Where do iridescent spots come from at the bottom of the pan after washing in the dishwasher?

Question

Physics

Despite the naivety of the question, he is quite busy. The first thought - a film from detergent - was not confirmed, the pan is dry. The second is oxidation, but I don’t understand what with what. Please tell me which way to look in order to understand the issue.
pot "stainless steel 18/10".

Answer 1

1. Rainbow streaks are almost always the result of interference in thin films.
2. About what exactly is a thin film at the bottom of your pan, you can assume for a long time. Several options:
- if stains are observed inside, but they are not outside, it can be assumed that this is a film of metal oxides that make up the steel, since the inner surface is heated in contact with what is being cooked, and the environment there can be very far from neutral;
- if stains are visible over the entire surface, then this may be a film of surfactant, which is part of the detergent.
3. There is probably no "kitchen" means to check this. Only if you take sandpaper, rub the bottom, and then watch when the streaks appear again, after a long time of use, or immediately after the first wash.

Answer 2

What is the saucepan from? :-)