When firing both lead bearing and lead free transparent flux (clear) directly on copper, the firing should be very hot and long enough for the enamel to absorb the copper oxide from the metal into solution with the glass. If the transparent color you fire looks dark or muddy, it could be that it is a color that needs flux underneath it, or it might be that you simply need to fire it hotter. You should experiment if you are not sure which colors will look good fired directly on copper.Make samples  and try refiring the color in a hotter kiln to see if the copper oxide will be absorbed in the glass and the color will clear. If they do not look clear fired directly on copper, you can always apply them over a properly fired clear flux undercoat.
Method #1: Fire the transparent colors directly on copper for two or three times at l400 to 1500 degrees for 3-5 minutes each. The piece will be VERY hot when the color is clear and because it is fired so hot, the enamel will pull from the edges. The edges will be dark with copper oxide and you may wish to apply more enamel i those areas if it is not an effect that you want.
Method #2: Fire for a shorter time in a kiln that is 1600-1700 degrees until the color is clear and brilliant. Do not use this method if you have solder joints on the piece that might come apart of become brittle at very high temperatures.
Method #3: Â Fire with a torch, holding it on the underside of the copper until you see bright orange red color from the heat and can tell that the enamel is very transparent, essentially over-firing as in method #1.
BEWARE! Do not get into the habit of over-firing all enamel as hot as you do when firing transparent colors directly on copper. Successive layers of transparent enamels should be fired at normal melting temperatures and times...just until the glass is fused. It is only on the first firing that the coper oxides at the interface of the base metal need to be absorbed into the glass during the hot, long, or multiple firings.
(Notes from Linda Darty)
Method #1: Fire the transparent colors directly on copper for two or three times at l400 to 1500 degrees for 3-5 minutes each. The piece will be VERY hot when the color is clear and because it is fired so hot, the enamel will pull from the edges. The edges will be dark with copper oxide and you may wish to apply more enamel i those areas if it is not an effect that you want.
Method #2: Fire for a shorter time in a kiln that is 1600-1700 degrees until the color is clear and brilliant. Do not use this method if you have solder joints on the piece that might come apart of become brittle at very high temperatures.
Method #3: Â Fire with a torch, holding it on the underside of the copper until you see bright orange red color from the heat and can tell that the enamel is very transparent, essentially over-firing as in method #1.
BEWARE! Do not get into the habit of over-firing all enamel as hot as you do when firing transparent colors directly on copper. Successive layers of transparent enamels should be fired at normal melting temperatures and times...just until the glass is fused. It is only on the first firing that the coper oxides at the interface of the base metal need to be absorbed into the glass during the hot, long, or multiple firings.
(Notes from Linda Darty)
