Dross

Dross is the mass of solid impurities that forms on top of or within a molten metal, produced mainly through oxidation on lower melting point metals and alloys such as tin, lead, zinc and aluminium. Different metals call for different removal methods: dross can be worked out of wrought iron by hammering and rolling, while sodium hydroxide pellets can dissolve oxides in molten tin or lead into a skimmable slag. Commercially, dross is not simply discarded: aluminium dross in particular is recycled and finds a second use in secondary steelmaking, where it helps deoxidize slag, and the distinction between dross, which stays solid, and slag, which becomes liquid, is an important one in industrial metal processing. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/