Dross is a mass of solid impurities floating on a molten metal bath. It appears usually on the melting of low melting point metals or alloys such as tin, lead, zinc or aluminium, or by oxidation of the metal(s). They can also consist of impurities such as paint leftovers. It can easily be skimmed off the surface before pouring the metal into a mold or casting flask.

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

Dross is distinguished from slag, which is a (viscous) liquid floating on the alloy, by being solid. Dross product is not entirely waste material, Aluminium Dross for example is used in Secondary Steelmaking for slag de-oxidation reducing processing costs by minimizing use of pure Aluminium.

Casting | Metallurgy