I have a bowl in my kitchen that turns black when heated (when it goes to the microwave oven or the washing machine). It is driving my crazy so I started to investigate. Here are the results: This is how it looks after cleaning it: After going to the microwave it becomes like this: The covered part of the bowl also became dirty so the black stuff seems to come from the inside of the bowl. Now it started to investigate at which temperature it becomes black. I put it in the oven and set the temperature to 120º C (248º F) and nothing happed. At 150º C (302º F) the black stuff started appearing at the bottom: Then took the oven to 220º C (428º F) and it stayed the same. Please help me solve this mistery! I have other bowl equal to this one that also suffers from this stange disease.