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.

Coating and Fusion

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:

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       percent

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       percent

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

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.