Over time and especially when the user has a pattern of sewing very slowly, the carbon from the brushes gradually rubs off onto the armature of the motor. As this thickens the carbon build up causes the motor to struggle and slow down. Eventually, the motor may fail completely. Replacing the brushes does not fix the problem because the residue must be removed. This may be done by burnishing the armature, but that can be risky. Running the motor at full speed for several minutes will often burn off the excess carbon. This is called a motor burn.