Part 193

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. {303}

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 per cent

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 per cent 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 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.»—A bad white may be due to its being insufficiently opaque. More oxide of tin is required. Cracks may be prevented by the addition of carbonate of ammonia. Insufficient luster can be avoided by adding to the quantity of soda and reducing the borax. If the gray shows through the white it proves that the temperature of fusion is too high or the viscosity of the mixing is too great. If the coating is not uniformly spread it may be due to the glaze being too thin; add magnesia. If the glaze separates from the gray add some bitter salt. Viscosity will be increased by reducing the quantity of borax. Immunity against chemical reaction is procured by increasing the quantity of borax. An improved luster will be obtained by adding native carbonate of soda. The greater the quantity of silicic acid the greater must be the temperature for fusion. To reduce the temperature add borax. Clay will increase the difficulty {304} of fusion. Oxide of lead will make a frit more easily fusible. A purer white can be obtained by adding a small quantity of smalt.

«Water.»—The character of the water used in the mixing of enamels is too frequently taken for granted, for unsuitable water may render a mixing almost entirely useless. Clean water, and with little or no sulphur present, is essential. For very fine enamels it is advisable to use carefully filtered water which has shown, after analysis, that it is free from any matter which is injurious to any of the enamel constituents.

«How to Tell the Character of Enamel.»—In the case of sign tablets the characteristics looked to are appearance and the adherence of the coatings to the iron. For the latter the tests are simple. The plate if slightly bent should not crack the coating. An enamel plate placed in boiling water for some time and then plunged into very cold water should not show any cracks, however small, even after repeated treatment of this kind.

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.

«Wasters and Seconds: Repairing Old Articles.»—In all enameling there must be certain articles turned out which are defective, but the percentage should never be very great. The causes which most frequently tend to the production of wasters are new mixings and a temperature of fusion which is either too high or too low. There are two ways of disposing of defective articles, viz.: (1) Chipping off the bad spots, patching them up and selling them as “seconds”; (2) throwing the articles into the waste heap. The best firms adopt the latter course, because the recoating and firing of defective parts practically means a repetition of the whole process, thus adding greatly to the cost, while the selling price is reduced. Overheating in fusion is generally shown by blisters or by the enamel being too thin in various places. Chipping may be also due to this cause, the excessive heat having practically fused the fundamental coating.