In arranging the bases with which to compose a glaze it is desirable to use one at least from each class, but it must be borne in mind that however many bases are introduced the total must always be unity. This unit is, for the sake of brevity, described as RO. For example the following groups may be set forth:
| 1. | PbO Lead Oxide | .7 |
| CaO Calcium Oxide | .3 | |
| —— | ||
| RO | 1.0 | |
| 2. | PbO | .6 |
| CaO | .4 | |
| —— | ||
| RO | 1.0 | |
| 3. | PbO Lead Oxide | .5 |
| ZnO Zinc Oxide | .2 | |
| CaO Calcium Oxide | .3 | |
| —— | ||
| RO | 1.0 | |
| 4. | PbO | .6 |
| ZnO | .1 | |
| CaO | .3 | |
| —— | ||
| RO | 1.0 | |
| 5. | PbO Lead Oxide | .6 |
| CaO Calcium Oxide | .3 | |
| K2O Potassium Oxide | .1 | |
| —— | ||
| RO | 1.0 | |
| 6. | PbO | .50 |
| CaO | .35 | |
| K2O | .15 | |
| —— | ||
| RO | 1.00 | |
| 7. | PbO Lead Oxide | .45 |
| ZnO Zinc Oxide | .10 | |
| CaO Calcium Oxide | .30 | |
| K2O Potassium Oxide | .15 | |
| —— | ||
| RO | 1.00 | |
| 8. | PbO | .35 |
| ZnO | .15 | |
| CaO | .35 | |
| K2O | .15 | |
| —— | ||
| RO | 1.00 |
The reason for the unit rule is that if one formula is to be compared with another there must be a uniform basis upon which to work and, furthermore, it makes no difference whether the silica combines with one, two, three, or four bases, the chemical action is the same and, so long as the sum of the bases is kept at unity, the same amount of silica will be required.
If two glazes be taken as an illustration this will be made clear:
| PbO | .6 | } | ||||
| CaO | .4 | } | Al2O3 | .2 | SiO2 | 1.6 |
| —— | } | |||||
| 1.0 | } | |||||
| PbO | .46 | } | ||||
| ZnO | .12 | } | ||||
| CaO | .28 | } | Al2O3 | .2 | SiO2 | 1.6 |
| K2O | .14 | } | ||||
| —— | } | |||||
| 1.00 | } |
Both of these formulae are bisilicates and each being properly fired, will stand, without crazing, on the same body.
The use of the formula is to give an insight into the composition of the melted glaze. It takes no account of volatile ingredients or losses in the fire but for this very reason it must be translated into the substances to be weighed before use can be made of it.
Of the ingredients given on pages 142, 143, some contain but one item of the formula, others contain several, as in the case of kaolin already cited. Feldspar, of the variety known as potash feldspar and named by mineralogists, "orthoclase," is a very useful ingredient in raw glazes, being, in fact, almost the only source of potash. The formula, page 142, shows that a molecule or equivalent of feldspar contains one molecule of potash K2O, one of alumina Al2O3, and six of silica SiO2. This fact is taken into account in calculating the mixture or batch weight.