Why is a metal pot suddenly iridescent? I have an old metal pot and for a few days every time I take the pot out of the dish washer it has been quite beautifully iridescent:


Any idea why this suddenly occurred and how I can explain it (to my son)?
I have read about iridescence, but saw only a description of iridescence in water (thin-film interference) but the metal doesn't seem to have the two layers necessary for the effect...
 A: I will assume from its appearance that this pot is made of stainless steel, which will neatly account for the colored fringes that you see in its bottom, as follows.
Stainless steel refuses to rust because it contains enough chromium to form a tightly-adherent, extremely noncorrosive layer of chromium oxide on the surface. the chrome oxide prevents the water from actually coming into contact with the stainless steel and similarly blocks oxygen from touching the steel surface. In this way, it halts chemical attack and the stainless steel remains bright, shiny, and nonrusting.
This layer of passive oxide, when grown at low temperatures, is extremely thin. But as you raise the temperature of the stainless steel, oxygen can diffuse down through the oxide and react with the chromium, which is simultaneously diffusing up through the (hot) oxide to react with the oxygen. This causes the oxide layer to get thicker, until it thickens to a point where diffusion is once again shut down, and its thickness stops increasing.
The thickness of the chrome oxide (which is optically transparent!) eventually gets thick enough to create interference fringes which appear as colored rainbows, as in your pot bottom. As the oxide thickens, the fringe colors cycle through the rainbow- in fact, with a calibrated chart you can deduce the oxide thickness simply by noting its color.
As the oxide grows thicker still, the color cycle starts over and repeats itself- and again, if you know both the color and the number of color cycles, you can deduce the total oxide thickness.
What happened to your pot to suddenly create this rainbow oxide? You accidentally left the pot empty on a hot burner. This started the oxide to thicken and re-equilibrate after running through two rainbow cycles.
In fact, the odd pattern of the oxide growth is a temperature map of the bottom of the pot while it was getting dry-fired on the burner!
