Cured hides and skins arriving at a tannery are trimmed to remove long shanks and other perimeter areas which do not go into the making of good leather. Trimmed hides are sorted for size and weight and formed into batches. The trimmed hides and skins are then ready to undergo the next treatment-Soaking.

The hides and skins lose a good deal of their natural moisture as a result of the curing process. The main object of soaking therefore, is to restore the lost moisture so that the chemical treatments that follow will achieve optimum results. In addition, adhering salt, blood etc are also removed duriong soaking.

Soaking operatin can be carried out in pits, paddles or drum. The method and duration of soaking vary according to the available equipment at a tannery and also depend upon the aonditin of the raw stock. Thus , a freshly slaughtered hide requires minimum soaking time, the wet-salted stock about 3-4 hours and the dry salted ones preferabley overnight with the addition of a preseravative to the soak liqour. For dried hides the soaking time varies from 36 to 48 hours and more. Best are achieved by soaking intitially in a salt solution followed by a second soak in an alkaline medium

Wate used for soaking should be soft to medium-hard and free of dissolved organic matter. Hard water softens hides more slowly than soft water and will precipates soaps from wetting agents. Water contaminated with organic matter willaccelerate bacterial growth which can damage the hide substance.

The hides and skins should be soaked in a minimum quantity of water which should be double the weight of the raw hide, to prevent thereduction of salt concentration. The bacterial spores present in the raw hides and skins multiply and attack the hide substance when the salt concentration decreasxes. In dirt soak bacterial spores are washed out along with dirt, blood and most of the salt used in curing the hides and skins.

The second phase of the soakingoperation is generally carried out in a quantity of water which is four to five times the weight of the hide. The enteire operation can be accelerated by using either a non-ionic or an anionic wetting agent. Some soaking agents have slight antiseptic properties.

The bacteria present in the soaking water and in the hides and skins can develop in normal tempreture of soak water during soaking, thereby causing loss of hide substance. To combat this bacterial growth, suitable bactericides like Chloros, Sodium trchlrophenate, ae used.

The pH factor of the soak liqour plays an important role in soaking. Comman alkaline soak liqours are made by adding caustic soda or sodium sulphide to the soaking bath. The pH of such liqour should be adjusted between 7 to 8 and should be not in any case exceed 9. A higher pH will give surface swelling and water uptake will get reduced due to constriction of hide capillaries. Due to internal structural change, hair Keratin becomes resistant to alkali and unhairing agents, in the higher pH range. This phenomenon is known as immunisation of Keratin and may cause difficulties in the subsequent liming and unhairing operations.

The temperature of soak liqour is also important in the soaking operation. On account of rapid multiplication of bacteria, hides and skins may putrefy at higher temperatures if the soaking period is not reduced considerably. The maximum allowable temperature for the soaking operation is 20^oC. If the soak aliqour temperature is kept above this limit the soaking time should be reduced by using heavy doses of wetting agents and bactericides.

The efficiency of this process depends upon care and attention given to the control of the bacterial content of soak water, its temperature and the duration of soaking time for a particular type of raw stock. A perfectly soaked hide will have a flexible and slippery handle.