Chrome Tanning by the "Double-bath" Method.—This method is commercially applied chiefly in the tanning of goatskins for the manufacture of glace kid; it is also used in the tannage of calfskins for willow calf ; and for mechanical leathers.
The prepared pelt is first treated with a comparatively weak solution of potassium or sodium bichromate acidified with hydro chloric or sulphuric acid. After this solution has been absorbed by the skins, they are transferred to a solution of sodium thio sulphate ("hypo"), to which hydrochloric or sulphuric acid is added as the reduction process proceeds. The free sulphurous acid produced reduces the chromic acid resulting from the action of the acid on the bichromate solution, into a basic chromium chlor ide or a basic chromium sulphate salt; the thiosulphate during the reduction process undergoing oxidation to sodium tetrathionate and sodium sulphate. At the same time, sulphur is liberated and deposited in the interstices between the fibres as well as in and on the fibres of the skins. The sulphur deposit is one of the principal features which distinguish leather tanned by the "two bath" process from that produced by the "one-bath" method.
The method now commercially practised is first to pickle the skins with sulphuric acid and common salt, and then drum in a solution of sodium bichromate and acid. The more complete the absorption of chromic acid resulting from the chemical action the better, as this has an important influence on the finished leather.
After the goods have been thoroughly impregnated, they should be of a bright orange yellow colour. They are then removed from the solution, and carefully drained for a minimum period of hours, when they are passed through a striking-out machine for the purpose of laying the grain of the skin perfectly flat and removing any wrinkles that may have appeared.
The reducing operation is done in the drum or paddle-wheel, but in the case of heavy goods it may be done in pits. The fol lowing is probably the best procedure :—The necessary quantity of sodium thiosulphate is dissolved and placed in the paddle. About one-third of the acid required, after dilution, is added to the solution of sodium thiosulphate. The mixture should be
well stirred either by paddling or by hand, with a wooden stirrer, when after a few minutes the solution will become slightly cloudy, as a result of precipitation of finely divided sulphur which shows that the reaction has begun. A further of the diluted acid is added, and after about i hour the remainder.
Generally, the reduction takes place in fairly well defined stages indicated by the colour of the goods. In the early stages there is a change from the original yellowish orange to a darker shade of yellow and to a lightly brownish yellow. Later the colour becomes a greenish olive, and finally a pale bluish green.
The following are suggested quantities for the two solutions:— Chrome Tanning by the "Single-bath" Method.—The "single bath" method is more largely practised than is the "two-bath" process, particularly in the case of calfskins intended for "box" and "willow," or suede finish; hides for side leathers; and sheep for gloving, upper leather, linings, and fancy purposes. The "sin gle-bath" process is simpler, and less liable to produce irregular results. The essential difference is that, whereas in the "double bath" process the basic salt of chromium is formed on the fibres, in the "single-bath" method the basic chromium salt is applied direct to the skins in solutions of gradually increasing strength.
One of the simplest methods of preparation of a "single-bath" chrome liquor consists of neutralising the acid of chromium sul phate or chrome alum by adding sodium carbonate. The follow ing proportions of sodium carbonate and chrome alum should be used to produce the salt, which is most generally suitable. For each T,000 lbs. pelt to be tanned, dissolve :-15o lbs. chrome alum in 5o gallons water, at a temperature of about 85 to 95 deg. C. When the whole of the chrome alum has gone into solution, cautiously add a solution, prepared by dissolving 16 lbs. soda ash or 6o lbs. washing soda crystals in 25 gallons hot water. When all of the sodium carbonate has been added, the solution should be thoroughly stirred.