SOAKING (WASHING)
Important: Tannery water may be infected or may contain salts of calcium and magnesium bicarbonate (temporary hardness) or calcium and magnesium chloride, and sulphates (permanent hardness) plus carbonic acid. Precipitates cause patchy stains on the leather.
The first process consists of soaking the skins in water, the aim being to allow them toreabsorb any water which may have been lost after flaying, in the curing process or during transport. This absorbed water re-hydrates any dried inter-fibrillary protein, loosening its cementing action on the fibres. The collagen fibres and keratin cells of the hair and epidermis also take up water and become more flexible. Due to the water returning to interfibrillar spaces the fibers may slip one against the other and an adequate plumpness is imparted to the hide.
Wet salted hides may be soaked for 8-20 hours (10-160C). The amount of water used ranges from 3 to 5 times the weight of hides (6-7 times for dried skins). Satisfactory soaking is judged by cleanliness and absence of salt. Salt is determined in the juice squeezed out of the skin, using a pocket refractometer (refraction increases linarly with concentration).
This process is not simple, because putrefying bacteria may act as soon as there is surplus water or curing agent is washed out.
Common additions to the soak liquor as disinfectants (bactericides) are:
1 part sodium hypochlorite per 1000 parts water or 1.5 part to 1 part trichlorophenate per 1000 parts water.
Formic acid and pentachlorophenate may also be used. Speeding up the water uptake of the skin reduces the chance of putrefaction This can be done by.
a) Mechanical action – rocking frames, paddles, drums, green fleshing.
b) Temperature – as warmed up to 380C, the rate of bacterial action may increase, if temperature exceeds 380C, protein fibers tend to shrink, skins loose area, protein fibers gelatinize.
c) Chemical additions:
– Acid addition (used when hair or wool is kept on the skin), 1-2 parts of formic, hydrochloric orsulphuric acid per 1000 parts of water at 160C.
– Alkali addition (more common as it looses hair), 1-3 parts of costic soda, or soda ash or washing soda or borax per 1000 parts soak water. Sodium sulphide also gives alkali solution, and speeds up loosening of hair and epidermis. Sodium polysulphide is less alkali and has a mild dispersing action on inter-fiber proteins giving smooth grains. Ammonia liquor has a safe, gentle swelling action, which opens up fiber structure. Ammonia and hydrogen peroxide, each about 2 parts per 1000 parts of water are favored for sheepskins, the wool not being loosened so much as with straight alkali.
There is the danger that if too much acid or alkali is used, the surface fibres of the skin will rapidly absorb it and swell so much that they distort the surface of the skin and block up the inter-fibrillary spaces, preventing the water from reaching the inside. This will give leather a loose grain.
– Salt (NaCl) solutions of 3 % concentration dissolve unwanted inter-fibrillary protein, thus speed up soaking.
– Wetting agents detergents 1-2 parts per 1000 parts of water (particularly if hides are greasy.
– Enzyme preparations (proteolytic action on the interfibre proteins)
To control the soaking properly, it is recommended to observe the following factors:
1) The pH of the solution, easing the swelling, and so the diffusion of bath components into the hide. Lowest degree of swelling is at isoelectric point.
2) Presence of salts (including NaCl), contained in soak water, as it influences water structure.
3) Surface tension at the water/hide interface, which is mainly depending on the fat content of the raw hide, and on the presence of surfactants in the solution.
