Читать книгу How to Enamel - Howard M. Chapin - Страница 5

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Enamel is generally bought in the form of hard chunks more or less flat and varying from the size of an acorn to that of a large dinner plate. When it is made it is poured into a mold where it hardens in the form of a disk or slab generally a foot or more in diameter, and later gets broken into smaller pieces during transportation.

These chunks of enamel may be readily broken up by a hammer, and if they are first wrapped in a piece of cloth the small particles will not fly about and get into one’s eye, and no enamel will be lost. When the enamel has been broken up so that none of the pieces are larger than a pea, it should be washed in clean water to free it from any dirt that it naturally contained or that it received from the cloth in which it was wrapped during the hammering process.

Cleanliness cannot be too much emphasized, for the slightest particle of dirt mixed in with the enamel may often completely spoil a piece of work.

Enamel if heated quite hot and then plunged into cold water will become so brittle that it can be broken up by the hands, but when treated in this manner is said to be harder to grind.

Fig. 2. Grinding with Weighted Pestle.

After the enamel has been broken up into small pieces it should be put in an agate or porcelain mortar and ground to a fine powder. This will take from twenty to twenty-five minutes of steady grinding, the length of time depending on the quality and quantity of the enamel, the strength used, etc. Water may or may not be mixed with the enamel during the grinding process, as the grinder prefers. If water is added the process may take a little longer but a very disagreeable noise is eliminated and the work may perhaps be slightly easier. Agate mortars are cleaner and so better than porcelain ones but are so much more expensive that they cannot be used for grinding large amounts. For this grinding machines or weighted pestles, such as Cunynghame describes in his “Art Enameling upon Metals,” should be used. The enamel should be ground until it is about the size of fine sand, and soft, not gritty, when rubbed gently between one’s thumb and forefinger. It should now be rinsed in clean (the purer the better) running water, the enamel being thoroughly stirred up and washed, the fine particles, “dregs” or “slimes” as they are called, being allowed to flow away. Often from ten to thirty per cent of the enamel is lost in this process. The remaining wet enamel powder is ready to be used. It may be kept for weeks under water in a glass jar or druggist’s salve box, but in time is said to deteriorate. If the enamel is to be used immediately it may be placed upon a clean copper or china palette which will be found very serviceable. It should be kept constantly wet and if possible under water. If, however, it becomes dry, it may be washed as described above and will probably be found in good condition; if not a slight grinding may assist matters.

If enamel is not ground fine enough it is apt to have pit-holes in it when fired, and if it is not absolutely clean it is likely to have both pit-holes and black spots in it. Sometimes enamel is washed in nitric acid as well as water but this is generally not necessary. It is very important, however, to use water that contains very little iron, for iron will cause black spots in the enamel.

If enamel is to be kept for any length of time it should be kept either as chunks preferably or as coarsely ground powder. When coarsely ground it may be kept dry in a dirt-proof jar or bottle.

The “dregs” and “slimes” if washed clean may be kept and used for paints in painting on enamel.

When enamel is bought in the form of a powder it is generally necessary to grind it finer and wash it thoroughly.