Mariam is a housewife. She lives in a small town. She has gone to visit her village house. Prior to her departure, she kept her utensils, crockeries, chopper, spoons, etc. under lock and key. She returned home after a few days. She saw that there was deposition of brown colour on her chopper. The cooking pots, spoons, remained the same as they were. Of the crockeries, some are made of metal and some are of non-metals.
Q. What is the name of brown coloured coating on the chopper and knife?