Could your "copper" have actually been a low zinc brass? Barbara Lewis has published a book that includes her method of torch firing enamel on low zinc brass. The colors of transparents on brass are different and quite lovely. The book is "Mastering Torch-Fired Enamel Jewelry," by Barbara Lewis. Other techniques are included in her book.

Has anyone found a source for a variety of copper alloys? I came across an old sheet of copper and used it for an etching project. The color of the copper on the finished enameled piece was quite different then my regular stock. More like silver. I am intrigued with experimenting with different alloys.