Definition
The non-metallic residue and impurities that separate from molten metal during welding, cutting, or smelting and rise to the surface, where they harden into a brittle crust on the finished bead or cut edge.
Plain English
The crusty leftover material that forms on top of a weld or cut as the metal cools. It is not part of the metal itself and is chipped or brushed off after the work is done.
Context Anchor
Seen in aircraft welding, weld inspection, and metal repair discussions.
Derivation
From Middle Low German 'slagge,' meaning the splinters or refuse thrown off when metal is hammered or smelted. The original sense — waste flung off during metalwork — is exactly what the word still means today.
Why Pilots Care
Slag left on a weld can hide cracks, porosity, or incomplete fusion underneath. On aircraft structures, an inspector cannot judge weld quality until the slag is removed, so this is a routine step in any welding repair the pilot might commission or sign off on.
Intuition Check
Slag is not the weld itself. It is waste material from the welding process that must be removed or accounted for during inspection.
Example Sentence 1
After completing the weld on the engine mount, the mechanic chipped away the slag before inspecting the bead for cracks.
Example Sentence 2
Any remaining slag on the repaired fuselage skin was brushed off before the dye-penetrant inspection.