The colour change in the copper foil is due to oxidisation - if the copper foil is completely deprived of oxygen it stays shiny and copper coloured. If you leave copper exposed at all it will go metallic blue or even bottle green, mostly it turns a lovely burgundy red colour- an intermediate oxidisation stage.
Through doing some experiments with art school students, I have found the speed of firing is critical in an electric kiln. In a gas kiln the speed is normally fast anyway and produces better results than an electric kiln. It also is a kiln with a reducing atmosphere rather than oxidising one of an electric kiln.
The main elements in keeping copper inclusions (and by extension, other metals) bright is to keep the metal from oxidising. Two elements are important in this:
- Keep oxygen from the metal
- Reduce the time the metal is exposed to high temperatures
Various methods are used to keep the metal from exposure to oxygen. Some of these involve:
- coating the metal with fluxes to reduce the amount of oxygen in contact with the metal.
- using a reducing atmosphere, such as a gas kiln.
- placing an oxygen hungry material in the kiln with the glass and metal.
- coating the metal with glass powder before encasing it within the glass.
Reducing the heat exposure of the metal also indicates that firing fast would provide better results. This requires very even heating within the kiln to avoid heat shocking the glass. This is where a gas kiln is most advantageous - it can be fired fast without breaking the glass and it has a reducing atmosphere within it.
In general, it is easier to make use of the effects of the oxidised metal rather than striving for bright metal inclusions.