or the lead chromate present in caustic potash have been devised, but the author does not consider that these give as satisfactory results as the scheme just described. The section on the analysis of Brunswick greens may be consulted. This pigment is the chromate of zinc, Zn Cr O4, which has the Zinc Oxide, Zn 0, Chromium trioxide, Cr 03, 100.00 44.63 per cent. 100 00 PREPARATION OF ZINC CHROME.-Zinc chrome is prepared in two ways-1st, by precipitating a solution of zinc sulphate with a solution of chromate of potash; 2nd, by treating zinc oxide with either chromic acid or potassium bichromate. 1st. By Precipitation. The preparation of zinc chrome by precipitating solutions of zinc salts is very difficult, and only by the exercise of some care is the operation a success; this arises from the fact that zinc chromate is a substance which, while insoluble in water, is readily soluble in acids and alkalies, and in various saline solutions. If the solutions used are slightly acid the zinc chromes will not form, consequently the acid bichromates cannot be used for preparing it. If the solutions are alkaline, then there is a tendency for the chrome to be decomposed, and for the white hydroxide of zinc to be formed. The materials used in the preparation of zinc chrome are the sulphate of zinc and the chromate of potash; the chloride of zinc may be used or the chromate of soda. The principal points are the use of neutral solutions as strong as possible. 61 lbs. of zinc sulphate are dissolved in as small a quantity of water as possible, and the solution is boiled; while boiling, a solution of 32 lbs. of normal sodium chromate in water is added, and the mixture boiled for one hour; the zinc chrome is precipitated, and can be collected on a filter, washed, and dried at a low temperature. The solution of zinc should be neutral; as zinc sulphate is liable to be slightly acid the solution should be tested with a piece of paper dipped in a solution of Congo red, and if found to be acid, shown by the paper turning blue, then the solution should be neutralised by adding sufficient caustic soda. The solution of normal sodium chromate can be made by dissolving 26 lbs. of sodium bichromate in water, heating to the boil, and adding sufficient caustic soda to just neutralise the bichromate; this is best ascertained by the use of test paper made from a solution of phenolphthalein in methylated spirit, the caustic soda being added until a drop of the chromate solution taken out just turns the paper a faint pink. The solutions are now ready, and can be used as described above. The chrome so prepared is of a fine, deep, lemon-yellow colour, with a good body. The reaction between the zinc and sodium salts is expressed in the equation: Zn S 04 + Na2 Cr 04 Zinc sulphate. Sodium chromate. = Zn Cr 04 + Na2SO4. Zinc chromate. That with the potassium chromate is similar. Sodium sulphate. Another plan of making zinc chrome, and which gives good results, is to dissolve 61 lbs. of zinc sulphate in water, and to add 261 lbs. of sodium bichromate dissolved in a small quantity of water; no precipitate will be obtained. The mixture is now boiled and, while boiling, a saturated solution of soda crystals is added in small quantities at a time, waiting between each addition until all effervescence has ceased; the addition is continued until there is little or no effervescence on adding more soda crystals; the whole is now boiled for about one hour, during which period the zinc chrome will gradually precipitate, and may be filtered and finished in the usual way. Care must be taken not to add too much soda crystals or, otherwise, only a precipitate of zinc carbonate will be obtained. In both methods potassium chromate may be substituted for the sodium salt. 2nd. From Zinc Oxide.-This method was the subject of a patent taken out by James Murdoch in 1847, who describes the preparation of three shades of zinc chrome. 1. A Marigold Tint.-125 lbs. of bichromate of potash are dissolved in 70 gallons of water and boiled; 60 lbs. of zinc oxide are mixed into a thin cream with water and then poured into the bichrome solution; the mixture is allowed to stand for from 24 to 30 hours; after which it is boiled up for one hour, allowed to settle, and the colour finished as usual. The liquor from the colour is kept and used in making the second tint. 2. A Lemon Tint.—The liquor from the last tint, together with the first washings, are put into a boiler, and a solution of 75 lbs. of zinc oxide in 44 lbs. of sulphuric acid of 65° Tw. is added and the mixture is boiled for one hour; the colour formed is allowed to settle, the clear top liquor run off and kept for making the third tint, and the colour finished as before. 3. A Pale Tint.—The liquor from the second tint is now mixed with a solution of 15 lbs. of oxide of zinc in 7 lbs. of sulphuric acid, boiled and treated as before. Clarke subsequently took out a patent in 1853, in which he describes the preparation of two shades of zinc chrome. 1. A Dark Chrome.-112 lbs. of bichromate of potash are dissolved in boiling water and 70 lbs. of zinc oxide, previously mixed with 35 gallons of water to a thin cream, added, and the mixture boiled for one hour; then the colour is allowed to settle, the top liquor poured off and used for making a paler tint (see below), and the chrome washed and finished. 2. Lemon Zinc Chrome-The liquor from the deep chrome is boiled down till it has attained a strength of 26° Tw.; to every 8 gallons of this liquor 40 lbs. of zinc oxide, previously dissolved in 24 lbs. of sulphuric acid, are added; the mass is now boiled for one hour, the chrome allowed to settle, and, after decanting off the top liquor, washed and finished as usual. Instead of potassium bichromate, chromic acid may be used with similar results. The following recipes may be used :- 1. A Deep Chrome.-Mix 100 lbs. of zinc oxide into a thin paste with 30 gallons of water; then add, by degrees, boiling the whole of the time, 130 lbs. of chromic acid; after the whole has been added, continue the boiling for one hour longer; then allow the colour which has been made to settle, decant the top liquor and wash. 2. Middle Chrome,-Use 200 lbs. of zinc white and then proceed as above. 3. Pale Chrome.- Use 300 lbs. of zinc oxide, and proceed as above. With all the processes just described there is great loss of chrome, as the liquors which are run off and the washings contain chromic acid, both free and in the form of chromates; these liquors are somewhat difficult to utilise; they might be used to make lead chromes, especially the oranges. This waste of chromic acid in making zinc chromes by these processes adds considerably to the cost. Another defect in the colours thus made is that they are very liable to be gritty, especially if the excess of chromic acid or alkaline chromate is not thoroughly washed out, for, if left in, this tends to crystallise in the colour; the presence of this chromate in the colour will lead to the colour, when mixed with oil, turning greenish, due to the oxidising action of the chromic acid on the oil. The lead chrome plant shown in Fig. 18 may be used for making the zinc chromes. PROPERTIES OF ZINC CHROME.-Zinc chrome is a yellow pigment of good colour and body; it is readily soluble in acids; all the common mineral acids dissolve it, even when dilute; it is also soluble in many organic acids. Caustic soda or potash, when mixed with it in small quantities, decompose it with the formation of zinc hydroxide and a chromate of the alkali; when in considerable excess they dissolve it. Ammonia will dissolve zinc chrome. Heat turns it a grey-violet tint, due to its decomposition into the oxides of zinc and chrome. As a pigment it is quite permanent, resisting exposure to light and air when well made; while sulphureous gases do not affect the colour. It can be mixed with all other colours without being affected by them in any way. Zinc chrome has usually a good colour and body, although in this respect it is scarcely equal to the lead chromes. ASSAY AND ANALYSIS OF ZINC CHROMES.-In assaying zinc chromes the points to be noted are:-1st, colour; 2nd, covering power; 3rd, texture or freedom from grit, so as to have a soft feel; these points can be tested for in the usual way. Zinc chromes are liable to adulteration with the lead chromes, as these are rather cheaper, or to form the pale shades the deep shades may be let down with a cheaper white base than zinc oxide. Pure zinc chrome is soluble in acetic acid without effervescence; any yellow residue indicates adulteration with lead chrome or yellow ochre, which can be distinguished by the application of special tests. A white residue indicates adulteration with barytes, china clay, &c.; while admixture with either whiting or white lead will be indicated by effervescence with the acid. Zinc chrome should not be discoloured on adding to it a little ammonium sulphide. A sample of zinc chrome analysed by the author had the composition: LEMON CHROME. BARIUM CHROME, This pigment is the chromate of barium, Ba Cr 04, prepared by precipitating a solution of barium chloride with potassium bichromate. Its use as a pigment has become obsolete owing to its having only a very pale yellow colour and to its want of body. The lemon chromes made from lead are much brighter and have more body. OCHRES AND SIENNAS. Next to the chromes the ochres and siennas are by far the most important of the yellow pigments. They form a group of natural pigments of inorganic origin, found in comparatively large quantities in many parts of the world. The ochres are generally of a good yellow colour, varying from a faint brownish to a reddish tint of yellow; the siennas (which are so called because they were first found near the town of Sienna, in Italy) are of a brownish-yellow tint, varying somewhat in depth of shade. Ochres occur in rocks of all geological ages; in the Oolitic rocks of Oxfordshire, the Mountain limestone of Derbyshire, the Silurian slates of Wales, the Granites of Cornwall, the Liassic and Cretaceous rocks of France, and in the volcanic rocks of Italy; so that geological age has no connection with their for mation. Ochres and siennas consist essentially of an earthy base, coloured, in the case of the ochres, by the hydrated ferric oxide; in the case of the siennas, by the hydrated ferric oxide and |