Chabad.org — English translation + Rashi
Torah Temimah (Menachos 28a): chatzotzros made from scrap silver are valid; from other metals invalid.
English: It was taught: the chatzotzros came from a block, from silver. If one made them from scrap pieces — they are valid; from other kinds of metal — they are invalid.
Material requirement — silver is required; the “mikshah” rule is lighter than for the Menorah.
The trumpets must be silver, but need not be beaten from a single unbroken mass in the same absolute way as the Menorah.