Black spots in porcelain coffee mug I bought a (fine china) coffee mug a couple of months ago and started using it recently. I saw 3 black spots in the inside of the mug and a lot more speck-like scattered on the outside.Initially, I assumed it to be just dust and tried washing with dishwashing liquid. It still remained and so,I tried my luck by soaking it in baking soda.Rinsed out the baking soda and filled with diluted vinegar. And then I tried scrubbing it with an brush but the spots do not seem to go away. What are these spots?
This is something that can happen during the manufacture of porcelain. It is due to iron particles in the clay used to make the porcelain, they get this look after being fired. It is part of the mug itself and cannot be removed. 

Customers don't like isolated black dots on white porcelain, so high end manufacturers only sell items without these dots. But if you bought something in the mid- or low-price range, or B-ware from the high end brands, these specks are common enough. 

It is only an optical defect, the mug itself is clean and you don't have to do anything more about it.