Dross

Dross is a mass of solid impurities floating on a molten metal bath. It appears usually on the melting of low melting point alloys such as tin, lead, zinc or aluminum, 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.

With tin and lead the dross can also be removed by adding caustic soda NaOH 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.