CATECHU.

495

by it, and the woven tissue is next placed in such a condition as is mostexpedient for the oxidation, and, consequently, transformation, of the colour-able principle into a brown insoluble and firmly-adhering substance. Theoxidation can be effected by :—

1. Exposure of the tissue to air,—a rather slow process,—with the simul-

taneous application of steam.

2. The transformation is greatly accelerated and aided by the introduction

into the colour mixture of such materials as are capable of acting asoxidating substances either more slowly or more rapidly if steam isapplied ; such substances are in this case salts of copper.

3. The oxidation is greatly accelerated by passing the tissue after the

catechuin—that is to say, the catechu, since as yet pure catechuin isnot commercially prepared in a state of chemical purity—has beenprinted on, into an alkaline solution, whereby the affinity of thecatechuin for oxygen is greatly increased.

4. By passing the goods through a solution of bichromate of potassa.

5. Lastly, a combination is made of two of these processes, viz., steaming

and passing into bichromate, or also ageing and steaming.

It has been suggested to wash powdered catechu in cold water, whichdissolves the tannin, leaving catechuin untouched. This latter substance,being soluble in hot water, is then fit for use. Sacc states that catechu, whentreated with sulphuric acid, is decomposed chiefly into glucose and a brownresin, which, on being dissolved either in moderately strong sulphuric acid, orin an alkaline solution, assumes a beautiful purple colour remarkable for itsstability. The brown resin is probably employed by some dyers and printersin place of catechu. Of course, if catechu alone is used, the manufacturerapplies the most rapid and complete method, which is undoubtedly thepassage through bichromate; but if it is mixed up with other colours, it isrequisite to adopt the process which suits the case, so as to have the desiredeffect upon the catechu, yet not to injure or alter the other colours.

The natural tint which catechu produces is modified by the introduction oraddition of divers metallic salts, such as those of iron, manganese, and others,among which the salts of copper play an important part, chiefly, however, for thefixing of catechu. This point has been purposely investigated by MM. C. Kcech-lin and Mathieu-Plessy.* As a result from their experiments, confirmed bythose of M. H. Schlumberger, we learn that sal-ammoniac has a specificinfluence in promoting the oxidation of the colourable matter of cutch by theoxy-salts of copper ; indeed the oldest receipts for the employment of catechu,given by Schagppler and Hartmann in 1806, and by M. Esslingerat Jouy in1 82g, recommend the mixing of a salt of copper and sal-ammoniac as a meansof fixing the dye upon cloth.

The mode of action of the sal-ammoniac is differently explained. Ac-cording to M. Persoz, it aCts by its tendency to form double salts, therebykeeping in solution divers salts and oxides; according to Schwartz, the sal-ammoniac should serve to prevent the catechu from combining with gum.The latter idea is of little value, the less as the same mixture is usefullyemployed in other instances for the very purpose of causing oxidation, as in the

* “ Bulletin de la Societe Industrielle de Mulhouse,” vol. xxii., p. 311 and following.