At a glance
- The colours come from a thin oxide layer, not from the steel itself changing colour.
- They only show on clean, polished steel.
- Time matters as well as temperature: a long hold at a lower heat can give a darker colour.
- Most makers temper knives in an oven at a measured temperature, and use colours as a check.
| Colour | About °C | About °F | Traditionally used for | |
|---|---|---|---|---|
| Pale straw | 220 | 428 | Razors, scalpels, engraving tools | |
| Light straw | 230 | 446 | Knife edges, small cutting tools | |
| Dark straw | 240 | 464 | Knives, wood-carving tools, punches | |
| Bronze (brown) | 255 | 491 | Axes, wood chisels, shears | |
| Purple | 270 | 518 | Cold chisels, surgical tools | |
| Dark blue | 290 | 554 | Springs, saws, screwdrivers | |
| Light blue | 320 | 608 | Springs for heavier use; too soft for most edges |
What are temper colours?
When clean steel is heated in air, a very thin layer of oxide forms on the surface. As the layer thickens with heat, it reflects light differently and shows a sequence of colours: straw, bronze, purple, then blue. They are the same colours used throughout this site.
Why do knife makers temper steel?
Hardened steel straight from the quench is very hard but brittle, and can crack or shatter. Tempering reheats it to a lower temperature, trading a little hardness for a lot of toughness. The higher the tempering temperature, the softer and tougher the steel becomes.
Knife edges are usually tempered in the straw range, around 180–240 °C depending on the steel and the use. Tools that take heavy blows, such as axes and chisels, are tempered hotter, into the bronze and purple. Springs are tempered to blue.
How accurate are temper colours?
They are a guide, not a thermometer. The colour depends on how long the steel is held at temperature, on the steel itself and on the light you see it in. That's why most makers today temper in a kitchen oven or kiln at a measured temperature, usually for about an hour and often twice, and use the colours only as a check.
Colours are still useful for differential tempering, where the spine of a blade is drawn softer than the edge. A smith heats the spine and watches the colours run towards the edge.
Colours on screen are an approximation. Real temper colours vary with the steel, the finish and the light.
Questions people ask
What colour should a knife be tempered to?
Most knife edges are tempered to a light or dark straw colour, roughly 200–240 °C. Tougher tools such as axes go to bronze or purple.
What temperature is blue on steel?
Steel shows dark blue at around 290–300 °C and lighter blue above about 310 °C. At blue, most knife steels are too soft to hold a good edge.
Do temper colours affect the hardness?
No. The colours are only an oxide film on the surface. The temperature and time cause the change in hardness; the colour is just a sign of the temperature reached.
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Related pages
Sources
- Machinery's Handbook (Industrial Press): tempering colours of steel
- Knife Steel Nerds (Larrin Thomas): tempering articles