Nonmetallic scum forming on molten metal during smelting or refining.

• Byproduct of separating metal from ore; fused impurities.

Hard stony residue from blast furnaces or mines.

• Used as aggregate or piled as industrial waste.

Vesicular volcanic rock ejected as cindery slag.

• Frothy, porous material around basaltic eruptions.

To form or accumulate slag during a metallurgical process.

• To remove or convert material to slag.

Informal, chiefly UK: to criticize or speak ill of someone.

• To denigrate or mock, often publicly.