CHROMIUM.

301

II. Four parts of chrome ore, containing 34 per cent, of protoxyde of chromium.

Two parts of potashes.

One part of saltpetre.

III. Four parts of chrome ore. — 34 —

Two of potashes.

four tenths of a part of peroxyde of manganese.

IV. Three parts of chrome ore.

Four parts of saltpetre.

Two parts of argal.

Some manufacturers have contrived to effect the conversion of the oxyde into an acid,and of course to form the chromate of potash, by the agency of potash alone, in a calciningfurnace, or in earthen pots fired in a pottery kiln.

After lixiviating the calcined mixtures with water, if the solution be a tolerably purechromate of potash, its value may be inferred, from its specific gravity, by the followingtable : —

At specific gravity 1-28 it contains about 50 per cent, of the salt.

1-21

33

1-18

25

1*15

20

1-12

16

Ml,

14

1-10

12

ho making the red bichromate of potash from these solutions of the yellow salt, nitricacia was at first chiefly used; but in consequence of its relatively high price, sulphuric,111 ~' a ^*c, or acetic acid has been frequently substituted upon the great scale.

a ere * s anot her application of chrome which merits some notice here; that of its greenyde to dyeing and painting on porcelain. This oxyde may be prepared by decomposing,tth heat, the chromate of mercury, a salt made by adding to nitrate of protoxyde of mer-r U’ chromate of potash, in equivalent proportions. This chromate has a fine cinnabar> when pure; and, at a dull red heat, parts with a portion of its oxygen and itseicu r ; a [ oxyde. From M. Dulong’s experiments it would appear, that the purest-»ate of mercury is not the best adapted for preparing the oxyde of chrome to bea , a ln Porcelain painting. He thinks it ought to contain a little oxyde of manganese®re | romate °f potash, to afford a green color of a fine tint, especially for pieces that>n a* 0 rece * ve a powerful heat. Pure oxyde of chrome preserves its color well enoughmuffle furnace; but, under a stronger fire, it takes a dead-leaf color.

Por i P reSn ox yde of chrome has come so extensively into use as an enamel color forable , Ul ’ that a f u U er account of the best modes of manufacturing it must prove accept-c to many of my readers.

p, e at oxyde, in combination with water, called the hydrate, may be economicallyflo\pe f * ) - v foiling chromate of potash, dissolved in water, with half its weight ofcttsilv S °* su ^Pbor, till the resulting green precipitate ceases to increase, which may beacc e ip ascerta ined by filtering a little of the mixture. The addition of some potashth e eh* 116 * t *' e °P erat * on - This consists in combining the sulphur with the oxygen ofc hromat° m - C aC ^’ so as to f° rm su lpb ur ic acid, which unites with the potash of theextra q 6 mto su lphate of potash, while the chrome oxyde becomes a hydrate. AnWon ofr 111 * 1 ^ potash facilitates the deoxydizement of the chromic acid by the forma-for o xv „ posu lphi te and sulphuret of potash, both of which have a strong attractionpure th 611 " Por this purpose the clear lixivium of the chromate of potash is sufficientlyThe’hS^^uldhold some alumina and silica in solution, as it generally does.a Pon it rat ®. may be freed from particles of sulphur by heating dilute sulphuric acidoarbonate u 10 * 1 dissolves it; after which it may be precipitated, in the state of aBy calcin'^ car * )ona t e °f potash, not added in excess,acid is also'd^ a m * x t ure °f bichromate of potash and sulphur in a crucible, chromicpartly conve t C d°™ P0Se ^’ an< * a hydrated oxyde may be obtained; the sulphur beinge!t Pense of th J, nto ? u ^Ph ur ot of potassium, and partly into sulphuric acid (at thesu Iphate. B v e chl f orn ‘ c _ acid), which combines with the rest of the potash into s.the chrome urep 0 ™ 1 ^ hxiviation, these two new compounds may be washed away, andLiebig and f ree d from the remaining sulphur, by a slight heat,

mad of\ beautiful ^ - ve lately contrived a process for producing a subchromate ofa gentle heat pure hue. Into saltpetre, brought to fusion in a crucible at

strong ebullition takes r °i me ye * low * s to he thrown by small portions at a time. Attnues so while it is hot a *r al: eac ^ addition, and the mass becomes black, and con-r emains undecomposed ' r '^ ae chrome yellow is to be added till little of the saltpetre*}* the mixture should been*™ taken not to overheat the crucible, lest the color

taring which the dense h” 6 - ° r ° wn - Having allowed it to settle for a few minutes,asic salt falls to the bottom, the fluid part, consisting of