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The presence of free sulphur in these pigments is likely to be a cause of want of permanence when used as a pigment.
ANALYSIS AND ASSAY.—For practical purposes it is not necessary to make a complete analysis of antimony vermilions. They are liable to adulteration by red lead, oxide of iron, or chrome orange. Red lead would be shown by the colour darkening when treated with hydrochloric acid, and, after solution, by crystals of chloride of lead separating out on cooling, and the application of the usual tests for lead. Oxide of iron can be distinguished by the solution in hydrochloric acid having a yellow colour and giving the characteristic tests for iron ; chrome orange can be detected by the colour of the acid solution being green and giving the tests for lead and chromium.
Antimony vermilions should be assayed for colour, both in the form of powder and when mixed with oil, and for covering power by the usual methods.
BRILLIANT SCARLET is the name given to the iodide of mercury, HgI2, prepared by carefully precipitating a solution of mercuric chloride with a solution of potassium iodide; it has a brilliant scarlet colour, but is very fugitive. It is rarely used as a pigment.
DERBY RED is the basic chromate of lead; its preparation and properties will be found on p. 116, et seq.
The CHROMATE OF MERCURY has been used as a red pigment and is prepared by precipitating a solution of mercuric chloride with potassium chromate; its cost and want of permanence has caused it to become obsolete as a pigment.
The CHROMATE OF SILVER has been proposed as a pigment; it has a dark red colour and is prepared by precipitating a solution of silver nitrate with a solution of potassium chromate. It is costly and fugitive.
The CHROMATE OF COPPER, a dark red coloured body prepared by precipitating solutions of copper with potassium chromate, has also been suggested, and probably used on a small scale, as a pigment; but it is fugitive, and, therefore, cannot be recommended for this purpose.
MAGNESIA PINK, prepared by calcining a mixture of magnesia and cobalt nitrate, has been little used ; it has but a pale colour and little body, although probably permanent.
THERE is a fairly large list of yellow and orange pigments derived from the vegetable, animal, and mineral kingdoms, the most important being the chromes and ochres; the others are only used in small quantities.
The chromes, as they are generally called, are a very important group of pigments varying in colour from a pale yellow through deep yellow, orange to bright red, and sold under a variety of names—primrose-chrome, lemon-chrome, chrome-yellow, orangechrome, scarlet-chrome, chrome-red, Derby-red, Americanvermilion, &c. The base of all these chrome pigments is the chromate of lead, Pb CrO, and its basic derivative PbO, PbCrO,.
Chromate of lead is capable of existing in the form of acid, normal and basic modifications; to the colour maker, only the last two are of any interest.
The normal chromate of lead is a deep-yellow coloured body having the composition
Lead, - - - - - - 63-99 per cent.
Chromium, - - - - - 16:23 ,
Oxygen, . - - - - - 1978 ,,
Lead oxide, Pb 0, . - - - 68.93 per cent.
Chromium oxide, CrO3, . - - 31-07 , ,
It is obtained as a bright yellow precipitate by adding a solution of bichromate of potash to one of acetate of lead ; the reaction is shown in the following equation —
Nitrate of lead may be used instead of the acetate, when the reaction becomes
From the above equations the equivalent quantities of the two compounds can be calculated, and are as follows:–650 parts of lead acetate or 662 parts of lead nitrate are equal to 295 parts of potassium bichromate, or 100 parts of lead nitrate require 44-5 parts of potassium bichromate, while 100 parts of lead acetate require 38.9 parts of potassium bichromate to precipitate them.
Lead chromate is insoluble in acetic acid and water, but soluble in moderately concentrated nitric or hydrochloric acids. When treated with a large excess of caustic potash or soda it dissolves, but when heated with a small quantity of the alkalies it is converted into the basic chromate, Pb, CrO3, Heated alone it first turns reddish-brown, and, finally, becomes greenish-grey with evolution of oxygen, while a mixture of the oxides of lead and chromium are left behind.
It has a specific gravity of 5.653.
The formation of the basic chromate from the normal chromate is shown in the equation
It is a scarlet-red powder of somewhat crystalline structure, with a specific gravity of 6.266; by friction it loses its crystalline form and changes colour, becoming orange; in other properties it resembles the normal chromate. It has the following composition –
Lead, - - - - - - 75-75 per cent.
Chromium, - - - - - 9-61 * >
Oxygen, ". - - - - - 14.64 ,,
Lead oxide, Pb 0, - - - - 81-61 per cent.
Chromium trioxide, CrO3, - - 18-39 * *
Both the normal and basic chromates when boiled with strong sulphuric acid are decomposed, lead sulphate and chromium sulphate being formed and oxygen evolved. Strong hydrochloric acid on boiling dissolves them, forming a green solution of lead and chromium chlorides, from which, on cooling, the lead chloride separates out, and the chlorine is evolved. When boiled with solutions of the alkaline carbonates, the chromates are decomposed, white carbonate of lead being formed and a solution of the alkaline chromate obtained. When boiled with solutions of caustic soda or potash the lead chromates dissolve ; on adding acetic acid to this solution a yellow precipitate of the normal chromate is obtained.
MANUFACTURE OF LEAD CHROMES.–In commerce the pigments having chromate of lead as the base are met with in a very great variety of shades from a very pale primrose-yellow to a deep red; as a rule, the deep shades are almost chemically pure, but the pale shades are obtained by mixing the pure chromate with the requisite quantity of a white base, such as sulphate of lead, barytes, gypsum, &c.; what are called “pure chromes” in the trade contain sulphate of lead, while the common chromes contain this body with barytes or some other white base in addition.
The preparation of these pigments may be grouped under three heads:—
1st. Preparation of chrome-yellows.
The same plant may be used in the preparation of all these chromes. The best form is that shown in Fig. 18 which, in its simplest form, consists of three tubs made of hard wood ; one of these, the largest, C, is placed on the floor, while the other two are placed above it on a small platform, P. In these two tubs (D D of the Fig.) are dissolved the salts used, the bichromate of potash and other precipitants in one, the lead salt in the other ; care being taken always to use the same tub for the same material. These tubs may measure 3 ft. 6 in. by 2 ft. 6 in. ; they will hold about 107 gallons each. The larger tub, C, may measure about 4 ft. in diameter by 3 ft. high, and will hold about 176 gallons. In this tub the colour is formed, and may, therefore, be called the precipitating or colour tub. The contents of D D may be heated by an arrangement of steam pipes, S S, and the colour tub should also be provided with a steam pipe, so that its contents can be heated up if required. At the bottom of D D are plugs, on opening which the contents can be run into the colour tub, C. For the purpose of running off the liquor from the precipitated colour in C, the latter is provided with a number of holes, H, H, H, H, at various heights in its side fitted with plugs, any one of which may be drawn out to permit the water or liquor to run away. In a plant of this kind about 2 cwt. of chrome may be made at one batch. It is advisable to provide several such sets of plant, which may be arranged as shown in the drawing, in which case one of the solution tubs, D, may be used along with two colour tubs, C. C. 1st. PREPARATION OF CHROME-YELLOWS.—The number of recipes which are in use for preparing chrome-yellows