When a number of very small articles are to be hardened, it is advisable to connect them with strong bailing wire before they are placed in the box so that they can all be removed at once. Beside holding the articles together, the wire will provide a means of testing the depth and quality of the process.

If only a thin coating of hardness is needed, or the labor and expense are excessive, the following method may be used: The article is heated thoroughly and evenly to about a bright red and thoroughly sprinkled with, or rolled in, cyanide of potassium. Then it is reheated so that the cyanide may penetrate as deeply as possible, after which it is quickly chilled in cold water. This is a good method of hardening small tack hammers made of soft steel, set screws, nuts, and very small tools.

Temperature and Color Chart to be Used in Tempering

ToolsTemperature (Fahr.)
Degrees		Color
Scrapers for brass430Very pale yellow
Light turning tools430Very pale yellow
Lathe and planer tools for steel430Very pale yellow
Steel engraving tools430Very pale yellow
Milling and boring cutters460Straw yellow
Screw-cutting dies460Straw yellow
Taps and reamers460Straw yellow
Punches and dies480Dark straw
Penknives480Dark straw
Twist drills500Bronze
Plane irons500Bronze
Surgical instruments530Dark purple
Cold chisels for steel540Dark purple
Cold chisels for cast iron550Dark blue
Cold chisels for wrought iron550Dark blue
Springs570Very dark blue

Suitable Temperature (Fahr.) for:

Degrees
Annealing tool steel900
Forging tool steel1200 to 1500
Hardening tool steel1200 to 1400
Casehardening iron or soft steel1300 to 1500

Colors and Corresponding Temperatures (Fahr.) for Iron

Bright red in dark 750 to 760
Red hot in twilight 880 to 890
Dark red hardly visible in daylight 970
Red visible by daylight1070
Brighter red by daylight1300
Cherry red by daylight1450
Bright cherry red by daylight1650
Light cherry red by daylight1800
Orange2000
Yellow2150
White heat2350
White welding heat2600
White welding and dazzling2800