From all these characters, taken together, the purchaser can form quite a correct idea of the value of indigoes in general; and the greater number of dyers, both in Europe and this country, are satisfied to make their purchases with only this physical examination. The most experienced dealers in this country make no other examination than the physical one. An eminent indigo broker in Boston has permitted me to copy the following memoranda for the physical examination of indigo from his notebook.
The chief signs of good indigo are its lightness, feeling dry when touched, and, when broken, appearing of a beautiful violet blue. Good indigo swims in water; if thrown upon burning coals it emits a violet-colored smoke, and leaves but little ashes.
In selecting indigo the large regularly formed cakes should be preferred,—those of a fine, rich blue color, extremely free from the white adhesive mould[1], and of a clean, neat shape. When broken, it should be of a bright purple cast, of a close and compact texture, free from specks or sand, and when rubbed with the nail should have a beautiful shiny coppery appearance; when burnt in a candle it should fly like dust; that which is heavy and dull colored should be rejected. Indigo is estimated and classed in commercial language, as follows: fine blue, ordinary blue, fine purple, inferior purple, and violet, strong copper, and ordinary copper. It is purchased by the factory maund (74⅔ lbs. The Bazaar maund is 82²⁄₅₀ lbs.), packed in cases containing on an average 2¼ cwt., dammered (pitched) and covered with gunny bagging.
------------------------------------------------------------- [1] Many experienced purchasers in this country pay no regard to this mould, as it weighs scarcely any thing.—Ed. -------------------------------------------------------------
Still, in making large purchases, as a measure of wise precaution the chemical test should be added. This is used to ascertain the proportion per cent of indigotine which a given indigo has. The determination of the quality of indigotine contained is not alone sufficient to fix the value of an indigo. With an equal yield of indigotine, the indigoes are always to be preferred which have a light and soft paste; and for the preparation of the indigo vat the preference should be always given to the violet red rather than to the clear blue indigoes.