Straw parts

[Kyrie]

Originally Posted by Norme

To appreciate the range of colors produced by heat treatment one need look no further than the hold-opens on Imperial era Lugers, some of which display a spectrum of colors (see photo).
Krieghoff Lugers are frequently encountered with a mixture of yellow, orange and vivid blue small parts, likely a result of erratic temperature control.
Norm

Or could be a good example of selective heat treatment. Plant the rear of the part in a heat sink, flame harden and draw the front of the part, and the boundary between the protected/unprotected sections of the part can show as that heat gradient.