An engobe must, of course, be put upon the unburned or green clay ware and this should be leather hard, not dry. The body with the engobe may be burned before glazing or the glaze may be put upon the unburned ware and the whole subjected to one fire only.
The ingredients in alkaline glazes are soda-ash, whiting, feldspar, flint and oxide of tin. The following is an example of a fritted glaze:
| Na2O | .60 | } | |
| K2O | .10 | } Al2O3 .10 | SiO2 1.30 |
| CaO | .30 | } | |
| Soda Ash | 64 | ||
| Whiting | 30 | ||
| Feldspar | 56 | ||
| Flint | 42 |
The entire batch is fritted and ground in a ball mill with the usual amount of water for fritt grinding, adding a tablespoonful of gum tragacanth mucilage to the batch after it is sieved. The glaze should be the consistency of heavy cream when used.
It is also possible to use an alkaline glaze in the raw or unfritted state. This necessitates grinding by hand in a mortar, but great care must be taken to mix the dry ingredients thoroughly before adding water and to stir the glaze constantly while pouring in the water, otherwise the soda-ash will cake and harden and be very difficult to break up. A batch of glaze can be ground by hand in fifteen or twenty minutes if done vigorously. It is then put through a 120-mesh sieve. The consistency is of importance. If too much water has been added and the glaze has become thin, it cannot be used successfully and should be discarded. Unfritted alkaline glaze does not keep well when moist but the ingredients can be ground dry and kept ready to be moistened as needed.
The following is an example of an unfritted alkaline glaze:
| Na2O | .59 | } | ||
| CaO | .21 | } | Al2O3 .20 | SiO2 1.6 |
| K2O | .20 | } | ||
| Soda Ash | 62 | |||
| Whiting | 21 | |||
| Feldspar | 111 | |||
| Flint | 24 |
For color add the following oxides to a batch.
1. Egyptian blue, opaque—from 5 to 8 grams of black oxide of copper—16 grams of oxide of tin.
2. Persian blue, opaque—from 8 to 10 grams of black oxide of copper—16 grams of oxide of tin.