(b) Boil 1 lb. of the dyestuff with ½ lb. alum in 1 gal. water, and precipitate by solution of carbonate of potash.
(c) Boil 4 oz. annatto and 12 oz. pearlash in 1 gal. water for ½ hour; strain, precipitate by adding 1 lb. alum dissolved in 1 gal. water till it ceases to produce effervescence or a precipitate; strain and dry.
For information concerning the numerous coal-tar colours now largely manufactured into lakes, the reader is referred to “Spon’s Encyclopædia of Industrial Arts.”
CHAPTER X.
LUMINOUS PAINTS.
The luminosity of minerals has an obvious practical value in the case of such substances as can be conveniently applied in the form of a paint to surfaces which are alternately exposed to light and darkness, such exposed surfaces emitting at one time the light which they have absorbed at another. Familiar illustrations are street plates, buoys, and interiors of railway carriages having to traverse many tunnels. The light absorbed may be either daylight or powerful artificial light. With this object, several compositions are prepared under the generic name of luminous paints. They are chiefly as follows:—
(1) Balmain’s.—This consists of a phosphorescent substance introduced into ordinary paint. The phosphorescent substance employed for the purpose is a compound obtained by simply heating together a mixture of lime and sulphur, or substances containing lime and sulphur, such as alabaster, gypsum, &c., with carbon or other agent, to remove a portion of the oxygen present; or by heating lime in a vapour containing sulphur. In applying this phosphorescent powder, the best results are obtained by mixing it with a colourless varnish made from mastic and turpentine; drying oils, gums, pastes, sizes, &c., may, however, also be used.
(2) A French compound.—100 lb. of a carbonate of lime and phosphate of lime produced by the calcination of sea-shells, and especially those of the genus Tridacna and the cuttle-fish bone, intimately mixed with 100 lb. of lime rendered chemically pure by calcination, 25 lb. of calcined sea-salt, 25-50 per cent. of the whole mass of sulphur, incorporated by the process of sublimation, and 3-7 per cent. of colouring matter in the form of powder composed of monosulphide of calcium, barium, strontium, uranium, magnesium, aluminium, or other mineral or substance producing the same physical appearances, i. e. which, after having been impregnated with light becomes luminous in the dark. After having mixed these five ingredients intimately, the composition obtained is ready for use. In certain cases, and more especially for augmenting the intensity and the duration of the luminous effect of the composition, a sixth ingredient is added, in the form of phosphorus reduced to powder, which is obtained from seaweed by the well-known process of calcination. As to proportion, it is found that the phosphorus contained in a quantity of seaweed, representing 25 per cent. of the weight of the composition formed by the five above-named ingredients, gives very good results.
The phosphorescent powder thus obtained and reduced to paste by the addition of a sufficient quantity of varnish, such as copal, may serve for illuminating a great number of objects, by arranging it in more or less thick coatings, or by the application of one or more coatings of the powder incorporated in the varnish, or by varnishing previously and sprinkling the dry powder upon the varnish. The amount of powder applied should not exceed the thickness of a thin sheet of cardboard.
The dry phosphorescent powders are also converted into translucent flexible sheets of unlimited length, thickness, and width, by mixing them with about 80 per cent. of their weight of ether and collodion in equal parts in a close vessel, and rolling the product into sheets, with which any objects may be covered which are intended to be luminous in the dark. The powders may also be intimately mixed with stearine, paraffin, rectified glue, isinglass, water glass, or other transparent solid matter, in the proportion of 20 to 30 per cent. of the former with 50 to 80 per cent. of either of these substances, and this mass is then reduced into sheets of variable length, width, and thickness, according to their intended applications. A luminous glass is also manufactured by means of the powders, by mixing them in glass in a fused state in the proportions of 5 to 20 per cent. of the mass of glass. After the composition has been puddled or mixed, it is converted into different articles, according to the ordinary processes; or after the manufacture of an object still warm and plastic, made of ordinary glass, it is sprinkled with the powders, which latter are then incorporated into the surface of the article by pressure exerted in the mould, or in any other suitable way.
It has been observed, after various trials, that the passage of an electric current through the different compositions augments their luminous properties or brilliancy to a great extent; this peculiarity is intended to be utilised in various applications too numerous to describe, but of which buoys form a good example. The current of electricity is furnished by plates of zinc and copper mounted on the buoy itself, when the latter is used at sea; but in rivers and fresh-water inlets the battery will be carried in the interior of the buoy. To secure the full effect, 10 to 20 per cent. of fine zinc, copper, or antimony dust is added to the phosphorescent powder described.