Dross

For higher melting point metals and alloys such as steel and silver, oxidized impurities melt and float making them easy to pour off.

With wrought iron, hammering and later rolling remove some dross.

[1] With tin and lead the dross can be removed by adding sodium hydroxide pellets, which dissolve the oxides and form a slag.

[2] The term dross derives from the Old English word dros, meaning the scum produced when smelting metals (extracting them from their ores).

[3] Dregs,[3] and the geological term druse are also thought to be etymologically related.