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 per cent |
| 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.
Defects In The Glaze Or White.
Water.
How To Tell The Character Of Enamel.
Culinary utensils, and those to hold chemicals, should not only look well, but should be capable of resisting the action of acids. Lead should never enter into the composition of enamels of this class, as they then become easily acted upon, and in the case of chipping present a menace to health. The presence of lead is easily detected. Destroy the outside coating of the enamel at some spot by the application of strong nitric acid. Wash the part and apply a drop of ammonium sulphide. If lead is present, the part will become almost black, but remains unchanged in color if it is absent.
Another simple test is to switch up an egg in a vessel and allow it to stand for about 24 hours. When poured out and rinsed with water a dark stain will remain if lead is present in the enamel. To test the power of chemical resistance is equally simple. Boil diluted vinegar in the vessel for several minutes, and if a sediment is formed and the luster and smoothness of the glaze destroyed or partially destroyed, it follows that it is incapable of resisting the attacks of acids for any length of time. There are several other tests adopted, but those given present little difficulty in carrying out, and give reliable results.