This base or strass is composed of silex, potash, borax, oxide of lead, and sometimes arsenic. The siliceous matter should be perfectly pure; and if obtained from sand, it ought to be calcined, and washed, first with dilute muriatic acid, and then with water. The crystal or flint should be made redhot, quenched in water, and ground, as in the potteries. The potash should be purified from the best pearlash; and the borax should be refined by one or two crystallizations. The oxide of lead should be absolutely free from tin, for the least portion of this latter metal causes milkiness. Good red-lead is preferable to litharge. The arsenic should also be pure. Hessian crucibles are preferable to those of porcelain, for they are not so apt to crack and run out. Either a pottery or porcelain kiln will answer, and the fusion should be continued 24 hours; for the more tranquil and continuous it is, the denser is the paste, and the greater its beauty. The following four recipes have afforded good strass:—
| Grains. | ||
|---|---|---|
| No. I. | ||
| Or, | Rock crystal | 4056 |
| Minium | 6300 | |
| Pure potash | 2154 | |
| Borax | 276 | |
| Arsenic | 12 | |
| No. II. | ||
| Sand | 3600 | |
| Ceruse of Clichy (pure carbonate of lead) | 8508 | |
| Potash | 1260 | |
| Borax | 360 | |
| Arsenic | 12 | |
| No. III. | ||
| Rock crystal | 3456 | |
| Minium | 5328 | |
| Potash | 1944 | |
| Borax | 216 | |
| Arsenic | 6 | |
| No. IV. | ||
| Rock crystal | 3600 | |
| Ceruse of Clichy | 8508 | |
| Potash | 1260 | |
| Borax | 360 | |
Topaz.
| Grains. | ||
|---|---|---|
| Very white paste | 1008 | |
| Glass of antimony | 43 | |
| Cassius purple | 1 | |
| Or, | ||
| Paste | 3456 | |
| Oxide of iron, called saffron of Mars | 36 | |
Ruby.
M. Wiéland succeeded in obtaining excellent imitations of rubies, by making use of the topaz materials. It often happened that the mixture for topazes gave only an opaque mass, translucent at the edges, and in thin plates of a red colour. 1 part of this substance being mixed with 8 parts of strass, and fused for 30 hours, gave a fine yellowish crystal-like paste, and fragments of this fused before the blowpipe, afforded the finest imitation of rubies. The result was always the same.
The following are other proportions for rubies:—
| Grains. | ||
|---|---|---|
| Or, | Paste | 2880 |
| Oxide of manganese | 72 | |
Emerald.
| Or, | Paste | 4608 |
| Green oxide of pure copper | 42 | |
| Oxide of chrome | 2 |