Matte is a term used in the field of pyrometallurgy given to the molten metal sulfide phases typically formed during smelting of copper, nickel, and other base metals.
[1] Typically, a matte is the phase in which the principal metal being extracted is recovered prior to a final reduction process (usually converting) to produce blister copper.
Mattes may also be used to collect impurities from a metal phase, such as in the case of antimony smelting.
[2] Molten mattes are insoluble in both slag and metal phases.
This insolubility, combined with differences in specific gravities between mattes, slags, and metals, allows for separation of the molten phases.