· by Michael Blair, Nashville's Masonry
The fired face of a brick is its protective layer and it is thin. Removing it with pressure or aggressive chemistry exposes the softer body, which then absorbs water and spalls.
Pressure is the main risk
High pressure erodes both mortar joints and the brick face. The damage is not obvious immediately, showing up as accelerated weathering over following years.
Low pressure with an appropriate cleaner and dwell time removes far more with none of the harm.
What each problem needs
Green algae and mildew, common on shaded north walls in our humidity, respond to a mild detergent or a dilute sodium hypochlorite solution and a soft brush.
Efflorescence — the white powdery bloom — is salt carried out by moisture. Brush it dry first, and address the moisture, because cleaning without that means it returns.
- Low pressure and dwell time, not high pressure
- Wet the wall before applying any cleaner
- Test in an inconspicuous area first
- Efflorescence: brush dry, then fix the moisture
- Never use hydrochloric acid on soft or historic brick
Acid cleaners
Muriatic acid is sold for brick and it burns mortar, etches soft brick and can leave a permanent yellow-green staining reaction with iron in the clay.
On modern hard brick with a heavy construction stain it has a place, heavily diluted, on a pre-wetted wall, tested first. On historic brick it does not.
Paint on brick
Removing paint from brick is difficult and frequently damages the face. Chemical strippers designed for masonry with a poultice work best; abrasive methods almost always cause harm.
It is worth being realistic that a full clean removal is often not achievable.