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:—

From lead acetate at 56 marks per 100 kilogrammes96marks
”” solution made in the works80”
”lead nitrateat 50 marks per 100 kilogrammes75”
”” solution made in the works64”
By the basic lead acetate method51”
””  chloride”40”
White lead method55”

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.