Chromate of lead, the chrome-yellow of the painter, is a rich pigment of vari ous shades, from deep orange to the palest canary-yellow. It is made by add ing a limpid solution of the neutral chro mate (the above granular salt) to a solu tion, equally limpid, of acetate or nitrate of lead. A precipitate falls, which must be well washed, and carefully dried out of the reach of any sulphureted vapors. A lighter shade of yellow is obtained by mixing some solution of alum, or sul phuric acid, with the chromate, before pouring it into the solution of lead ; and an orange tint is to be procured by the addition of sabacetate of lead, in any de sired proportion.
Lately great use has been made of the green oxide to dyeing and painting on por celain. This oxyde may be prepared by decomposing, with heat, the chromate of mercury, a salt made by adding to ni trate of protoxyde of mercury, chromate of potash, in equivalent proportions. This chromate has a fine cinnabar red, when pure j and, at a dull red heat, parts with a portion of its oxygen and its mer curial oxyde. From M. Dulong's ex periments it would appear, that the purest chromate of mercury is not the best adapted for preparing the oxyde of chrome to be used in porcelain painting.
he thinks it ought to contain a little oxyde of manganese and chromate of potash, to afford a green color of a fine tint, especially for pieces that are to re ceive a powerful heat. Pure oxyde of chrome preserves its color well enough in a muffie furnace ; but, under a stronger fire, it takes a dead-leaf color.
An improved method of making this valuable color for enamelling, is to mix intimately 45 parts of gunpowder with 240 parts of perfectly dry chromate of potash, and 35 parts of hydrochlorate of ammonia (sal ammoniac), reduce to pow der, and pass through a fine sieve; fill a conical glass or other mould with this powder, gently pressed, and invert so as to leave the powder on a porcelain slab of any kind. When set on fire at its apex with a lighted match, it will burn down to the bottom with brilliant corns cations. The black residusn, being eln triated with warm water, affords a fine bright green oxide of chromium.