Skin Tanning Process

 

 

It is quite feasible to consider performing good qualtiy tanning of small animal skins at home or on the farm. Since the intrinsic value of most small skins is the FUR (rabbit and fox etc) or the WOOL (lamb and sheepskins) or the HAIR (goat, calf, kangaroo and deer), this article will only consider tanning of skins with intact "hair" (hair, wool or fur). The size and thickness of cattle hides precludes their processing outside a tannery where specialised expensive equipment is generally essential.

Most of the equipment required for tanning a small number of skins is available at home or is inexpensive to purchase and all necessary basic chemicals are available from specialist suppliers - details are provided later. Definitions
Before commencing a description of all the processes it is helpful to become familiar with some tanning terminology:

  • Flaying - removal or stripping the skin from the animal.

  • Fleshing - removal of fat, muscle, veins and loose surface from the reverse side (to "hair") of the skin. This reverse side (or inside) is called the flesh side.

  • Curing - temporary preservation of the skin until tanning can be commenced. This can be salting, drying or other chemical treatments. However, only salting will be recommended here. If kept dry, the salted skin is stable for long periods.

  • Scouring - removal of dirt and natural grease from the "hair" or wool.

  • Pickling - a salt/acid treatment which is essential before chrome tanning. The acid pre-conditions the skin for tanning and the salt prevents the skin from swelling and damage due to acid.

  • Tanning - this is only one step in the overall conversion of raw skins to finished leather, although the whole process is generally referred to as "tanning". This is essentially the chemical conversion of raw skin from a form easily damaged by bacteria and heat, to a form which is stable to heat, bacteria and washing. After tanning and fatliquoring (see below) the skin will dry to a soft flexible form which we know as leather. Because of the ease of use and the stability of the final products, only chromium sulphate tanning will be considered here, even though there are many other chemicals capable of tanning skins.

  • Fatliquoring - the application of special oils in a water-dispersed emulsion. This is essential for soft and flexible leather and applciation must be before the skin is dried out after tanning.

  • Drying - the slow drying of skins (after tanning and fatliquoring) away from heat and direct sunlight.

  • Staking - the softening of skins after drying.

  • Degreasing - solvent dry-cleaning for removal of skin grease and fat and small amounts fo residual wool grease. Essential for sheepskins if the final product is to be odour free and may be necessary for lambskins and other skins.

  • Combing - combing with a special comb to straighten "hair" or wool and to remove tangles and small amounts of vegetable matter such as burr.

Equipment Requirements

  • Fleshing knife (from leather or craft shops)

  • Large plastic rubbish bins (3)

  • Plastic buckets

  • Plastic measuring jug. Measuring cyclinder

  • Plastic storage containers for liquids

  • Scales (eg kitchen scales)

  • Wooden poles for draining/drying

  • Wool comb (metal)

  • Rubber gloves and safety glasses

Processing Note
All wet processing is carried out in a plastic bin or rubbish bin. At all times, it is essential that the skins are fully immersed in the processing liquors. The volume recommended throughout this article is 20 litres per skin. However, if the skins will not easily immerse then the volume can be increased until the skins are covered. For any increase in the volume of water used, all chemicals must be increased proportionally - this is why all amounts are calculated "per litre" of water used. Agitate skins by moving gently with a smooth stick or gloved hands.

Processing Sequence

Step No


Details

Time

1.

Flaying

Remove skin as soon as possible after slaughter avoiding skife cuts on the flesh side and minimising amount of fat and muscle removed with the skin.

Time of slaughter

2.

Washing

Wash in 3 changes of cold water (20 litre per skin) each of about 20 mins. Remove blood and loose dirt.

Time of slaughter

3.

Fleshing

Place skin on table or board and cut off fat and muscle etc by scraping parallel to the skin surface so as not to cut into the skin structure. Wash and drain for about 30 min.

Time of slaughter

4.

Curing

Lay the skin on a flat drained surface (shearing shed floor is ideal) and spread fine salt over the complete surface until about 1 cm deep and rub salt into the flesh. Allow to dry in shade. The skin is safe for months if kept dry.

IMPORTANT NOTE: Owing to bacterial attack on skin and subsequent loosening of "hair" and putrefaction, the skin must be salted on the same day as slaughter and as soon as possible during summer.

Time of slaughter

5.

Washing

Wash 20 litre per skin with cold water and soak for a few hours. Drain.

After storage

6.

Scouring

Scour skins at 38°C for 1 hour in a bath containing 4 gram/litre of a scouring paste. Drain and repeat scour in a fresh bath. Agitate frequently and gently for lambs and sheep - the longer and finer the wool the more care required to prevent felting. 60's wool is usually the finest wool handled by commercial tanneries and finer wool requires very careful handling. Re-flesh if necessary.
 
  Note: Never exceed 40°C in any bath with untanned skins.


7.

Rinse

20 litre per skin at 38°C. Drain


8.

Pickle

Prepare a bath of 40 gram/litre salt and 6ml per litre of 33% sulphuric acid (battery acid) (refer safety notes). Add skins to salt/acid mixture and agitate occasionally over 2 days. Remove skins and drain back into rubbish bin if pickle liquor to be re-used (see later). Drain for a few hours. DO NOT WASH SKINS OR EVEN SPLASH WITH WATER WHEN IN THE PICKLED FORM.

Days 1 & 2

9.

Tan

Prepare in advance a bath containing 24 gram/litre of chrome tanning powder, 40 gram/litre of salt and 20 gram/litre of aluminium sulphate. Add skins and agitate as frequently as possible (say 1 minute every few hours) during the day over 3 days. At the neck end, make a cut about 4 cm long and 1 cm in from edge of skin and check that the skin is green through the complete thickness of the skin. If not, continue tanning until this is so. Remove skins and drain until this is so. Remove skins and drain back into the bin to collect tanning liqor if it is to be re-used.

Day 3 to Day 6

10.

Rinse

Three rinses with 20 litre water per skin cold, each rinse about 30 minutes and draining between rinses.


11.

Neutralis

To further stabilise the tannage, make up a fresh batch of water at litre per skin containing 1 gram/litre sodium bicarbonate. Agitate occasionally for the rest of the day and stand overnight.

Day 6

12.

Drain

Drain overnight over a pole or similar support.

Day 7

13.

Fatliquor

For each skin about the size of a lambskin, use 100 mls of fatliquor. Increase or decrease amount for larger or smaller skins. Prepare the fatliquor by stirring it into 2 times its volume of hot water to give a homogeneous milky product. Paint onto flesh side of skin and allow to soak in. Repeat application.

Day 8

14

Dry

Hang, or stretch and nail out on a board to dry (this must be in the shade).

Day 9+

15

Stake

When still slightly damp (1-2 days), draw flesh side of skin over the edge of a table, fence etc and work until soft and flexible. Allow to dry fully and stake the skin again.

Day 9+

16.

Finish

Comb wool or "hair" and trim off edges for appearance.


17.

Dry Clean

If fat patches appear in the skin or on the surface, or if the skin has an unpleasant odour, it is recommended that the skin be commercially dry-cleaned. It must be established with the dry-cleaner whether any fine-woolled skins are safe to dry-cleaned without the risk of felting.


18.

Re-finish

Comb and soften as necessary for appearance.


Re-Use of Chemicals (see example, Appendix 1)
Pickling and tanning baths are not completely used up. Exact measurements are necessary in commercial production, but for re-use at home estimates can be made for "making-up" to starting strength.

Pickle Liquor
For the amount of liquor remaining, add half (½) the original amount of sulphuric acid (ie 3.0 ml/litre of battery acid) and quarter (¼) the amount of salt (ie 10 gram/litre). Make up to the original volume with full strength pickle.

Tanning Liquor
For the amount of liquor remaining, add quarter (¼) the original amount of chrome powder, and make-up to original volume with full strength liquor.

Owing to inaccuracies in this technique and, partiuclarly, if any variations in tanning appear, it is recommended that re-use for only two or three cycles be considered.

SAFETY NOTES
Sulphuric acid - this is particularly dangerous and must be handled with care using gloves and glasses. ALWAYS dilute by adding the acid slowly to a large excess of water (NEVER ADD WATER TO THE ACID).

Use gloves and glasses when handling pickle or tanning liquors - they may irritate the eyes and a few people are sensitive to these liquors.

Sodium bicarbonate - neat sodium bicarbonate could be harmful to the eyes and skin so use gloves and glasses when mixing.

Kitchen equipment - wash well in hot water after use.

PROCESS NOTES
Do not wet the pickled skins with water (don't even splash them with water). They must go into a salt-containing bath (pickle or tan liquors) whilst in an acid condtion.

PROCESS CHEMICALS
Various amounts of following chemicals may be available from local rural supply shops or chemical distributors.

  • Salt (sodium chloride)

  • Wool scouring agent

  • Fatliquor (type suitable for woolskins)

  • Chrome tanning powder (33% basic chrome sulphate)

  • Aluminium sulphate

  • Sodium bicarbonate

  • Sulphuric acid is best purchased as a diluted solution from car battery shops where it is sold as a 33% solution. Tanning kits (with their own instructions) are available from craft shops.

SUMMARY
This process can produce skins with a high degree of stability to washing. The pickling (step 8) and tanning (step 9) processes could be extended without harm and it could be beneficial - they may also be shortened in time with experience and care but, initially, a safe, lengthy process is recommended. Where small pieces of skin are available, it is recommended that small scale trials be carried out so as to become familiar with the technique.

APPENDIX 1

EXAMPLE OF RE-USE OF CHROMIUM TANNING LIQUOR

Two skins tanned and drained into a plastic bin (initial volume = 40 litre for two skins)

Final volume 35 litre (measured with a calibrated bucket)

Therefore base calculations on this volume -

Add (¼) of the original chrome powder = 35 x 24 x (¼) gram = 210 gram

Make-up to original volume = 5 litre of full strength liquor

 


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