The chrome yellows are the lead, zinc or barium salts of chromic acid. In describing the different methods used in making these colours, lead chrome yellow will be taken first, since this is the one most largely used.

Lead Chrome Yellow.

Both neutral and basic lead salts of chromic acid are known. Neutral lead chromate is found in nature as the somewhat rare mineral crocoisite, which is found in very small but perfectly shaped crystals in many lead mines.

Neutral Lead Chromate, PbCrO₄, is formed as a very heavy precipitate of a fine deep yellow colour when a solution of potassium chromate or bichromate is added to a solution of a lead salt in water. When exactly equivalent quantities of potassium chromate and lead solution are used, and the strength of the solutions is the same, the product has the same shade each time the operation is performed. It is not immaterial whether the one or the other salt is in excess, or whether strong or weak solutions are used; all these conditions modify the shade of the chrome yellow produced. Many colour makers are apparently of the opinion that some particular skill of the workman is necessary to produce chrome yellow of a particular shade. This is, however, not the case; manufacturers who know the simple conditions which are important in making chrome yellows may produce any desired shade without difficulty.

Neutral lead chromate readily parts with half the chromic acid it contains. When treated with alkalis, such as lime or caustic soda, or even when digested with finely ground litharge, the neutral salt gives up half of its chromic acid, and is converted into the basic chromate or chrome red, Pb₂CrO₅.

Basic lead chromate has, as the name chrome red indicates, a fine red colour. If the quantity of lime or caustic soda used is sufficient to decompose only a portion of the neutral lead chromate, a mixture of the yellow neutral and the red basic chromate is formed, the shade of which will incline to yellow or red according as it contains a preponderance of one or the other compound. The pigment known as orange chrome is a mixture of approximately equal parts of the neutral and basic lead chromates.

In order to brighten the deep yellow shade which distinguishes neutral lead chromate, it is either mixed with a white pigment, or a white substance (lead sulphate) is precipitated from the solution simultaneously with the lead chromate. In this manner all the imaginable pale yellow shades, lemon yellow, sulphur yellow, etc., can be obtained.

Just as the quantities of the solutions used and their strength influence the shade of the chrome yellow they produce, so it also appears to be not immaterial which lead salt is employed. Colour makers are generally agreed that the finest product is obtained from neutral lead acetate. Any lead salt, even insoluble in water, may be used for the preparation of chrome yellow. The affinity of chromic acid for lead is so great that an interchange of constituents occurs between the insoluble salt and the potassium chromate. Chrome yellow may be made from lead acetate, chloride or sulphate; the resulting substances are the same, but there is a considerable difference in regard to the beauty of the product. The finest chrome yellow, which leaves nothing to be desired in beauty of shade, is obtained by proceeding in the following manner: Lead acetate is dissolved in water, the solution diluted with an equal volume of water, and then mixed, under constant stirring, with a similarly diluted solution of potassium chromate or bichromate. The precipitate is immediately formed, and quickly sinks to the bottom in consequence of its high specific gravity. It is washed with clean water so long as this removes soluble salts. The precipitate is then drained on cloths and dried in the air.

The finest product is obtained by working with the following proportions:—

If lead sulphate is used the following quantities are to be employed:—

In the case of lead chloride the following is the proportion:—

The chrome yellows prepared from insoluble lead salts have no particular beauty, but they may be used for mixed colours such as the spurious chrome green.

Preparation of the Lead Solution.—Many makers of chrome yellow do not use commercial lead acetate, but prepare its solution themselves. The preparation of this solution requires neither much space nor labour, so that considering the high price of lead acetate this procedure may be regarded as advisable, but only when acetic acid is obtainable at a low rate.

The following is the method by which lead acetate solution is made. Lead is granulated by pouring the molten metal from a height of several yards into cold water, which is kept in rapid motion. The smaller the particles of lead, the larger surface they will possess, and the more quickly they will dissolve. For the solution of the lead small tubs are used, 50 centimetres in diameter and 90 to 100 centimetres in height, provided with a tap immediately above the bottom to run off the liquid. Four of these vessels are so placed, one above the other, that the contents of each may be run into the one next below it. The lead in the top vessel is covered with acetic acid; in a few minutes this is allowed to flow into the second vessel, and similarly after a few minutes from the second to the third, from the third to the fourth, from which it runs away into a receiver below. After this treatment it contains but a small quantity of lead acetate; the object of the operation is to start the oxidation of the lead, which quickly follows when air has sufficient access, the lead particles lose their metallic appearance and become covered with a white layer. When this is the case, the acetic acid is pumped from the receiver back into the top vessel, where it is left one to two hours in contact with the lead. It is then run off into the second, and thence into the third and fourth, remaining in each vessel for about the same time; the resulting liquid is an almost completely neutral solution of lead acetate. The treatment with acetic acid is continued so long as lead remains undissolved.

The solution of potassium chromate is prepared in a tub. The salt is easily soluble, and warming is unnecessary if the chromate is placed in a basket, lined with close linen cloth, hung in the liquid so that it is immersed to half its depth. The salt rapidly dissolves, its solution has a greater density than water, in consequence of which it sinks and fresh quantities of water continually come in contact with the salt.

Precipitation of the Chrome Yellow.—Before the chrome yellow can be precipitated it is necessary to estimate the quantity of lead acetate contained in the solution, since upon this depends the quantity of potassium chromate solution to be used. If the lead solution contained only acetate and water, its strength could be simply found by means of the hydrometer. It contains, however, varying quantities of acetic acid, on which account the hydrometer would give very inaccurate results. The test by which the relation between lead solution and potassium chromate solution is found is performed in the following manner: The lead solution is measured off in a cylinder divided into 100 divisions; the same volume of potassium chromate solution is measured and placed in a high narrow vessel; the lead solution is gradually added to the chromate solution so long as a precipitate is formed. The precipitate settles rapidly, and there is no particular difficulty with a little practice in finding with sufficient accuracy the quantities required for the precipitation. In order to precipitate 100 litres of potassium chromate solution, there are required as many litres of lead solution as were used divisions of the cylinder.

The preparation of the chrome yellow is now a very simple matter. Whilst steadily stirring, the measured quantity of lead solution is run into the solution of potassium chromate; the precipitate is allowed to settle, is well washed and dried. It does not make any difference to the colour whether potassium chromate or bichromate is used; the same product is obtained in each case.

It is stated by Dullo that chrome yellow prepared by the preceding process alters in colour on long keeping. This is ascribed to the formation of a basic compound. According to the same author, a chrome yellow free from this objectionable property is obtained by using lead nitrate in place of acetate and an excess of potassium chromate solution. The writer has kept chrome yellow, made from lead acetate, for years without observing the slightest alteration in the colour. On chemical grounds, it is incomprehensible that a chrome yellow prepared from lead nitrate should have different properties to the same substance prepared from another soluble lead salt, and freed from foreign substances by sufficient washing.

The product of this process is that which par excellence is known as chrome yellow, the chemist’s neutral lead chromate; it exhibits a characteristic deep yellow colour, a shade which is known as chrome yellow. Under the microscope chrome yellow is seen to be a crystalline mass; it will possess greater covering power the smaller the crystals. Now, the motion of a liquid in which crystals are forming prevents the production of large crystals, thus the reason is clear for the rapid stirring of the solutions in the preparation of this pigment.

According to C. O. Weber, who has published an exhaustive account of chrome pigments (Dingler’s Journal, 282), the cost of the lead chrome pigments varies greatly according to the raw materials employed. Assuming that 100 kilogrammes of litharge cost 35 marks, 100 kilogrammes of 30 per cent. acetic acid 25 marks, and 100 kilogrammes of 60 per cent. nitric acid 26 marks, Weber calculates that 100 kilogrammes of litharge, in a form suitable for making chrome yellow, will cost as follows:—

The Pale Chrome Yellows.—When the solution of the chromate used for the precipitation of the lead solution is mixed with sulphuric acid, then a mixture of lead sulphate and lead chromate is formed on precipitation. Lead sulphate is white, so that the colour of the precipitate would be paler according to the quantity of sulphuric acid added to the potassium chromate solution. There are, however, compounds of lead chromate and lead sulphate, of which we know two. Their composition is expressed by the formulæ:—

PbCrO₄.PbSO₄ and PbCrO₄.2PbSO₄.

The former is a beautiful lemon yellow shade, the latter nearly approaches sulphur yellow. By corresponding alterations in the quantity of sulphuric acid added, all intermediate shades can be obtained. On the works these shades are made in the following manner: buckets are used for taking the potassium chromate solution out of the vessel in which it was made; these buckets hold 12·5 litres. Now, if the solution of potassium chromate has been made from 25 kilogrammes of the salt and 750 kilogrammes of water, one of these buckets holds exactly 0·43 kilogramme of chromic acid. In order to obtain the lemon yellow compound, 0·39 kilogramme of sulphuric acid must be added to a bucketful of solution; 0·78 kilogramme must be added to obtain the sulphur yellow chrome. These liquids are prepared by pouring the sulphuric acid in a thin stream into the potassium chromate solution made as above, the liquid must be stirred whilst the sulphuric acid is being added. The lemon chrome has the peculiar property of increasing considerably in volume soon after formation, a property to which regard must be had in the manufacture. A description of the rational preparation of the lemon and sulphur yellow shades of chrome yellow on the manufacturing scale follows.

In making lemon chrome, the tub in which the precipitation is to take place is two-thirds filled with water, the lead solution is stirred in, and then the chromate solution, mixed with the proper quantity of sulphuric acid, is run in; the liquid is well stirred so that the precipitate may form as quickly as possible throughout the whole liquid. The precipitate is allowed to settle, the liquid drawn off, and the colour washed twice with water as quickly as possible. The paste is then removed from the tub and poured on a strong linen strainer. At first, the fine precipitate goes through the strainer, the liquid is poured back on the strainer until the size of its pores is so far diminished by the precipitate itself that only clear liquid runs through. The precipitate is left on the strainer until it forms a stiff mass, which can be easily spread out upon boards by spatulas.

To obtain a good, that is, a loose product, it is necessary to carry out the processes so quickly that the swelling mentioned above does not take place while the colour is being strained, but when it has been spread out on the boards. This swelling only takes place completely when the layer of precipitate is fairly thin. Large boards should be used, upon which the precipitate is spread out in a very thin layer. To prevent the mass—which is still fairly fluid—from running off the boards, they are provided with raised edges, and the paste is spread out smoothly in these flat trays. If the operations have been properly performed, the precipitate at once begins to swell and becomes of a loose nature. When it has acquired a buttery consistency it is cut up, by means of a thin sheet of brass, into prisms, which are placed near one another standing on the narrow side, and dried first in the shade and then in the sun. It is necessary that the drying should take place slowly at first, or the cakes will crack or even fall to pieces. The precipitate cannot be washed completely in the tub, because it often begins to swell on the strainers, consequently a crystalline crust covers the surface of the cakes during drying. This layer must be removed by scraping the cakes of colour, in which operation small quantities of chrome yellow dust become suspended in the air. To protect the workman against poisoning by this lead compound, precautions must be taken against breathing in the dust. The simplest and most efficacious is to tie a wet sponge over mouth and nose; this retains the particles of dust in the inspired air. The dust, scraped off the cakes, is put into water, in which the salts dissolve, whilst the chrome yellow sinks to the bottom. In this process for preparing chrome yellow the solution of potassium acetate left after the precipitation contains a considerable quantity of free acetic acid, which may be utilised to dissolve lead.

When sulphur yellow chrome is to be made, the process is substantially the same as for the preparation of the lemon shade, but with the difference that everything is done to prevent the swelling of the precipitate. The precipitation and washing of the precipitate are done as quickly as possible: the washed precipitate is filled into press bags and strongly pressed. Care must be taken in the pressing that the pressure is only gradually increased; if powerful pressure is applied at once, even the strongest cloths will be burst. The more thoroughly the precipitate is pressed, the closer will be the fracture of the chrome, a property which is regarded as a sign of good quality in this species of chrome yellow.

The manufacture of chrome yellow is intimately connected with that of a number of colours, varying from orange to dark red, which are known under the names of chrome orange or chrome red. In accordance with the division adopted of pigments according to their colour, chrome orange and chrome red will be considered among the red mineral pigments.