premise: the <mask> of metals
hypothesis: the best context for <mask> here is The act or operation of melting or rendering fluid by heat; the act of melting together; but not An inclosed carriage or litter, commonly about eight feet long, four feet wide, and four feet high, borne on the shoulders of men by means of two projecting poles, -- used in India, China, etc., for the conveyance of a single person from place to place..