Lemon Yellow

Synonyms:

Lemon Yellow, Baryta Yellow, Barium Chromate, Yellow Ultramarine, Permanent Yellow, Jaune d'Outre mer, Zitronengelb

Of all the chromates which have been used in painting, barium chromate is the most stable. It has a pure yellow colour, with a not inconsiderable degree of opacity. It works smoothly.

Lemon yellow is often made by mixing solutions of neutral potassium chromate and of barium chloride, both liquids having been previously heated to 100° C. A still better plan is to take equivalent proportions - namely, 25 3/4 parts by weight of pure crystals of barium chloride and 21 1/2 parts of pure crystals of neutral potassium chromate - of these two compounds, and to grind them together to very fine powder. Continue the grinding, and then add gradually sufficient pure water to convert the mixture into a thin paste. The paste is then heated to 100° for fifteen minutes, thrown on a filter, washed with abundance of pure water, dried, and ground.

Properly prepared lemon yellow may be mixed with most other stable pigments without suffering change. It is not blackened like the lead chromes by sulphuretted hydrogen, but it has a tendency, as a water-colour, to become greenish when long exposed to this gas or to impure air. In oils it is very useful, for although some organic pigments may give it a greenish cast by reducing it in part to green chromic oxide, yet it may be safely associated with aureolin, with madder carmine, and with Prussian blue. Lemon yellow may be used in fresco.

Strontium chromate is very often - we may say generally - substituted for true lemon yellow, but it is less stable, and has the further defect (for water-colour work) of being decidedly soluble even in cold water, so that light washes of it may be found to sink into the paper and to partially disappear. The most common adulteration of lemon yellow is with pale chrome; of course, sulphuretted hydrogen detects this falsification by darkening or blackening the pigment. Strontium chromate is distinguished from barium chromate by its dissolving in boiling water to such an extent as to yield a solution having a strong yellow colour. It may be prepared in the same way as the chromate of barium. Zinc chromate and calcium chromate are yellow pigments of inferior value. A mixture of zinc chromate with barium chromate is sold as primrose yellow.