COCH′INEAL. Syn. Coc′cus (B. P.), L. Grana fina, Span. The Coccus Cacti (Linn.), an insect found upon the cactuses of Mexico. It is of great value as a dye stuff. The female insects, when matured, are brushed off the plants and dried by artificial heat. The entire insect is used. There are two varieties known in commerce—silver cochineal, which has a purplish-grey or silver-grey colour; and black cochineal, which is smaller, and of a reddish or purplish-black colour. The former is that commonly met with.
Adult. Genuine cochineal has the sp. gr. 1·25. It is commonly increased in weight by slightly moistening it with gum-water, and then rouncing it in a bag, first with sulphate of baryta, and then with finely powdered bone-black. In this way its sp. gr. is raised to 1·35, in consequence of being loaded by about 12% of useless foreign matter.
Herr Durwell, a German chemist, states that he found a sample of cochineal adulterated with sulphate of zinc. He thinks the sophistication was probably effected by immersing the cochineal in sulphate of zinc, and then in an alkali, whereby the white pulverulent aspect of the genuine article was imparted, and the weight increased.
The following is a method which has been given for estimating the value of samples of cochineal:—Grind the samples to be tested to a fine powder, weigh out 2 or 21⁄2 grammes, and boil this amount in a capacious narrow-necked flask, with 750 c. c. of water for 1 hour; filter immediately through dry paper filters, and allow it to cool. To test it 50 c. c. are measured in a flask of that capacity, and poured into another flask of about 200 c. c., and the measuring vessel rinsed with a definite quantity of water, say 10 to 15 c. c. A weak solution of permanganate is then run in from a burette with a glass cock, the flask being shaken after the addition of every 10 c. c. So much permanganate solution is then added that the cochineal extract shall be charged from its original colour to a pink of the faintest shade—almost yellow, in fact, but never reaching a full yellow. This pink shade should be persistent, that is, it should not turn yellow after standing fifteen minutes; and after a little practice it will be found very easy to obtain the tinge, which shows that the colouring matter is almost but not quite destroyed.
When a number of samples are to be compared, arrange an equal number of 200 c. c. flasks and test-tubes on the table, a tube standing in its rack in front of each flask. Then the same number of c. c. of the permanganate solution (which should be, at least, so weak that bulk for bulk of this and the cochineal solution will be required) is run into each flask, taking care to use too little to completely destroy the colouring matter in all.
The flasks are well shaken and allowed to stand for ten minutes. Part of the contents of each is then poured into the corresponding test-tube, and a glance of the tubes as they stand side by side will show which is the least affected by the bleaching liquid. This sample having been selected to serve as a standard, the contents of the test-tube are returned to the flask, and more permanganate solution is cautiously added, until a very faint pink tinge, which a fraction of a c. c. will turn to a full yellow, is obtained. The number of c. c. used having been noted, a fresh trial is made, in which the c. c. required, minus one, are used, the flask agitated, and the last c. c. or part of it, as the whole may not be necessary added.
If the two results agree, the next sample is treated in the same way, and so until all are tested.
A final trial may be made by measuring 50 c. c. of each solution into its flask, running in the permanganate in the ascertained amount into each as quickly as possible, letting the flasks stand ten minutes, and then making a comparison of all in the test-tubes. If the shades are not exactly alike, a pretty good guess can generally be made of the fractions of c. c. required, which should be added, the contents of the tubes being joined to that in the flasks, and a second or third comparison thus made.
This is a rather long description of what is in practice a very simple and good process, the three principal points to be borne in mind being—
1st. To use a weak solution of permanganate.