What is the brown scum which appears at the top of the black oxide tank?

The brown scum is impurities from the chemicals along with grease and oil, which comes to the surface during processing. Remove the brown scum with a stainless steel strainer and place into a plastic pail and dispose of according to local, state, and EPA regulations. It is suggested to skim brown scum at the beginning of the day and again half way through the day.