Complex sentence: Since alchemists had shown that when stibnite, an antimony sulphide ore was heated with scrap iron, metallic antimony was produced, the typefounder would typically introduce powdered stibnite and horseshoe nails into his crucible to melt lead, tin and antimony into type metal.
Split into simpler sentences: Since alchemists had shown that when stibnite, an antimony sulphide ore, was heated with scrap iron, metallic antimony was produced. The typefounder would typically introduce powdered stibnite and horseshoe nails into his crucible to melt lead, tin and antimony into type metal.