Tanning Process

Soaking is done in open baths withs paddles for intermittent circulation of the materials & water in the bath. Soaking of the raw salted skins restores the water content & removes blood from the hides. 

This process is continued in paddle baths. The soaked hides are treated with a milk of lime with or without adding sharpening agents, for 36 hrs depending upon the type of skin. This process removes the hair, nails, hooves and keratinous matters that enters the fibrillary soluble proteins in addition to condition the collagen for satisfactory tannage.   

Post liming, the hides are cleaned of the adipose flesh and fat by a fleshing machine. It clears the superfluous matter & assists in better permeability of the chemicals in subsequent baths. 

The fleshed hide is inserted in Wooden drums for a series of processes culminating in tanning, which takes about 18 hrs. The limed and un-haired skins called pelts are repeatedly washed with ammonium sulphate or equivalent chemicals to reduce its alkalinity. This ensures proper tannage and making the surface of leather produced smooth and malleable grainy. 

Bating is a process required for soft leathers. Bating agent compromising activated animal enzymes acting on the pelts to remove remaining lime & thereby producing a silky grain. It also imparts breathing property to the goat/sheep skins. 

The pelts are treated with a mild solution of acids in a buffer medium of sodium chloride to put the skin at a Ph level Between 2.8 to 3.2 to satisfy the binding capacity of the collagen. It would ensure uniform and proper intake of chrome salts in pelts for tanning. 

The objective of tanning is to convert the putrescible protein of raw into non putrescible leather that can be used for years without the chances of of any decay. It is done either using controlled chromium sulphates or vegetable tanning agent. Preservatives are also added to prevent bacterial or fungal damage. 

The tanned pelts are subjected to a slow process by sodium bicarbonate or sodium hydroxide to fix the chrome salts or other tannins into the pelt by a complex chemical bond and to provide it a neutral Ph state.  

By now the color of raw hides becomes blue, hence the name. These pelts are piled for draining the excess water on removal from the tanning drum. Shamming and Setting machines are used for a quicker drainage. The Blu is selected based on its quality and constituency for further processing into finished leather. 

Thick Blues are often split into two or more slices and the intervening slices called 'splits' by a Splitting machine. The Split is recycled and used as a by product source for cheaper leather. The corium side is thereafter brought to the desired thickness by shaving machines which shaves off the additional thickness.   

The shaved Blue is again dropped in drums for re-tanning and addition of syntans and Fat-Liquour to produce the finished leather of desired supple softness and constitution as well to impart it with any special properties which varies widely in nature of parameters of the chemicals used. The fat liquored  leather is transformed into 'Crust' by drying is by a toggle heater arrangement.  

The option for finishing of crusts are diverse which can include staking machine, spray booth systems, finiflex, polishing, plating, rotopress and roller plating machines. Most cruts are coated with water-based pigments with acrylic, protein or cellulose binders followed by a protective coating of nitrocellulose thinner or polyurethane compound for yielding the desired touch, feel, and grain display.