It has been oberved how the hides and skins after soaking become pliable conducive to further chemical treatment. The next process after soaking is Liming.

The following objective s are acheived in the liming process:

1. Unhairing - to remove the epidermic layer and at the same time to loosen the hair.
2. Removal of intertibrillary proteins(cementing substances) to make the final leather pliable ad soft to the touch
3. Swelling of fiber bundles into individual fibers and splitting of fibers into into fibrils in order to expose their surrounding surface to the subsequent action of tanning materials, thereby converting them into leather
4. To remove the natural fats in leather by saponification in order to obtain non-patchy and uniform surface in the final leather.

Lime is the most widely used agent for hair loosening. It is made from natural materials containing calcium carbonate likelime stone or sea-shells. These are burnt in a kiln to form calcium oxide (CaO) which, when combined with water gives calcium hydroxide (Ca(OH)2). Lime (CaO) is sparingly soluble in water. Its solubility decreases with the rise in temperature. 100 cc of water dissolves 0.135 gms. of calcium oxide at 5'C, while at 100'C only 0.059 gms. of calcium oxide goes into the solution. At 20'C saturated lime solution (limewater) is approx. N/20 strength with a pH of 12.60. During liming, lime water absorbed by hides and skins is atomatically replenished by dissolved lime. Thus the alkalinity of lime liqour is also automatically maintained and its action remains uniform and steady.

Lime water derives its power to loosen hair from its alkaline character. Due to its limited solubility in water, it gives an alkaline solution which hass a mild action on the hides and skins.

It takes about 8 to 12 days when lime alone is used (process know as straight liming) as a depilatory agent, depending on the temperature to loosen the hair. This longer duration of time causes growth of bacteria in the lime liqour. Lime also reacts on hair(keratin) and epidermis. These hydrolysed protins form a culture medium for further growth of bacteria. Consequently bacteria attack the cementing substances and if the process is prolonged, it may attack collagen fibers (true skin). Bacterial action associated with staight liming is not harmful.

Straight liming procedure is generally adopted for vegetable tanned leathers. The space opened up by bacaterial action are filled up by vegetable tannins and non-tannins absorbed by leather inconsiderablequantities during the tanning process and resultant leather is full and pliable.

Whenever rapid liming is desired as in the case of tanning with a mineral tanning agent, the unhairing process is accerlerated by adding sharpening agents like sodium sulphide, sodium sulphydrate, dimthylamine, etc.

Sodium sulphide is commonly available in 30% crystalline or 60% fused form. It is readily soluable in water and dissolved, gives casustisc soda and sodium sulphydrate.

• Na2s + H20 = Naoh + NaSH
• Sodium Sulphide + Water = Caustic Soda + Sodium Sulphydrate

SH ion in sulphydrate has a depilatory action in the presence of OH ion from casustic soda or lime

Hence when lime is sharpened with sodium sulphide, sulphydrate causes rapid loosening of hair, whereas caustic soda solution causes vigorous plumping of the hide fibers due to its strong alkalinity and also saponifies the natural fat in the hides and skins. High level of sodium sulphide may cause excessive plumping which can damage the pelt. Addition of buffers like calcium or sodium chlorides to sharpen lime liqour reduces swelling effect.

Lime solution sharpened with sodium sulphide makes a strong unhairing agent. A solution of 5% or even a lhigher strength not only loosens hair quikly but attacks it and reduces it to a pulp. This pulped hair can be removed by simply wahsing hides and skins in a drum.

The action of sodium sulphide on cementing substances and collagen fiobers is however negligible.

Sodium Sulphydrate (NaSH):  Sodium sulphydrate is commercially availableas 38%solution in liquid form or 78% conc. in powder form. As already seen, sodium sulphydrate is produced when sodium sulphide is used as a sharpening agent in Liming. However, it has to act on hides and skins along with causetic soda which has an inordinate swelling action on the pelt, making the grain coarse. Sodium sulphydrate alone has no swelling action and acts as a very good depilatory agent in the presence of lime (OH group). For this reason it is regarded a very good sharpening agent for line grain leather.

 Surface Active Agents:  Non-ionic or anionic surfactants are particularly useful for liming fresh hides. Theseproducts assist in removing excess grease from hair, promote efficient hair burning without resorting to high usage of depilatory agents and contrivute generally to clean, clear grain.

These then are the various methods followed for the liming procedure. The process can be carried out in pits paddles or drums, employing the above chemicals. Amount, tempreture and concentration of the float frequency of the agitation and the total period of liming has a decisive effect on the degree of liming and needs to be controlled. If the purposed is to save the hair for its commercial value, a along procedure using weaker solutionns and a lower tempreture is adopted. Sharpened lime with ore agitation will remove the hair quicker.