Chrome Yellow

Synonyms:

Chrome Yellow, Chrome, Chrornate Of Lead, Jaune De Chrome, Chromgelb

This pigment, when of a pure yellow hue, is the neutral lead chromate. By associating it with an additional quantity of lead oxide it may be obtained of various orange and reddish orange hues. It may be made by the mutual action of a soluble lead salt, such as the acetate or nitrate, and the chromate or bichromate of potassium. Or white lead in fine powder (2 kilos.) may be boiled with a solution of bichromate of potassium (1/2 kilo.) in water (10 litres). Alum and baryta-white, or lead sulphate, are also employed in the preparation of some of the paler chrome yellows. Lemon chrome is a mixture of lead chromate and sulphate. Orange chrome and chrome red are prepared from a mixture of lead acetate (6 3/4 kilos.), litharge (5 1/2 kilos.), neutral potassium chromate (6 kilos.), caustic potash being sometimes used in addition. Chrome red may be obtained also by the direct action of caustic soda in solution upon the yellow lead chromate: its chemical formula is PbCrO₄, PbO, or Pb₂CrO₅.

The chromates of lead are peculiarly liable to change, and are quite unfitted for use in tempera or water-colour painting. In oil, especially if protected by varnish, or locked up in a resinous vehicle, these pigments show a certain measure of permanence, except when they are mingled with paints of organic origin. In fact there are two causes which militate against the integrity of the lead chromates. One of these is the tendency which they possess towards reduction, that is, the loss of oxygen by their chromic constituent, by which the green or lower oxide of chromium is formed. This change is brought about by many kinds of organic matter, notably by such animal or vegetable pigments as are themselves prone to oxidation. The other cause of deterioration is the presence of certain sulphur compounds which act upon the lead chromates in the same way as they act upon white lead, producing lead sulphide of a dark brown, or a grey colour.

Of late years the respective merits as oil-paints of cadmium yellow and chrome yellow have been warmly contested; the tendency at present, especially among artists rather than among chemists, is to give a verdict in favour of the latter pigment.