476

DYEING AND CALICO PRINTING.

called a “reserve” or “resist,” that is, a composition which prevents thecolour from fixing itself on the fibre ; the chief ingredient for that purpose issulphate of copper, which adts by prematurely oxidising the indigo, and thusprevents its fixation. In both cases the pieces are passed through a weaksulphuric acid bath to insure the perfedt fixation of the indigo. Formerly thecopper thereby liberated from the fabrics was completely lost, but Mr. J. Leesedevised, some years ago, a method of saving this metal; to effedt this, thediluted solution of sulphate of copper is made to filter through vessels con-taining wrought-iron turnings ; the acid dissolves the iron, which may beused as sulphate of protoxide of iron for future operations, whilst the copperdeposited may be used again either to prepare sulphate of copper or utilisedin another manner. As an instance of the importance of small savings, itmay be stated that by this ingenious but apparently trifling improvement onefirm alone saved at least £3000 a year.

Dr. F. C. Calvert effedted, some years ago, an economy in this branch ofcalico printing and dyeing, consisting in extradting from the cold indigo-vats,which were considered to be exhausted, a certain percentage of the indigooriginally employed. Dr. Calvert observed that a green insoluble flocculentmatter, which was considered by chemists and printers to be simply oxide ofiron, was in reality a compound of indigo and iron. In order to recover thishe devised the following simple method :—The green pulp is conveyed fromthe exhausted vats into a general receptacle, and there mixed first with asmall quanity of hydrochloric acid, so as to remove the excess of lime,allowing the green pulp to settle and running off the liquor. The green pulpthus purified is next treated with strong hydrochloric acid, whereby chlorideof iron is produced and indigo liberated, which, after having been washedwith water, is again fit for use.

Although in a later portion of this chapter more details will be given onprinting with indigo, we give here the following brief outline, which is one ofgreat difficulty, but successfully carried out by various methods : —That usuallyemployed consists in mixing very finely powdered indigo with orpiment orprotochloride of tin and a caustic alkali, a process which is further facilitatedby printing the pieces in an atmosphere of coal-gas, as first devised byMr. Bennett Woodcroft, and carried out by Messrs. T. Hoyle and Sons, ofManchester. Of late years Mr. J. Leese, of the firm of Kershaw, Leese, andCo., has succeeded in applying the following method, first devised by Fritzsche:—The indigo is ground to an impalpable powder, and then mixed with glucose,lime, and caustic soda, in such proportions as are needed to produce the shaderequired. These materials are all mixed cold, and the cloth when printed ispassed through a steam chest, wherein it is exposed for from 30 to 60 seconds.In this short period the indigo is completely reduced, and thereby renderedsoluble in the alkalies. It enters, penetrates, and combines* with the fibre.On emerging from the steam chest it becomes oxidised and fixed by exposureto the atmosphere ; or the pieces may be immersed in a solution of an

* According to a series of experiments made in Java, the results of which have not beenmade public, by the late Dr. Fromberg, aided by Dr. Croockewit and other scientific andpractical men, vegetable fibres, also wool, silk, and hair, are not only penetrated by porositythrough the fibre, but actually enter into chemical combination with the white indigo, andform a definite compound therewith.