As has been already explained, the glaze is much more important than the fundamental coating. Discoloration or slight flaws which could be tolerated in the latter would be fatal to the former. {303}
In glazes, oxide of lead need not be used. It should never be used in a coating for vessels which are to contain acids or be used as cooking utensils. It may be used in sign-tablet production.
For pipes the following glaze gives good results:
| I.— | Feldspar | 33 per cent |
|---|---|---|
| Borax | 22.5 per cent | |
| Quartz | 16.5 per cent | |
| Oxide of tin | 15 per cent | |
| Soda | 8 per cent | |
| Fluorspar | 3.75 per cent | |
| Saltpeter | 2.25 per cent |
For sign tablets the following gives fair results, although some of the succeeding ones are in more general use:
| II.— | Cullet | 20 per cent |
|---|---|---|
| Powdered flint | 15 per cent | |
| Lead | 52 per cent | |
| Soda | 4.5 per cent | |
| Arsenic | 4.5 per cent | |
| Niter | 4 per cent | |
| III.— | Frit of silica powder | 30 per cent |
| Oxide of tin | 18 per cent | |
| Borax | 17 per cent | |
| Soda | 8.6 per cent | |
| Niter | 7.5 per cent | |
| White lead | 5.5 per cent | |
| Carbonate of ammonia | 5.5 per cent | |
| Magnesia | 4 per cent | |
| Silica powder | 4 per cent |
The following are useful for culinary utensils, as they do not contain lead:
| IV.— | Frit of silica powder | 26 per cent |
|---|---|---|
| Oxide of tin | 21 per cent | |
| Borax | 20 per cent | |
| Soda | 10.25 per cent | |
| Niter | 7 per cent | |
| Carbonate of ammonia | 5 per cent | |
| Magnesia | 3.25 per cent | |
| This should be ground up with the following: | ||
| Silica powder | 4.25 per cent | |
| Oxide of tin | 2.25 per cent | |
| Soda | 0.5 per cent | |
| Magnesia | 0.5 per cent | |
| V.— | Feldspar | 41 per cent |
| Borax | 35 per cent | |
| Oxide of tin | 17 per cent | |
| Niter | 7 per cent | |
| VI.— | Borax | 30 per cent |
| Feldspar | 22 per cent | |
| Silicate powder | 17.5 per cent | |
| Oxide of tin | 15 per cent | |
| Soda | 13.5 per cent | |
| Niter | 2 per cent | |
Borax will assist fusion. Quartz mixings require more soda than feldspar mixings.
| VII.— | Borax | 28 per cent |
|---|---|---|
| Oxide of tin | 19.5 per cent | |
| Cullet (powdered white glass) | 18 per cent | |
| Silica powder | 17.5 per cent | |
| Niter | 9.5 per cent | |
| Magnesia | 5 per cent | |
| Clay | 2.5 per cent | |
| VIII.— | Borax | 26.75 per cent |
| Cullet | 19 per cent | |
| Silica powder | 18.5 per cent | |
| Oxide of tin | 19 per cent | |
| Niter | 9.25 per cent | |
| Magnesia | 4.5 per cent | |
| Soda | 3 per cent |