Heat the copper red hot. Use a propane torch, and make sure it is heated red all over. Keep the torch on one side of the cell, do not let the flame touch the other side. Keep it red for a 2 min 40 sec. This will cause two types of copper oxide to form on the side away from the flame.
Cupric oxide is the black flaky stuff that forms.
Under this is pinkish cuprous oxide. Be sure not to damage the cuprous oxide. If the whole side of the cell is not covered, repeat the process. If it is not covered with cuprous oxide on the edges, just cut off the edges. Cover the side with the cuprous oxide with a silvering solution.