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CHAPTER IX

DETECTION OF DYES ON YARN OR PIECES.

YERS generally judge of the composition of any colour which they meet

with, either by simple inspedlion or by certain routine processes whichhave been traditionally handed down from generation to generation. Even atthe present day there are many dyers who believe that the colouring mattersof commerce cannot be detected with certainty. Hence they employ theknown reactions of tindtorial substances as little as possible. In order toshow how far even a slight knowledge of chemistry will aid in the recognitionof dyes, we give simple diredtions, arranged under the six heads of Black,Blue, Yellow, Red, Green, and Violet.

The apparatus employed is very simple. For steeping the tissues orskeins of silk, cotton, or wool, a small white porcelain capsule is needed, andfor the application of heat a small iron tripod, with a spirit-lamp or Bunsengas-burner. The white colour of the porcelain renders it easy to observe thechanges of colour under the influence of reagents. in order to reduceportions of the tissue to ashes, a small platinum capsule or piece of platinumfoil is used, which may be heated to redness in the flame until all combustiblematter is employed and only the ash remains. The colour of the ashindicates the charadler of the dye, of which more below. It may be green,blue, grey, white, or red.

A. BLACKS.

1. Chrome Blacks.—Chlorine-gas, or solution of bleaching-Iime, turnsthese a kind of chestnut-brown. If exposed to bright sunshine they becomegreenish. If thoroughly burned they leave a greenish ash.

2. Logwood with Iron Mordants.—Dilute hydrochloric acid turns theswatch a cherry-red. The prompt application of a dilute alkali restores theblack. Solution of bleaching-lime discharges the black, leaving a yellowishground (oxide of iron). The cloth leaves a yellowish-brown ash.

3. Galls with Iron Mordants.—-This dye is chiefly found on the betterclass of silks. Hydrochloric acid gives no immediate red colour. Bleaching-liquor discharges the black, leaving the iron ground as above.

4. Logwood with Iron on a “ Woaded,” i.e., Vat-Blue, Bottom.—Hydrochloric acid has no immediate adtion, but gives in a few minutes adull purplish red spot. A mixture of hydrochloric and oxalic acids extradls a