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How does bismuth help type-metal alloys make clean castings?

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A labeled periodic table element box for bismuth shows atomic number 83, symbol Bi, atomic weight 208.9804, electron configuration, and properties including solid state, rhombohedral crystal structure, and weakly acidic oxides. — bismuth Properties of bismuth.

Bismuth is a valuable component in type-metal alloys because it possesses the unique property of expanding upon solidification. This expansion ensures that the molten metal fills the details of a mold completely as it cools, resulting in neat, clean castings.1