494
DYEING AND CALICO PRINTING.
When the crude catechu of commerce is exposed to a temperature of ioo°,it fuses, becomes transparent, losing from 4 to 5 per cent of its weight ofwater; on ignition in a platinum crucible, there is left from 3 to 4 per cent ofmineral incombustible matter. Catechu being an extra#, is liable to beadulterated with various other substances, among which sand, clay, ochre,dried blood, sugar, and coarse starch are the chief; the careful examinationof the physical and organoleptical characters of the substance, as well as ofthe quantity and nature of the residues insoluble in water and alcohol, thequantity and nature of the ash, may give some idea of the purity of thesample, which ought to be entirely soluble in hot water, and then give a brown,not blackish, solution. It ought not to be totally soluble in cold water, for thatis an indication of heating and partial decomposition. The hot, waterysolution should, on cooling, deposit a portion of the catechu in a fine,granular state. The only reliable test for the quality of catechu is to makesome colour from it, or to dye up samples from it in comparison with an equalweight of a known sample.
Catechu is employed* in medicine, for tanning skins, and for dyeing.For this latter purpose, catechu has been used in India from time immemorial.It was not employed in Europe until in 1806, when MM. Schaeppler andHartmann first used it as a brown colour in conjunction with madder work.Their method of application described by Dr. Dingier f was, however, notadopted ; and catechu was hardly used for dyeing or printing to any extentuntil- 1829, when M. Barbet, of Jouy, re-introduced it. Catechu yields,when boiled with water, a turbid decoCtion; the infusion above named iscommonly also turbid : but when there are added to the water 8 parts ofvinegar (sp. gr. 1*007, containing 4 per cent of anhydrous acetic acid), and theboiling continued for some time, a rather clear solution is obtained. Catechuis also readily soluble in caustic soda lye of moderate strength. An aqueousdecoCtion of catechu yields, with calico mordanted with alum, a deepyellow, and with iron mordants an olive. This shade becomes brighterand far more beautiful by the addition of glue; while by being passed througha solution of bichromate of potassa, the shades of both mordants are madebrown. When a small quantity of acetate of copper is added to the decoCtionof catechu, it yields with alumed calico a kind of oak-brown colour, and withan iron mordant a deep cinnamon-brown. If the solution or decoCtion ofcatechu is aCted upon by a solution of bichromate of potassa previous tobeing fixed upon the cloth, it becomes unsuitable for use.
Generally speaking, catechu is of no use for dyeing purposes unless theoxidation takes place upon the tissue itself; indeed, the colourable matterpresent in cutch (the catechuin) is only soluble and capable of penetratinginto the fibres as long as it is not oxidised, for when once the oxidationof catechuin has taken place, it becomes totally inert and insoluble, and isthen only capable of being mechanically fixed. The processes by whichcolours derived from catechu are obtained are based upon the following prin-ciples The catechuin is applied in solution on the fibre which is penetrated
* “ Bulletin de la Societe Industrielle de Mulhouse,” vol. xii., pp. 354, 373; vol. xiii., p. 291;vol. xiv., p. 197.
t Dingler’s “ Polyt. Journ.," ii., No. i, 1815. This mixture is printed on the calico, is nextsteamed, and afterwards passed through a solution of bichromate of potassa.