[Ron Wood]

Tim,
Yes, the strawing does occur simply by means of the heat applied. It really is a controlled oxidation process. To say it occurs "simply" is perhaps an over simplification. A lot has to do with the preparation of the metal surface and the control of the temperature. As John's instructions indicate, thin portions of the metal tend to heat up faster than the thicker parts so the heating process needs to be applied uniformly, monitored carefully and the parts cooled/quenched relatively quickly when the correct color is attained.

An old trick is rather than place the parts on a pan and heat them up, fill the pan with clean (emphasis on clean) fine sand. When the sand has attained the approximate 450 degrees, push the part to be strawed into the sand (needle nose pliers or surgical hemostat are handy tools). The sand bath provides a uniform heating medium. Remove the part after a few seconds, check the color, and repeat as necessary to achieve the proper color. Quench immediately to stop the process. Like many things, color hardening is as much an art as a science.