Copper is one of the most popular metals to enamel on. Whether using transparent, opaque or opal enamels you usually need a flux, as you do when enameling on most metals. There are some colors of enamel you will find, you can get by with out flux but you will have to run a test to find out.
When applying your flux coat on copper use very fine ground enamels, 150 mesh, this will ensure quick fusing of the enamel coverage and eliminate oxidation. =little brown spots on the metal.
Copper is the one metal that can be fired higher also. I fire my enamel on gold or fine silver at 1425 degrees where as copper is better suited at 1500 and even 1525 degrees, and I have see enameling artist fire at 1550 degrees.
One complaint of copper is the oxidation. But some artist use this to their advantage of design. Leaving the oxidation and enameling over it. You get a rough look and can get some very beautiful colors in the copper, which can add to the enamels.
It is best when making jewelry with copper and enamel, to apply enamel to both sides of the metal and fire at once. You can do this by adding Klyre-Fire to the enamel on the back. This is called the counter enamel. A few drops of Klyre-Fire added to your wet enamel, makes the enamel stick to the back so you can flip it over and apply enamel to the front and have one firing.
Above is a cleaned piece of copper, of 18 ga. by using penny brite. In the center photo the wet counter enamel is applied that has Klyre-Fire added to it so once I have used the brush to remove the excess moisture I can flip it over. In the third photo enamels are being sifted on. Once you have full coverage fire at your desired temperature from 1500 – 1550 degrees for approx. 1 minute and 15 seconds. Or hydraulic the enamels appear smooth.