Hard Brush used for cleaning heat management devices (HMD)
As soon as you smoke with any HMD burnt tobacco will settle on the underside over time.
Proper cleaning doesn't take long and will leave your HMD looking almost new.
Soak your HMD in warm water for about 30-60 minutes. Then go over the underside of the HMD with Hard Brush HMD Cleaning until most of the burnt tobacco is gone. The longer and more thoroughly you do this, the better the result