Many metals can be enameled, but the most practical ones, those most used in jewelry, are gold, silver, copper, and their alloys. Hence we will confine our descriptions to these metals.

Whatever metal is used, however, must first be made chemically clean before it is “charged” or covered with enamel. This may be done in a number of different ways, of which the following is considered the most practical.

The metal, whether gold, silver, or copper, must be annealed by being heated to a red heat and allowed to cool slowly in the air. When cool it will be found to be covered with a black oxide.

The metal must now be plunged for half a minute or more into a solution called a “dip,” consisting of two parts sulphuric acid (H₂SO₄), two parts nitric acid (HNO₃), and a slight touch of hydrochloric acid (HCl); a few drops of the latter to the gallon will suffice. This dip will clean the surface of the metal and make it bright. As the solution gets old it will become weak and a longer dip will be required. The ordinary commercial acids are suitable for making these dips. Care should be taken not to inhale the fumes of the acids, as they are quite poisonous.

This dip may be used cold, but better results are obtained when it is heated to 170°F. A convenient way to heat it is to set the jar containing the dip in a larger jar containing water and then to heat the water in the larger jar.

Many prefer to use first a hot and then a cold dip, rinsing the metal in clean water between the dips. Experience will soon teach one to tell when the metal is clean by looking at it, and this in fact is the only satisfactory way, as the length of time required will vary with the condition of the metals and the strength of the dip.

After the dip the metal must be thoroughly rinsed in clean water and then dried in clean sawdust or hot air. If rinsed in very hot water the metal will dry itself almost immediately. Some prefer to rinse the metal in ammonia as well as in water in order to rid it of all trace of the acid, but this is not generally necessary. If the metal is copper or gold it is now ready to be charged, but if it is silver it must be “fire-stained.”

There are many other possible ways of cleaning metals, but the above means are generally used and will doubtless be found the most convenient. If small quantities only are being “cleaned,” a copper pickle pan and a Bunsen gas burner will suffice, while for large quantities earthen pickle pots and large jars will be found advantageous. Metals if “wriggled,” that is, scraped or engraved all over with a graver, are generally clean enough to be charged.

If silver is used it must be “fire-stained,” or have the “fire” taken out of it. This is done in the following manner. The silver is submerged for a minute or more in a solution of four parts nitric acid and one part water, heated to 170°F. The acid will react on the silver, which will become covered with a brownish scum. The silver should now be rinsed in clean water and brushed with a metal brush in a solution of soap-tree bark and water. This will clean the scum off the silver and make the latter bright. After being rinsed in clean water, the silver is ready to be charged with enamel. Sour beer, or a solution of licorice root and water may be used instead of soap-tree bark. It will be very convenient to have the metal wire brush a circular one revolving on a lathe.

If red enamel is to be used, the metal should always be either wriggled or engine turned or else have some design cut sharply in it, in order to bring out the brilliancy of the color. Engine turning and sharp engraving or relief in the metal background will always enhance the brilliancy and give an additional sheen to any transparent colors. As the depth of the enamel layer has a great deal to do with the shade of the enamel, different effects can be obtained, even when only one enamel is used, if the metal background has been cut lower in some places than in others.

If the silver is not “fire-stained,” red enamel will often become maroon when fired, and fondant and other delicate colors will become streaked or spotted. If the fire-staining and dipping have not been carefully done, the enamel is likely to chip out, have pit-holes in it, or contain black blotches.

As the acid fumes eat the flesh quite rapidly, it is advisable to wear rubber gloves when dipping the metals.