slag; dross
Refers to the waste material left over from smelting or refining metal.
At the steel mill, a large amount of slag is produced.
iron rust
Specifically refers to rust from iron; less common than the general term さび.
Iron rust was falling off the old lump of iron.
Rare kanji form; literally 'metal excrement'.
Rare kanji form; literally 'iron excrement'.
Compound of 金 (kana, 'metal') and 屎 (kuso, 'excrement'), literally 'metal excrement'. The exact historical derivation is uncertain, but the word has been used to refer to waste products from metalworking.