I visited Marshall at his home on Lopez Island about a year ago and was so impressed that I bought his book and built my own cupola. The other suggestions are correct, however.
Cupolas are primarily for iron and higher temp non-ferrous metals such as bronze.
I strongly recommend the Gingery series of books, as well as Lindsay Publications as an excellent resource of how-to books and videos on the subject. Feel free to e-mail me any time if you need other ideas.