11

the consistence of a thick paste. By this means all excess of ammonia is expelled. This préparation is used for dyeing silk, and to produce violet and mauve colours on woollen goods.

Carminé Lakes.—These very beautiful pigments are prepared from a décoction of cochineal, and not from carminic acid, the animal matter which the insect con-tains, appearing to be necessary to their production. The mode of preparing the finest qualifies iskept a secret bythe manufacturers ; but I will describe two processes, which give very satisfactory results. The first consists in boiling one pound of ground cochineal with two gallons of water, to which has been added one ounce of alum. It is then boiled for three minutes, the liquor is allowed to settle, and, after having been kept for several days, about an ounce of a bright carminé lake is pro-duced. For the alum employed in this process cream of tartar can be substituted. The second process consists in boiling for three hours, two pounds of powdered cochineal in thirty gallons of water. To this is added three ounces of pure saltpetre. The liquor is then boiled again and left to settle. The clear liquor is run off, and after two or three weeks’ yields a fine carminé lake.

As these lakes are expensive, they are often adulterated with starch, kaolin, vermillion, &c. The complété solubility of pure carminé lakes in ammonia affords a ready means of detecting these adultérations.

Kermes.—This colouring matter is also derived from a variety of Coccina, which lives on the species of oak called Quercus coccifera. The young female animal fixes itself under the epidermis of the leaves or young shoots of the oak, in the early part of spring. As the insect grows, it gradually swells out the epidermis, covering the surface of the leaves or branches with a multitude of excrescences. During this period it deposits its eggs.

In Spain and the South of France, during the month of June, or just before the eggs produced would be hatched, the animais are removed, and destroyed by placing them in the steam from heated vinegar.

Although this colouring-matter is seldom used in England, it is extensively employed in the South of France, in Spain, Morocco, and Turkey, to dye morocco leather, and to dye woollen cloth with that particular shade which characterises the cap called “fez,” worn by the Asiatics.

If the colour is not so brilliant as that of cochineal, it has the advantage of not being changed by soap or dilute alkalies. It is also employed at Milan, Rome, and Florence to colour a very favourite beverage known as alkermes. The colour-giving principle of this insect is identical with that of cochineal, and has been used as a dye in the East from time immémorial.

Gum-lac.—This is another variety of the Coccina, which lives especially on the Ficus, or fig tree. They reproduce themselves with such rapidity and in such numbers that they entirely cover the surface of the branches of the trees on which they are deposited. Owing to a resinous fluid which they secrete, they form solid masses, which are often a quarter of an inch. thick, all round the branches, and adhere very firmly to them. The natives break off these branches just before the hatching season of the animal, and expose them to the sun to kill the insect. These gum-lac twigs are sold under the name of stick-lac. Those of Siam are considered the best, those of Assam next, and those from Bengal the worst.

There are three kinds of lac in commerce—stick-lac, which has just been described, seed-lac, and shell-lac. The resinous concrétion is taken from the twigs, coarsely powdered, and triturated with water in a mortar. The greater part of the coloring principle is thus dissolved.

The granular portion which remains is dried in the sun, and constitutes seed-lac. Shell-lac is obtained by melt-ing seed-lac, and straining whilst hot. It is then dropped upon smooth stems of the banyan tree, and so runs into thin plates, which are known in commerce under the name of shell-lac.

These three lacs have the following composition :—

Stick-lac. Seed-lac. Shell-lac.

Resin 68'0 88-5 90-9

Colouring matter 10-0 2:5 -5 -

Wax 6-0 4-5 4-0

Gluten 5-5 2-0 2-8

Foreign matters 6-5

Loss 4-0 2-5 i-8

100-0 100-0 100-0

The colouring matter of the insect is identical with that of cochineal and kermes, and it has been employed as a scarlet dye-stuf in the East, from time immemorable.

Lac-lake and Lac-dye are préparations imported into this country from India since 1796. They are both prepared by acting on stick-lac by a weak alkaline solution, to which is then added a solution of alum. This produces a precipitate, which, when washed and dried, is ready for use. Although both these lakes are prepared with the same substances, lac-dyeis considered much superior in quality. This is due to the greater care bestowed on its préparation. The details of the process are kept secret.

To dye woollen cloths with them, they are dissolved in a weak solution of vitriol, to which is added a little oxy-muriate of tin, and the cloth dipped in when the liquid is near the boil. It only requires washing and finishing to be ready for market.

Some years ago, Messrs. E. Brooke and Co., of Manchester, introduced a lac-dye much superior to that imported from India, which they préparé by treating stick-lac with weak ammonia, and adding chloride of tin to this solution, when a fine red precipitate is formed, which, collected, is ready for use.

Murexide or Foman Prirplc.—Although this colour has now been superseded by those derived from coal-tar, I call your attention to it as an example of the assistance rendered by the progress of chemistry to the art of calico-printing.

In 1776, the illustrious Swedish chemist, Scheele, dis-covered uric acid in human urine. In 1817, Brugnatelli found that nitric acid transformed uric acid into a substance which he called erythric acid, but which was after-wards named by Wohler and Liebig alloxan. In 1818, Dr. Prout found that this latter substance gave, when in contact with ammonia, a beautiful purple red colour, which he named purpurate of ammonia—the product known by the name of murexide since the researches of Liebig and Wohler in 1857. These discoveries remained dormant in the field of pure science until the year 1851, when Dr. Saac observed that when alloxan came in contact with the hand it tinged it red. From this he inferred that it might be employed to dye woollen fabrics red, and further experiments showed that if woollen cloth, prepared with a sait of tin, were passed through a solution of alloxan, and then submitted to a gentle beat, a most beautiful and délicate pink colour was obtained.

In 1856, MM. Depouilly, Lauth, Meister, Petersen, and A. Schlumberger, applied it as a dying material to silk and wool, and succeeded in producing red and purple colours, by mixing the murexide with corrosive sublimate, acetate of soda, and acetic acid.

For printing upon cotton, a mixture of murexide with nitrate of lead or acetate of zinc, properly thickened, was printed on the fabric, which was then allowed to dry for a day or two, when the colour was fixed by passing them through a mixture of corrosive sublimate, acetate of soda, and acetic acid.

The uric acid required for the préparation of such large quantifies of murexide, was obtained from Peru-vian guano. The guano was treated with hydrochloric acid, and washed. The insoluble mass was then treated with nitric acid of spécifie gravity 1:40. When the action of the acid was completed, the mass was treated with warm water to dissolve out the alloxan. It was then care-fully evaporated to such a degree that it became solid on cooling. The solid mass had a brown or violet colour.

---

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 98,89 %.

La langue de reconnaissance de l'OCR est le Français.

---