What caused the black and white dots on decorating coating?

1.The black dots are the contaminants on the surface of working pieces, including the residuals left when cleaning and the deposits from dusts, racks, furnaces and heating tubes, and those from the dusts of evacuation furnaces. When coating, these contaminants will not be coated but become dark bulges, and the substrates will be exposed when the black dots are cleaned out.

2.The white dots are exposed when contaminants on working pieces are blown off after coating. White dots on IP black coating are very distinct. Darker the color is, more the dots are, because white dots will been seen if the coating is not firm enough on the contaminant coatings.