Inclusions of metals can be achieved with care. Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass. This note provides some things you might consider when planning to include copper in your fused pieces.
The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin. Copper leaf can be very faint if a single layer is used. Placing several layers of leaf improves the colour, but often provides wrinkles. In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.
Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles - sometimes a single large one.
Copper can provide several colours.
Copper sheet normally turns burgundy colour when oxidised. This means that there is enough air reaching the copper to oxidise it to deep copper red. This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.
To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing. Reduction of air contact with the metal helps to retain the copper colour. There are two methods I have used. Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface. Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.
In certain circumstances you can get the blue green verdigris typical of copper in the environment. This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation. The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures. Experimentation is required to achieve this consistently.
The spaces under and over the copper give the opportunity for bubbles to form.
This means that the copper needs to be as flat as possible for one thing. Burnishing the copper can have a good effect on reducing the undulations in the copper. Thinner copper is easier to make flat than thicker. If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat. Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal. This remains the case even after burnishing to be as flat as possible.
The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass. The spread of the powder over the glass assists in giving places for the air between layers to escape.
These two things combined with a long slow squeeze can reduce the amount of bubbles you get. It cannot totally eliminate them.
Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.