I’ve been curious about the effect of copper in glass. In some experiments, I’ve gotten a bright blue color, and in others there is very little change. I think it is based on how much oxygen reaches the copper during firing, before the copper is covered with glass.
As a rough experiment, I decided to cast a cross using 14 gauge copper wire. The wire is suspended between two layers of glass, so there should be some room for the air to reach the copper. There was some reaction, but still less blue than I had been expecting.
Here is the sequence of steps.