34 &

DYEING AND CALICO PRINTING .

For industrial purposes—that is to say, in dyeing—are used (i), the woodreduced to powder or shavings; (2), the cold infusion; (3), the decodlion ;(4), the extracts, in a more or less concentrated state, and sometimes eveninspissated so as to form a dry material. The tindtorial value of these sub-stances is estimated by a process of dyeing on the small scale precisely similarto that described for the Brazil woods ; but since logwood is richer in colouringmatter, only 5 grms. weight of that wood are taken for 25 square centimetresof tissue. The mode of manufacture of the extracts of logwood on the largescale will be afterwards separately treated. Logwood yields, with aluminousmordants, deep violet-grey shades ; with proto-salts of iron a bluish-black, andwith per-salts a black of a more brownish shade ; a mixture of both yields afiner black than either separately. With chromates of potash logwood yieldsa black colour, which on exposure to light gradually takes a greenish shade.These colours are, however, unstable, and readily destroyed by the aCtion oflight, soap, alkalies, and acids. Logwood violet or logwood black is thereforevery readily recognised by simply applying to the dyed fabric a drop of a some-what concentrated acid, which has the effeCt of turning these colours red,while if the black be that of nut-galls the colour is changed, but not turned red.Logwood is very extensively used in the dyeing of cotton, wool, silk, leather,and in the preparation of some steam-colours: the tone of the colours variesaccording to the mode of application.

Logwood blue is produced upon cotton by working the goods in a beck con-taining a liquor or decodtion of logwood, varying in specific gravity from 1*007to i*oi6, with the addition of acetate of copper. The liquid is raised from anordinary atmospheric temperature to 50° in one hour. The blue thus pro-duced resembles a vat-blue, and is very intense: this process of dyeing maybe executed by alternately passing the woven fabric through a copper bathand logwood bath.

In order to produce uniformly black-coloured grounds upon cotton, it is firstmordanted with a suitable mixture of pyrolignite of iron and acetate ofalumina, sometimes with the addition of nitrate of soda, to promote theoxidation of the iron. The dyeing operation is performed starting at 30°, andheating up to boiling; sumach or quercitron are occasionally added to thedye-beck. Uniformly black-dyed fabrics generally admit of being discharged iftreated with oxalic acid. The printing of logwood colours upon cotton doesnot differ from the method just described herein, excepting that the aluminousor ferric mordant is printed on by the engraved roller instead of being uniformlypadded. In order to fix the chrome logwood black, as it is termed, a mixtureof a logwood liquor, properly thickened, is printed on, and, after drying, thefabric is placed in a bath of bichromate at 40°. The black colour thusproduced is fast as compared with the other logwood blacks, and is the resultof a special oxidation which haematoxylin undergoes under the influence ofchromic acid : the lake thus produced contains oxide of chromium. Anotherkind of logwood black is obtained by printing a mixture of a logwooddecodtion to which protosulphate of iron and persulpho-nitrate have beenadded, and by passing the cloth so prepared through tepid milk of lime. Bythe aid of the bichromate process cotton-yarn or woven fabrics may be verycheaply dyed black: to 500 litres of the logwood extradt, 1*007 sp. gr., areadded 1*5 kilos, of bichromate of potassa, previously dissolved in water, to