Complex sentence: The powder melts, flows, and then hardens to a smooth, durable coating on metal, and also glass or ceramics, although the use of the term "enamel" is often restricted to work on metal, which is the subject of this article.
Split into simpler sentences: The powder melts, flows, and then hardens to a smooth, durable coating on metal, or on glass or ceramics. The term "enamel" is most often restricted to work on metal, which is the subject of this article.