Sap Green.—This lake, also known as bladder green, is obtained from unripe yellow (Persian) berries. It can be used as a water colour, but not in oil painting. This pigment differs from the ordinary yellow berry lake in colour and chemical composition; the yellow lake is a compound of xanthorhamnin with the metallic oxide, whilst sap green contains an uncrystallisable bitter substance, rhamnocathartin.
The lake is obtained from yellow berries which are not quite ripe. They are broken up and left in a warm place. The mass soon ferments; after about ten days it is pressed. Four parts of the liquid are then mixed with 0·5 part of alum and 0·5 part of potassium carbonate. The salts are dissolved in boiling water, and the solutions added to the hot sap. The mixture is then evaporated to the consistency of a thick syrup, which is generally packed in animal bladders, hence the name of ”bladder green”. Some care is necessary in the evaporation to prevent the burning of the soft mass on the bottom of the vessel. If the evaporation is carried somewhat further the mass solidifies on cooling; it is then black, and only transmits green light at the edges. The use of potash in preparing sap green is attended with the disadvantage that the hygroscopic nature of the salt prevents the colour from drying. If magnesia is used instead of potash the lake dries far more rapidly, but has much less covering power. Sap green is principally used for colouring paper and leather.
Orange-coloured lakes also can be obtained from yellow berries by precipitating the decoction with stannic chloride. These lakes are not directly employed, but are produced immediately upon the fabric in dyeing.
Chinese Green, Lokao.—Under this description a green lake has recently been imported from China. It is a valuable pigment. Chinese green is sold in flat cakes, which are blue, with a green or violet lustre. The powdered lake is pure green and partially soluble in water; it contains colouring matter, water and inorganic matter consisting chiefly of clay and lime.
Chinese green is made in China in a peculiar manner from the twigs of certain species of Rhamnus. According to report, the bark is boiled with water and a cotton cloth immersed in the decoction. The fibre fixes a colourless substance which becomes green on exposure to air. The cotton is repeatedly dipped into the decoction until it has absorbed a large quantity of colouring matter. It is then washed with cold water and boiled with water upon which cotton yarn is laid. The colouring matter suspended in the liquid adheres to the yarn, which is washed with a little cold water, and the colouring matter then collected upon paper and dried.
If Chinese green is really made by this process, it is not a lake but a vegetable colouring matter, to which clay has been added to increase its weight or to make it plastic. Very fine lakes can be obtained from Chinese green by dissolving it in a solution of alum, and adding soda solution. When Chinese green is dissolved in acetic acid and ammonium chloride and a zinc salt added, a blue lake is produced on the further addition of sodium acetate. A bluish violet lake is obtained by treating Chinese green with a strong reducing agent and adding calcium acetate.
Charvin’s Green.—The very high price at which genuine Chinese green was sold whilst it was still a novelty, gave rise to attempts at imitation. Charvin, of Lyons, succeeded in producing a pigment from Rhamnus which has the characteristic property of Chinese green of retaining its colour in artificial light. He plunged the bark of common buckthorn into boiling water, boiled for a few minutes, and allowed the whole to stand for twenty-four hours. Lime water was added to the brown liquid, which was then exposed to the action of the air in shallow vessels, when it gradually turned green and deposited a green precipitate. When this appeared, the liquid was brought into glass vessels and potassium carbonate solution added so long as a precipitate resulted, which after drying had all the properties of genuine Chinese green.
Although for some time Chinese green was a very fashionable colour, its use is now almost discontinued, although it can be used with advantage as an artists’ pigment, and its preparation by Charvin’s method is neither difficult nor specially costly.
CHAPTER LXIV.
BROWN ORGANIC PIGMENTS.
Asphaltum is found in large deposits in several regions of the earth. It is a compound of carbon and hydrogen, and varies in appearance from cobblers’ wax to tar. Asphaltum is of organic origin and is closely related to the mineral oils. Among the pigments usually employed by artists asphaltum plays an important part; it produces very warm shades between brown and deep black. The preparation of asphaltum for artists’ use is very simple. Good uniform lumps, free from sand and other impurities, are coarsely powdered and mixed with a solvent in a well-stoppered flask. Asphaltum readily dissolves in essential oils, and also, though with more difficulty, in fatty oils. It dissolves slowly at the ordinary temperature, but quickly on warming. On account of the inflammability of the essential oils generally used, certain precautions must be taken. The asphaltum is mixed with turpentine in a large flask which is well closed and heated by hot water. In a short time a viscous mass is formed which is mixed with the solvent by shaking.