When steel and iron are heated to a high temperature they first become red, then orange, and then white. The temperatures may be approximately told by the color as follows:

First sign of red

957° F.

Dull red

1290°

Cherry red

1655°

Bright red

1870°

Dark cherry

1470°

Dull orange

2010° F.

Bright orange

2190°

White heat

2370°

Bright white

2550°

Dazzling white

2730° to 2910°

The following table represents the colors which appear on a polished steel surface when heated in the air.

Color

Approximate Temperature

Used in Tempering

Very pale yellow

430° F.

Hammering tools, lathe and planer tools, scrapers.

Straw yellow

460°

Taps and dies

Dark straw color

480°

Reamers, knives, punches, and dies

Brownish yellow

500°

Drills, stone-cutting tools

Yellow tinged with purple

520°

Angles, draft pins

Light purple

530°

Cold chisels for steel, augers

Dark purple

550°

Springs, screw - drivers, cold chisels for iron, hatches

Dark blue

570°

Saws for wood

Pale blue

610°

Blue tinged with green

630°