If lead sulphate is used the following quantities are to be employed:—
| Lead sulphate | 100 |
| Potassium bichromate | 25 |
In the case of lead chloride the following is the proportion:—
| Lead chloride | 100 |
| Potassium bichromate | 27 |
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.