Martindale Carbon Brush Seating Stone, Commutator Cleaner – Brush Seater BRSRX12FH, 4/pkMotor Brush Carbon Brush Seating Stone and commutator cleaning Chalk stone. 1 4 x 1 2 x 4 3 4" soft white. Carbon Brush Seating Stone 4 in Box This helps clean the motor armature commutator and reduces sparking. This stone is specifically not intended for the initial "seating" of motor brushes. It is designed for application during and subsequent to the seating process to effectively remove carbon buildup.