After enamel has been charged on a piece of metal, it is “fired,” that is, heated until the enamel is fused, after which, on cooling, it becomes hard. It is best fired in a muffle furnace, although with care a nice job may be done with a blowpipe, a Bunsen burner, or even an alcohol lamp.

In case a Bunsen burner or lamp is used, it is well to have a metal tripod supporting a sheet of metal or wire mesh on which to set the work, as holding it by a pair of tongs becomes very tedious.

A muffle may be heated by electricity, gas (with or without an air blast), oil, coal, or wood. Electricity or gas, however, is preferable, as they are not accompanied by dirt. The muffle itself, which is simply a small oven, is generally made of fire clay, although both fused quartz and nickel are sometimes used. A door to the muffle is not necessary, although it is an aid to cleanliness. If, however, a door is used it should have a hole in it large enough to allow the enameler to watch his work constantly. It is very important to keep the enamel work away from the oxidizing effect of the fire; so that any cracks that appear in a muffle must be immediately repaired. It will be found very advantageous, too, to have the doors of the furnace on the side of it opposite to that containing the mouth of the muffle.

The pieces to be fired should be laid upon a tray of a size convenient to pass easily through the mouth of the muffle. These trays are made preferably of sheet nickel for this metal will not scale or flake off when heated and so contaminate the enamel. As these trays become quite hot it is advisable to handle them by means of a pair of long-handled tongs.

Before placing a tray of pieces in the muffle to be fired, it is well to leave it near the mouth of the muffle for a minute or two so that the enamel may become entirely dry before it is placed in the muffle, for if damp the enamel will be blown off of the metal by the rapid evaporation of the water which it contains.

When the pieces are fully dry, the tray may be placed in the muffle, which should be heated to a bright red heat of about 1400° F. With such a heat it should take from half a minute to two minutes to “fire” or melt the enamel. The time required will vary with different enamels and can only be learned by experiment. The enamel should be kept in the muffle until it begins to melt and its surface becomes fused and comparatively smooth and glossy, when it should be removed from the furnace and allowed to cool in the air. It is not necessary or even desirable to fire the enamel perfectly smooth in the first firing. It generally takes at least two coats, that is two chargings and two firings, before a piece is ready to be stoned. If, however, only one coat is to be used, it should be fired as smooth as possible.

When the pieces cool, the metal (unless it is fine, i.e., pure, unalloyed) will be found covered with a black scale caused by oxidation. This may be removed by simply brushing it, which operation in most cases will clean the piece sufficiently for the second charging. Otherwise the piece may be “pickled,” or cleaned with acid.

Subsequent firings do not differ from the first one except that in the last firing the enameler should try to obtain as smooth a surface as possible.

After the last firing the pieces should be submerged for a minute or two in a mild pickle consisting of one part sulphuric acid and one part water. This will clean off the oxide and make the metal bright.

If red enamel is used it is best to throw the piece into a cup of thick heavy machine oil to cool instead of letting it cool in the air. This will give the red a greater brilliancy. Red enamel, too, often requires a greater heat than other colors, although too low a heat tends to destroy the gloss on any enamel. Many delicate colors which are opaque when fired at a low heat, will become opalescent and in some cases transparent if fired at a higher temperature. Red enamel loses its color if fired too many times, and a large number of firings have a weakening effect on high-karat gold.