Remove Stubborn Stains
For stubborn stains, use something stronger than soap and water. Muriatic acid is an abrasive chemical used to clean fireplaces.
Safety Alert: Muriatic acid is a harsh chemical, and is also toxic. Follow all safety precautions printed on the bottle.
Mix one part muriatic acid with 10 parts water, and apply the solution to the brick or stone surfaces of the firebox.
Let the solution remain on the surface for about 30 minutes before scrubbing with a wire brush (Image 1).
Also use a metal brush to remove rust and debris from the damper (Image 2). The damper may be difficult to access, so wear old clothing, eye protection and gloves for this job. Make sure that the damper mechanism is working properly so that the damper can open and close freely.