Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low-melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. For higher melting point metals such as steel, oxidised impurities melt and float making them easy to pour off. (Persian: خُبث، سرباره)
With wrought iron, hammering and later rolling removed a few dross. With tin and lead the dross can be removed by adding sodium hydroxide pellets, which dissolve the oxides and form a slag. If floating, dross can additionally be skimmed off.
Dross, as a solid, is distinguished from slag, which is a liquid. Dross product isn't entirely waste material; for example, aluminium dross can be recycled and is used in secondary steelmaking for slag deoxidation.
Etymology and usage
The term dross derives from the Old English word dros, meaning the scum produced when smelting metals. By the fifteenth century it had come to refer to rubbish in general. Dregs, and the geological term druse are additionally thought to be etymologically related. The most popular usage of the word is now as an adjective for poorly written or even plagiarised journalism.