This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Chipping is the most disastrous. This may be prevented by the addition of some bitter salt, say from 3 to 4 per cent of the weight of the frit.
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.
Difficulties of either may generally be done away with by reducing the magnesia used in the frit to a minimum.
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.
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.
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:
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:
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
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:
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
Feldspar....... 41 per cent
Borax......... 35 per cent
Oxide of tin. .. . 17 per cent
Niter.......... 7 per cent
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 percent
Borax will assist fusion. Quartz mixings require more soda than feldspar mixings.
Borax......... 28 per cent
Oxide of tin... . 19.5 per cent Cullet (powdered
white glass) .. 18 percent
Silica powder. . . 17.5 per cent
Niter.......... 9.5 per cent
Magnesia...... 5 per cent
Clay.......... 2.5 per cent
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
To No. VII must be added—while being ground—the following percentages of the weight' of the frit:
Silica powder. . . 18 per cent
Borax......... 9 per cent
Magnesia...... 5.25 per cent
Boracic acid.. .. 1.5 per cent To No. VIII should be similarly added the following percentages of the frit:
Silica powder. .. 1.75 percent
Magnesia...... 1.75 per cent
Soda.......... 1 per cent
This mixing is one which is used in the production of some of the best types of hollow ware for culinary purposes. The glaze should be kept in tubs mixed with water until used, and it should be carefully protected from dust.
 
Continue to: