction with aldehydes, produced with the
      oxidation of oils; used by the American Indians
   D. Vegetable Tannage
      (Hebrews used Oak Bark; Egyptians used Babul Pods; Arabs used
      Barks and Roots)
      1) Classic Method: Pelts are placed in a pit or vat in
         alternating layers with ground vegetables, tanbark, pod,
         leaf, wood or root.  Water is poured to cover it; the curing
         process takes 6 to 12 months.
      2) Bag or Bottle Method: Pelt is made into a bag, filled with
         vegetable tanning matter and water and tied to a pole.  The
         water leaches from the material inside the bag and the tannin
         diffuses osmotically through the pelt.  Curing is complete in
         3 to 6 days.
   E. Mineral - Alum Based
      1) Mud and Alum was used by the Chinese long before Christ;
         later it was also used by Assyria, Babylonia, Phoenicia,
         India, Greeks and Moors
   NB: Two or more processes are often combined
      1) Alum and Oil
      2) Alum and Gallnut or Sumac Leaf - produces soft nappa leathers
   F. Chemical - Modern - Chrome salts, Zirconium salts, Aluminum salts

6. WASHING - wash the hide in warm, mild soapy water and thoroughly rinse

7. DRYING, SOFTENING - place hides over rods in a warm, semi-moist,
   shady area (you want the hide to dry slowly).  As the hide slowly dries
   out, carefully pull the hide back and forth across the rods (this gently
   stretches and softens the hide while it is drying).  If it is drying
   unevenly, moisten the dryer areas with moist sawdust or rags.

8. FINISHING - process of obtaining the proper thickness, moisture,
   lubrication and aesthetic appeal.
   A. Trim off any undesirable edges and shave any thick spots in the
      leather.
   B. Conditioning - use damp sawdust to obtain uniform moisture (ideally
      the hide should be 20% moisture).  Stretch and soften as
      described above.
   C. 1st Finish Coat - applied to the grain surface
      1) Egg Albumin (10-20%), water, Glycerin (5-20%)
      2) Shellac and Casein (10-20% - made sol. in mild alkali), and
         Beeswax (5-20%)
      NB: can add to either of the above formulas - 6% pigment, 0.5-2% preservative
   D. 2nd Finish Coat - applied to the grain surface
      1) Waxes
      2) Silicones


WHAT I DO FOR HAIR ON SKINS:

1. During the skinning process it is very important that you do not
   stretch, tug or yank on the skin; pull gently, use a sharp knife to
   separate where necessary.  If the skin is stretched during skinning, the
   hair follicles get stretched and hair will fall out causing thin or bald
   spots on the finished skin.

2. Cure it immediately in the Pickling Brine for 3 or 4 days.

3. Clean the flesh side of the skin.  Using a very sharp, flat blade
   knife, remove all flesh, fat, tendons as well as the epidermal layer.
   It takes a lot of experience to know how much to remove as well as how
   to work the knife properly.  It is very easy to slice too far, or pull
   the epidermal layer too hard, again causing hair loss.

4. If the skin or the hair is dirty from slaughter, wash the hide in
   luke warm, mild soapy water, rinse thoroughly.

5. Tan skin in a new, clean Pickling Brine for 2 to 4 weeks (small skins
   for shorter time, larger skins and hides for a longer period of time)

6. Wash in luke warm, mild soapy water, rinse thoroughly, dry and soften
   as described above.

7. Finish by rubbing a mixture of egg whites and olive oil into the
   flesh side of the skin till soft.


WHAT I DO FOR DE-HAIRED SKINS:

1. Curing - wet salt method for 4 to 5 days

2. Clean the flesh side of the hide, as described in my hair on method.

3. Wash in very warm, mild soapy water and rinse thoroughly - it takes a
   couple of hours to clean the hair of a large hide, so this serves as a
   re-hydration process.

4. Sweat the hide for 4 to 5 days.

5. If the hair does not pull, soak in wood ash and warm water or lime
   and warm water for a minimum of 24 hrs.

6. While pulling the hair, keep the hide sweating in very warm to hot
   water (this helps to loosen hair follicles).

7. If Ash or Lime was used, re-wash in soapy water, rinse and soak in
   water and 1 quart of vinegar for 1 to 2 hrs., rinse several times.

8. Tan in Pickling Brine, wash, rinse, dry and soften as described above.

9. Finish by rubbing a mixture of egg whites and olive oil into the
   grain surface, then final finish with Wax or Saddle Soap.


FORMULAS:

Pickling Brine: 2 lbs. salt, 1 lb. alum, and 2 gallons of water

Modern Brine: 2 oz.  Reagent strength Sulfuric Acid, 1 lb. 10 oz. carton
              of salt, and 1 gallon of water

NB: use non-iodized salt for both formulas


BIBLIOGRAPHY:

Encyclopedia Britannica:  Vol 10, pgs 759-764, 452
                          Vol IX, pgs 809-810
                          Vol 1, pg 696
                          Vol 5, pgs 523, 1029
Raising Rabbits, by Ann Kanable
Webster's Dictionary