Chromium in the chrome liqour is trivalent and if all the three valencies are attached to the OH group, as in the case of chromium hydroxide (Cr(OH)₃), the compund is 100% basic. Conversely, chromium sulphate (Cr₂(SO₄)₃) is 0% basic.

Any basicity within the range of 0% to 100% is theoretically possible but in actual practice, chrome liqour upto 60% basicity can be made esily. At 66.66% basicity, chromium gets precipitated. The tanning power of any given basic chrome salt increases with the addition of alkali or any substance which reacts to acid and reduces by the addition of acid.

The most suitable basicity is about 33.3%. Too low a basicity gives an empty,flat, hard and tinny leather with a tendergrain, while too high overloads the fiber with tans which obstruct further penetration of chrome into the leather, giving under tannage in the middle layer and a cracky grain.

The skill of the tanner lies in striking a golden mean and keeping the optimum range of basicity from the initial to the final stage of tanning, according to the type of leather desired by him.

Chrome tanning liqour used in the initial stage of tanning should have a low astringency for uniform penetration of tan liqour in the skin. The stringency of chrome liqour is gradually increased during the course of tannage by makingit progressively basic by adding weak alkalis. This tratment gives a greater fixation of chrome on leather and a better exhuastion of chrome bath.

During the tanning process a simple basic chrome salt taends to fix on the surface of the leather, because of its high affinity to collagen fiber and produces comparatively flatter leather. This affinity of chrome tanning salts to collagen can be decreased by adding salts of weak organic acids, like calcium or sodium formate, to the basic chrometanning agents. These salts enter into the chrome complex and reduce its affinity to collagen. This effect is known as "Masking" and the organic salts are known as "Masking Agents". A masked chrome liqour is therefore able to penetrate deeply and distribute itself uniformly into the hide substance before fixation with protein takes place. For this reason, masked chrome liqours produces a leather of increased fullness with uniform chrome pentratation and dostribution throughout the thickness of the hide.

While masking chrome liqours, care should be taken to add only an adequate quantity of the masking agent. Excessive use may give chrome leathe a very poor quality and in extreme case, may evn preavent the tanning potention of the chrome liqour. For making a full and good grain leather, tannage should be mellow at the start in order to obtain a deep penetratin of chrome liqour which would result in a fine, tight grain. As the tanning progresses achrome liqour of increased affinity should be added toachieve greater fixation of the chrome towards the end of the tanning process.

Leather can be chrome tanned either of the two methods, single bath or double bath. In the single bath process, bated and pickled hides are treated with basic chrome salt solution (chrome solution). The pelts gradually absorb the chromeliqour and permanently fix it, where by they get tanned. In the double bath, bated skins are first treated in a solution of sokium bichromate and sulphuric or hydrochloric acid. Chrome acid is formed inthe solution which impregnates the skin. The pelts which have absorbed the chromic acid are not tanned. For complete tannin, chromic acid impregnated skins are trated a bath containg a reducing agent usually am acidified solution of sodium or ammonium thiosulphate. The sulphurous acid thus formed reduces chromic acid thus formed reduces chromic acid in the pelts to basic chrome which in turn combines with hide collagen and tans it.

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The advantage of a one bath chrome tanning is its simple, easily-controlled and safe application. For this reason most chrome leathers are now manufactured by the one bath process. The two bath process is restricted dto some speciality leathers like glazed kids, glove leather etc. But here again, there is a distinct trend to replace the process by the one bath method.

Single bath chrome liqour is generally prepared from sodium or potassium dichromate and sulphuric acid. Dichromate is dissolved in minimum quantity of wate in a lead-lined vessel and the required quantity of suphuric acid is added. The chromic acid thus formed is then reduced to a basic stateby adding very slowly but with vigorous stirring some suitable reducing agent. Each reducing agent gives its own distinctive characteristic to the final leather.

Generally, glucose or molasses is used as arducing agent but other substance like hypo, sawdust, fleshing, glycerine, etc. can also be used. When sulphur di-oxide is passed through the dichromate solution without adding acid, 33%basic chromeliqour is produced. This method is aplied for making commercially, ready to use basic chromepowders. Thechemical reaction involved is represented by the following general equatin, where X represents the reducing agent.

    k₂Cr₂O₇ + 3H₂SO₄ + X   -->   2Cr(OH)SO₄ + K₂SO₄ + 2H₂O + XO₃

The reducing agents gets oxidised by oxygen liberated from dichromate with the formation of carbon di-oxide, organic acids and many other unknown compounds. These products have a definite role to play in tanning and can considerably influence the characteristics of leather. The reaction is exothermic and the rate of reaction depends upon the addition of the reducing agent. Preparation of chromeliqour involves rigid control of the procedure in order to achieve uniformity in the final product. Any deviation in the quality and quantity of the starting chemicals as well as in the mode additions, charges the nature of achrome liqour which in trun reflects on the final quality of the leather produced. More and more tanners have now discontinued preparing their own chrome liqours since the method involved is time-consuming and prefer to use standardized chrome preparations available commercially.

Chrome tanning operation is performed in tanning drums. It can be executed in the same drums that were used for bating and pickling, making use of the pickle liqour floats as soon as the latter operation is complete. However, if tanning is to be done in a fresh bath, some amount of salt is added to the fresh taning bath, to check acid swelling. The pickled petlts are then entered into the dfrum and the drumming is started. Next, the proper amount of chrome liqour is added through a hollow axle in a number of feeds. Where a ready-to-use basic chrome powder is used, they required quantity of powder can be added undissolved, provided the solubility of powder is very good.

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Chrome imparts bluish green colour to the skins. This property is useful in assessing the extent of the penetration of chrome liqour inside the leather. When considered adequate, the alkalinity of the system is slowly altered by adding alkalies like sodium bicarbonate or soda ash in a diluted form in several feeds. This reduces the acidity of chrome liqour and at the same time, increases its affinity for protein fiber. The entire tannage takes anything between four to eight hours to complete, depending upon the thickness of the hides.

The completion of the tanning process can be tested by a simple "boiling test". An untanned piece of hide held in boiling water will shrink in size quite noticeably. Chrome tannage imparts heat resistance to leather which appreciably increases as tanning proceeds. Chrome tanning precess is usually considered complete when a piece of tanned leather held in boiling water for three to five minutes, does not shrink.

After the leathers are fully tanned, they are taken out of the drums and piled for removing excessing moisture to bring them to a proper condition for the next operation of splitting and shaving. Excess moiture can also beremoved by working the hides on a samming machine. Sammed hides are the split and shaved to desired thickness.

Syntans can be directly intorduced into the chrome tanning operation to help modify the basic chromium sulphate, which in trun produces a light-coloured chrome complex of superior tanning prpoerties. Leather so tanned is fuller, rounder and depending upon the syntan used, is of a lighter colour throughout its entire thickness. It requires very little neutralising prior to coloring. Colours are brighter, cleared and generally more uniform within a skin and also from skin to skin. Leathers tanned in this way release water more easily, thus facilitating splitting and shaving.

The method of application of syntans in chrome tanning system is very simple. An acid syntan may be added with chrome tan or a neutral syntan, after the chrome has thoroughly penetrated. When the syntan has run about 30to 45 minutes, sodium bicarbonate or soda ash is gradaully added to bring the stock upto the desired pH.

Application of syntans in chrome tanning also has an excellent dispersing action on the excess natural grease in the stock which is washed away in the rinse before colouring or fat-liqouring.

By and largeal tannersand expecially those engaged in making leather for export, live with the problem of mould growth on chrome tanned leather in a wet-blue condition.

Mould growth produces stains on chrome tanned stock that discolour the leathe so badly that it is difficult to process leather into white or lighter shades and it becomes necessary to put them into a black dye-an undesirable production limitation. Mould growth in chrome-tanned stock may be effectively prevented by suing 0.25% of preservatives on pelt weight.

The product may be aded at any stage of chrome taning. However, it is preferable ato add it at or near completion of tanning and before basification. It may also be added with the diluted sodium becarbonate feeds, or just prior to the time the drum is stopped at the the finish of the chrome tanning operation and basification.

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