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White powder on brick: what efflorescence means and how to remove it

That white, chalky bloom on a brick wall is usually salt left behind by water. It is rarely the problem itself, but it tells you where water is moving. Here’s what the National Park Service says about it.

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What it is

The National Park Service’s masonry glossary defines efflorescence as “a whitish haze of soluble salts on masonry.” Water moving through brick and mortar carries dissolved salts with it. When the water evaporates at the surface, the salts stay behind as a white powder or crust. NPS Preservation Brief 2 describes the same process: moisture leaving a wall deposits soluble salts “either on the surface as efflorescence or below the surface as subflorescence.”

Where the salts come from

The NPS glossary lists several sources, and an old Philadelphia rowhome can have more than one:

  • The ground. Capillary action can pull salts up into the wall from the soil, including chlorides from winter salting of streets and sidewalks and nitrates from fertilizers. Bloom along the bottom few courses of a front wall or garden wall often fits this pattern.
  • The mortar and the air. Carbonates from lime mortar, and pollutants in the air or in rainwater, can leave sulfates on the surface.
  • The brick itself. Sulfates from the curing and firing process are, in the glossary’s words, “a common source of efflorescence in brick.”
  • Cleaning. Efflorescence can be salt residue left by chemical cleaning, a cleaning solution that was too strong, or poor rinsing.
  • New mortar. Brief 2 notes that new construction “bloom” sometimes appears in the first few months after repointing. It also warns that some additives, such as antifreeze compounds, can introduce salts that cause efflorescence later.

Is it harmful?

Usually not by itself. The glossary says efflorescence “may be more unsightly than harmful,” and Brief 2 calls salts on the surface “usually relatively harmless.” The catch is what it signals:

  • Water is getting in. On an older building, the glossary says efflorescence “often serves as a warning, indicating that water has found a point of entry into the structure,” and that more serious damage can usually be predicted once that has happened.
  • Salt may be building up under the surface. The glossary calls that subflorescence “potentially damaging” and “most definitely a matter of concern.” As salts crystallize inside the brick, especially through freeze-thaw cycles, they build pressure that can make the face of the brick spall off or delaminate.

So the useful question isn’t only “how do I get it off?” but “where is the water coming from?”

Find the water first

Look at where the bloom is, and what is above or below it:

  • Under a window sill or cornice, or below a leaking gutter or downspout
  • Around flashing, a parapet or the top of a wall
  • Along the base of the wall, which points to ground moisture or splash-back
  • On a wall that was painted or coated. The Rowhouse Manual suggests considering removing paint on exterior masonry, which “can trap moisture inside and cause damage.”
  • Near clogged weep holes at the base of a brick wall, which the Manual says to clear so moisture can drain

Our checklist Does my brick need pointing? covers the other warning signs. NPS warns that repointing alone won’t fix problems caused by leaking roofs or gutters, settlement or rising damp, so the source of the water has to be fixed too.

How to remove it, per NPS

For new construction bloom after repointing, Brief 2 says it “usually disappears through the normal process of weathering.” If it doesn’t:

  1. Dry brush first with a stiff natural-bristle or nylon brush.
  2. Then wet brush with plain water.
  3. Don’t use metal brushes. Brief 2 says “metal bristle brushes should never be used on historic masonry.”
  4. Don’t use muriatic acid. Brief 2 says hydrochloric (muriatic) acid “is generally ineffective, and it should not be used to remove efflorescence,” because it may free up more salts, which can lead to more efflorescence.
  5. Go easy on water pressure. On newly repointed walls, Brief 2 recommends only very low pressure (100 psi) water washing with stiff natural or nylon brushes.

Expect it to come back if the water source isn’t fixed. Brushing removes the salt on the surface, not the moisture that brought it there.

Sealers, coatings and historic houses

It is tempting to seal a wall that keeps blooming. Be careful: a coating that stops the wall from drying can trap moisture, the same reason the Rowhouse Manual cautions about paint on masonry. If the property is on the Philadelphia Register of Historic Places, the City says project reviews are required for “masonry cleaning, painting, pointing, repair, replacement, alteration, or removal” and for “painting, coating, staining, or sealing surfaces except wood and metal trim.” Check with Historical Commission staff before you clean or coat a historic facade. See our pointing rules page.

What to ask a mason

  • Where do you think the water is coming from, and what would fix that?
  • Is any brick spalling or flaking under the bloom?
  • Do the joints need repointing, and with what mortar? (See lime vs. Portland mortar.)
  • How will you clean the wall, and will you avoid acids and metal brushes?
  • If my house is historic, will you handle the Historical Commission review?

Sources

  1. National Park Service: A Glossary of Historic Masonry Deterioration Problems and Preservation Treatments (Grimmer, 1984), Efflorescence and Subflorescence (PDF)
  2. National Park Service, Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)
  3. City of Philadelphia: Philadelphia Rowhouse Manual (PDF)
  4. Philadelphia Historical Commission: Get approval for work to a historic property

Checked November 4, 2026. Rules, programs and prices change, so confirm with the source. Cost figures are published ranges, not quotes.

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