If only part of the surface is to be dulled or etched, the part that is not to be etched should be covered with shellac, or a mixture of three parts beeswax and one part Burgundy pitch, and allowed to dry before the piece is put in the acid. The acid must be kept either in lead or wax bottles, as it eats glass very rapidly. Its fumes are very dangerous and destroy human tissues.
If for some reason a piece has been enameled wrong, so that it is desirable to take the enamel off the metal, this can best be done by leaving the piece for a few hours in a solution of hydrofluoric acid which will eat the enamel, so that it can easily be brushed off the metal, leaving it bright.
Plique-à-jour enamel may be done in several ways. The simplest is to lay the filigree work upon a piece of mica or fire clay and charge with enamel, as if the mica or fire clay were the background and the filigree work the sides of the piece. The process continues the same as in ordinary enameling, except that just before the last firing the mica or fire clay should be removed.
Copper or silver foil may be used instead of mica, but in this case the filigree work must be either of gold or platinum, for the copper or silver foil can only be removed by being dissolved in nitric acid.
The most difficult way is to mix a little gum arabic or gum tragacanth with very finely ground enamel and charge a layer on the inner sides of the wires which make up the filigree work. The gums will hold the enamel in place after it is dried. When the piece has been fired, another layer of enamel should be charged and so on, until by degrees the entire space between the wires has been filled up. This process is tedious and requires much skill and care, but gives very satisfactory results.
CHAPTER VIII
ENAMEL PAINTING
Enamel painting is usually done on a background of opaque white enamel, although other opaque colors are sometimes used. The metal must be prepared in the regular manner, and one or two coats of enamel charged and fired, the last coat being fired to as perfect smoothness as possible. Instead of flushing the enamel smooth it is often advisable to stone and fire it, by which processes a smoother surface is obtained.
After this last firing, the black flakes of metallic oxide which will be found on the metal should be brushed away, leaving the piece clean and ready to be painted. It is desirable not to have too thick a layer of enamel, as in that case it is apt to chip off in some of the later firings. If, on the other hand, the enamel layer is thin, it is quite difficult to get it smooth.
