will make quite a difference and
cause confusion. It is these possible sources of error, coupled
with the ever present chance of human error, that makes it advisable
to draw the temper of tools in an oil bath heated to the proper
temperature as shown by an accurate high-temperature thermometer.

Another table, by Gilbert and Barker, runs to much higher temperatures.
Beyond 2,200 deg., however, the eye is very uncertain.

TABLE 26.--COLORS FOR TEMPERING TOOLS
-----------------------------------------------------------------------
Approximate |
color and | Kind of tool
temperature |
--------------|--------------------------------------------------------
Yellow | Thread chasers, hollow mills (solid type) twist drills
430 to 450 deg.F.| centering tools, forming tools, cut-off tools, profile
| cutters, milling cutters, reamers, dies, etc.
--------------|--------------------------------------------------------
Straw-yellow | Thread rolling dies, counterbores, countersinks. Shear
460 deg.F. | blades, boring tools, engraving tools, etc.
--------------|--------------------------------------------------------
Brown-yellow | Taps, Thread dies, cutters, reamers, etc.
500 deg.F. |
--------------|--------------------------------------------------------
Light purple | Taps, dies, rock drills, knives, punches, gages, etc.
530 deg.F. |
--------------|--------------------------------------------------------
Dark purple | Circular saws for metal, augers, dental and surgical
550 deg.F. | instruments, cold chisels, axes.
--------------|--------------------------------------------------------
Pale blue | Bone saws, chisels, needles, cutters, etc.
580 deg.F. |
--------------|--------------------------------------------------------
Blue | Hack saws, wood saws, springs, etc.
600 deg.F. |
-----------------------------------------------------------------------

CHAPTER X

HIGH-SPEED STEEL

For centuries the secret art of making tool steel was handed down
from father to son. The manufacture of tool steel is still an art
which, by the aid of science, has lost much of its secrecy; yet
tool steel is today made by practical men skilled as melters,
hammer-men, and rollers, each knowing his art. These practical
men willingly accept guidance from the chemist and metallurgists.

A knowledge of conditions existing today in the manuf