"Richard J Kinch" wrote: More to it than just that. The thickness (1)
equilibrates instead of just growing, and (2) differently so with differing
temperatures, and (3) reversibly so since the thickness follows the
temperature both up and down.
^^^^^^^^^^^^^^
Your additional explanation answers some of the questions I have had, but it
raises others that I wish I had answers to. We know that steel oxidizes in
contact with air. We know that chemical reactions are accelerated at higher
temperatures. So, I always assumed that as the steel is tempered, the oxide
film grew continuously, but more rapidly as temperature went up. According
to your esplanation, this is not true--instead the film thickness reaches an
equillibrium thickness depending on temperature. Now I think I understand
why the temperature can be judged by the color, and time does not enter into
it.

Now the part that baffles me. If the film thickness tracks the temperature
reversably, as you say, why doesn't the film/color disappear as the steel
cools?