Silica is added during smelting in order to remove iron oxides from copper sulphide and oxides. During roasting the iron sulfides react with oxygen (in air or oxygen enriched air) to produce iron oxides (mainly as $F$ eO, but with some magnetite $\left.\left( Fe _3 O _4\right)\right)$ and sulfur dioxide.

Copper sulfide and iron oxide can mix, but when sufficient silica is added, a separate slag layer is formed.

The slag forming reaction is: $FeO + SiO _2 \rightarrow FeO \cdot SiO _2$

Slag is less dense than matte, so it forms a layer that floats on top of the matte.