Interference colors from an oxide film that thickens with temperature: pale straw near 200 °C, dark straw around 240 °C, bronze and purple near 260 to 280 °C, and blue around 300 °C. Toolmakers and smiths use them to draw the spine of a blade or the shank of a chisel while the edge stays hard. Time and alloy shift the colors, so they are a guide, not a measurement.
Run the torch along the spine till it goes blue, but keep the edge at straw, those are your temper colors.
Also heard as
- tempering colors
- oxide colors
- heat tint
Related terms
Draw
Shop word for tempering, as in drawing hardened steel back to a softer, tougher hardness.
Tempering
Reheating hardened steel below its lower critical temperature to trade some hardness for toughness.
Cherry red
Color name for steel glowing at roughly 750 to 800 °C, near where most hardening heats begin.
Bright
Describes parts that come out of the furnace clean and unoxidized, with no scale or discoloration.