A Midtown Atlanta building came to me after a cleaning contractor had taken a hundred years of grime off its brick in two days. He had also taken the fired skin off about a third of the units. The wall was cleaner and it had lost maybe forty years of service life.
Start by identifying the soil
You cannot pick a cleaning method without knowing what you are removing. Five broad categories cover most of it:
- Atmospheric particulate — carbon and dust bonded lightly to the surface. The easiest to remove.
- Biological growth — algae, lichen, moss. Needs a biocide, not abrasion, and it will return if the moisture source stays.
- Metallic staining — rust from anchors or railings, copper runoff. Requires specific chemistry, and the source has to be dealt with first.
- Efflorescence and salts — dry brushing or poulticing, never acid, which can make it worse by introducing new salts.
- Paint and coatings — chemical strippers or poultices; almost never abrasion on historic units.
Getting this wrong is how walls get hurt. Acid on limestone or marble dissolves the stone. Alkaline cleaner on brick can leave salts that bloom for years. Bleach on lichen kills the surface growth and leaves the roots.
Pressure, and the number that matters
Most damage in masonry cleaning comes from pressure. The rule of thumb for historic brick is 200 to 400 psi at the surface with a fan tip held well back — closer to a garden hose than to the 3,000 psi machine on the truck.
Two variables people ignore: tip distance, which changes effective pressure dramatically, and dwell. A 400 psi wand held six inches from a soft brick will do more damage than 1,200 psi held two feet away and kept moving.
Sandblasting has no place on historic masonry at all. It removes the dense fired skin that protects the porous body underneath, and the damage is permanent and accelerating.
The sequence that protects the wall
- Test panels. A three-foot square in an inconspicuous spot, several methods side by side, left to dry and assessed after a week. Non-negotiable on anything historic.
- Gentlest method that works. Water and a soft brush first. Escalate only if it fails.
- Pre-wet the wall before any chemical, so the cleaner stays at the surface rather than being drawn deep into the pores.
- Neutralize and rinse thoroughly. Residual chemical in a wall keeps working and shows up as staining months later.
- Poultice for stains that are in the material, not on it — clay or cellulose plus the appropriate solvent, applied wet, covered, allowed to dry and draw the stain out. $8 to $20 per square foot and slow, but it is the only method that removes what is inside the stone.
Cost bands: low-pressure water washing $1.50 to $4 per square foot, chemical cleaning $3 to $9, poulticing $8 to $20, paint removal $6 to $18. Anyone quoting a large building for under a dollar a foot is planning to use pressure.
On commercial buildings the work often goes to a general cleaning firm rather than a restoration specialist, which is where the mismatch starts. The question worth asking an Atlanta industrial cleaning company before mobilization is what pressure they intend to use and whether test panels are in the scope — those two answers predict the outcome better than any reference photo.
Frequently asked questions
Is soda blasting safe for old brick? Gentler than sand but still abrasive, and sodium bicarbonate leaves salts in the masonry that can cause efflorescence for years afterward. It has legitimate uses on some substrates. On soft historic brick, test it carefully or avoid it.
Should a wall be cleaned before repointing? Generally after, not before — repointing is messy and you would clean twice. The exception is when the soil layer prevents you from assessing the joints or matching the mortar color, in which case a test area is enough.
Will cleaning make the wall more vulnerable? A correctly cleaned wall is not more vulnerable. An over-cleaned one is, because losing the fired skin on brick or the case-hardened surface on some stones exposes a softer, more absorbent material underneath. That is why the gentlest effective method is the rule.