slag
slag
/slæɡ/
Stony waste matter separated from metals during smelting or welding; or to produce slag.
In arc welding, flux melts into a floating glass-like crust over the weld pool called slag, which shields the liquid metal from atmospheric oxygen as it freezes and must be chipped off afterward.
The story behind the word. Slag was borrowed into Middle English from Middle Low German 'slacke,' related to words for hitting or striking, because blacksmiths knocked off the cooling waste crust from forged iron pieces.
Word relationships
- dross: Dross refers to solid scum floating on liquid metal, whereas slag refers to glass-like vitreous waste produced by flux reactions or smelting.
- pure metal: Pure metal is the refined desirable element, while slag is the discarded stony byproduct.
Commonly confused with
- sludge: Sludge is wet muddy sediment, whereas slag is stony, glassy mineral waste formed at high temperatures.
Word family
- slaggy (adjective): Containing or resembling metalworking slag
Collocations
- slag heap
- chip slag
- slag inclusion
- molten slag
Example sentences
- "After completing the weld pass, the fabricator used a chipping hammer to knock off the brittle layer of slag covering the bead." Welding inspection
- "Hills of black slag surrounding the abandoned steel mill served as quiet monuments to the city's industrial past." Industrial history
Memory hook
SLAG is the SAGGING glass crust you SLAP off the weld. Chipping off the glass waste crust.
When not to use
In British slang, 'slag' is an offensive insult—ensure technical register in metalworking contexts.
Fun facts
- Crushed blast-furnace slag is not wasted today—it is widely ground into fine powder and added to modern concrete mixes to make them stronger and more water-resistant!
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