A highly resistive glaze may form on brush surface, requiring periodic cleaning. This is sometimes also called a “patina” and forms a visible black non-conductive band on the shaft. It is also one of the disadvantages of using a carbon block brush for shaft grounding.  It renders the carbon block brush ineffective.

Carbon Block Brush causes Non-Conductive Coating on Motor Shaft "patena film"

Carbon Block Brush causes Non-Conductive Coating on Motor Shaft “patena film”

What to do? Clean the shaft where the carbon block brush contacts the shaft and install the AEGIS Shaft Grounding Ring in its place.