Clean carbon steel heated in air grows a thin oxide layer whose color reports its temperature — the oldest thermometer in the smithy, and still the fastest way to read a temper or diagnose a grinding burn. Colors below are for clean, bright steel; scale, oil and alloy shift them.
| Color | Approx. temp | What it means on a blade |
|---|---|---|
| Faint / pale straw | ~390–410°F | The classic low temper — maximum hardness territory for most carbon steels (razors, fine slicers) |
| Straw | ~420–440°F | The everyday knife temper zone — the usual 1-2 point HRC trade for stability |
| Dark straw / bronze | ~450–470°F | Working knives wanting extra forgiveness; many camp blades live here |
| Brown-purple | ~480–500°F | Big-blade and impact territory – hardness clearly traded for toughness |
| Purple | ~510–530°F | Spring and tool tempers; softer than most blade edges should run |
| Blue | ~540–570°F | Springs, spines in differential draws – and the classic grinder-burn verdict on an edge |
| Pale / gray-blue | ~590–640°F | Fully drawn soft; screwdriver country |
Reading a grinder burn
Color at the edge mid-grind is this chart applied in reverse: compare the color’s temperature against your tempering temperature — straw on a 400°F-tempered blade is cosmetic, purple or blue means that zone is now over-tempered soft, and sanding off the color does not restore hardness. Full triage in Burned Edge on the Grinder.
Tempering by color, honestly
Torch-tempering by color is real and traditional (spine draws on differential blades use it deliberately — see the differential guide), but an oven with a verified thermometer beats eyes for whole-blade tempers: color reads the surface’s peak, not the soak that hardness actually wants. Use the chart to diagnose and draw; use the oven to temper. The full process lives in the heat treat guide.
Browse more tools in the Calculators section.