| V.— | Silica | 65 per cent |
|---|---|---|
| Borax | 14 per cent | |
| Oxide of lead | 4 per cent | |
| Clay | 15 per cent | |
| Magnesia | 2 per cent |
No. V gives a fair average of several mixings which are in use, but it can be varied slightly to suit different conditions of work.
Defects In The Gray Or Ground Coating.
The addition of magnesia when it has been omitted from the frit may also act as a preventive, but it should only be added in very small quantities, not exceeding 2.5 per cent, otherwise the temperature required for fusion will be very great.
Coating And Fusion.
A soft surface is always the outcome of a mixing which can be fused at a low temperature. It is due to too much lead or an insufficiency of clay or silica powder.
A hard surface is due to the quantity of lead in the mixing being too small. Increase the quantity and introduce potash, say about 2.5 per cent.
The gray or fundamental mixing should be kept together in a condition only just sufficiently liquid to allow of being poured out. When required to be applied to the plate, the water necessary to lower it to the consistency of thick cream can then be added gradually, energetic stirring of the mass taking place simultaneously in order to obtain uniform distribution.
The time required for fusion may vary from 15 minutes to 25 minutes, but should never exceed the latter. If it does, it shows that the mixing is too viscous, and the remedy would be the addition and thorough intermixture of calcined borax or boracic acid. Should this fail, then remelting or a new frit is necessary.
A highly glazed surface on leaving the muffle shows that the composition is too fluid and requires the addition of clay, glass, silica powder or other substance to increase the viscosity.