Although patina tends to fade away over time, and I have not been able to find a reliable way to preserve it, I am still curious about what makes it happen. I have tried both kinds of carbon, new and old, and the results were the same. I did notice, though, that the more pieces I have in the box, the less patina I get. The amount of carbon, relative to the number of pieces in the box, seems to make a difference.
In the past few days I have been emptying the box into a sieve while the carbon was still very hot. The colors were amazing. Here are the colors on fired copper right off a very hot box:
If you try it, be careful. Wear a mask and goggles and hold the box with insulating gloves. Take the box out with a kiln fork and lay it on an incombustible surface like concrete or bricks.