Antimony metal (atomic number 51) in powder/granular form. Antimony has a melting point of 1167 F and has the unusual property of expanding when cooling. 'Hardened Lead' is made by adding small amounts (typically 6-12%) of Antimony metal to molten Lead. Antimony metal powder is also used in some specialized pyrotechnic formulations to produce unusual effects. Under the proper conditions, it burns quite brilliantly when heated producing Antimony Trioxide.
CAUTION: Antimony Metal is toxic. Use normal chemical safety precautions when working with Antimony Metal.