476 TEXTILE FIBRES, YARNS, FABRICS
mixture of a mordant colouring matter fixed by means of chromium and
an ordinary acid colouring matter may be identified by extracting in a
suitable apparatus with pyridine, which removes the acid colouring matter,
whereas the mordant colouring matter remains totally fixed on the fibre
and may hence be identified. The pyridine is distilled off and the acid
colouring matter in the residue then fixed on wool and identified by means
of the corresponding table.
(/) To separate indigo and other vat colouring matters from mordant
and acid colouring matters the following procedure may be followed: In
an ordinary extractor connected with an air condenser is placed a piece
of the sample previously dried in an oven, this being covered with a layer
of wool and a thermometer arranged with its bulb in contact with the sample.
Extraction is carried out with a mixture of 100 parts of cresol (commercial
97-98% cresylic acid) and 30 parts of " solvent naphtha " (crude xylole)
having the b.pt. 125-140°, or with a mixture of 75 parts of cresol with 25
parts of heavy petroleum benzine of b.pt. 155-170°. By this means
the temperature of the extractive liquid in contact with the sample is about
100-105° and should not exceed 110°. This treatment results in the extrac-
tion of the vat colouring matters, whilst most of the acid and mordant
colouring matters remain on the fibre and may then be identified.
(g) Logwood colours are separated from acid, mordant and vat colour-
ing matters by boiling with dilute hydrochloric acid, which usually takes
up only the logwood giving a crimson extract.
Red-wood and other dye-woods generally behave like logwood. Acid
alizarin reds are also extracted.
(h) With " vatted blacks " (mixtures of indigo and logwood) the indigo
alone is extracted by the above cresol or pyridine mixture, or the logwood
alone by boiling dilute hydrochloric acid.
(i) If an acid black is present in addition to the indigo and logwood,
the logwood is first extracted with dilute hydrochloric acid, the indigo
being then removed by the above cresol mixture, so that only the acid
colouring matter afterwards remains on the fibre.