Restoring Historic Brick and Stone Without Blowing Out the Mortar

Old masonry is beautiful. It's also fragile in ways that aren't obvious until someone hits it with 120 PSI and aggressive media. We've been called in to fix jobs where a well-meaning contractor destroyed 100-year-old mortar joints trying to clean brick with equipment and techniques meant for structural steel.

Here's how we approach historic masonry and brick restoration.

The Problem with Standard Blasting on Old Masonry

Historic brick and lime mortar are significantly softer than modern materials. Pre-1920s brick was often fired at lower temperatures and is more porous. The mortar between those bricks is typically lime-based (not Portland cement), which is softer still — around 2.0-3.0 on the Mohs scale.

Standard sandblasting at 80-120 PSI with aggressive media will:

• Blow out mortar joints. The mortar disintegrates under pressure, leaving gaps that allow water infiltration.
• Pit the brick face. The hard outer fire skin of old brick is only a few millimeters thick. Blast through it and you expose the soft interior, which absorbs water and spalls in freeze-thaw cycles.
• Destroy surface detail. Carved stone, decorative cornices, and ornamental brickwork patterns get rounded off and lose definition.

Once this damage is done, it's extremely expensive to repair. Repointing an entire building with historically appropriate lime mortar is a major undertaking.

Low-Pressure Crushed Glass: The Right Tool

We use fine-mesh crushed recycled glass (70/140 or finer) at 15-30 PSI for masonry restoration. At these settings, the media has enough energy to remove surface contaminants — soot, paint, biological growth, efflorescence, and graffiti — without cutting into the brick or mortar.

Why crushed glass and not something softer?

• It's angular. The sharp edges cut through surface contamination efficiently even at low pressure.
• It's inert. No chemical reaction with the masonry. No residue that needs to be neutralized.
• It's consistent. Manufactured to uniform mesh sizes, so the cleaning action is predictable.
• Zero free silica. Made from recycled bottle glass, not silica sand. Safe for the operator and the environment.

For extremely delicate work — thin-set terra cotta, soft sandstone, or already-damaged brick — we can drop to 10-15 PSI or switch to glass bead media, which is rounded rather than angular and even less aggressive.

Graffiti Removal

Graffiti on historic masonry is a common call for us. The instinct is to reach for chemical paint strippers, but many of those products contain acids or solvents that can stain or etch old brick and stone.

Low-pressure crushed glass removes spray paint, marker, and other graffiti without chemicals. The process is:

1. Test patch. Always. Different brick absorbs paint differently, and some graffiti has been there long enough to penetrate the pores.
2. Blast at 15-25 PSI with fine glass media, working from the edges of the graffiti inward.
3. Inspect. Some deep-penetrating paints may leave a ghost image in porous brick. A follow-up application of a poultice-type cleaner can draw out remaining pigment.
4. Rinse. Light water rinse to remove media dust.

The advantage over chemical strippers: no runoff of caustic chemicals onto adjacent surfaces, landscaping, or into storm drains.

Cleaning Soot and Pollution Buildup

Buildings in urban areas accumulate decades of soot, diesel exhaust, and atmospheric grime. This black crust is mostly calcium sulfate (from acid rain reacting with limestone) and carbon particulate. It's bonded to the surface but not deeply embedded.

Low-pressure glass blasting removes this buildup efficiently. A building face that looks black can come back to its original warm brick or stone color in a single pass. The transformation is dramatic and it's one of the most satisfying jobs we do.

What We Check Before Starting

Before we blast any historic structure, we assess:

• Mortar type. Lime mortar vs Portland cement. This determines our maximum pressure.
• Brick hardness. We do a scratch test to gauge how soft the brick is.
• Previous repairs. Mismatched repointing with hard Portland cement next to soft lime mortar creates stress points. We need to know where those are.
• Paint history. Lead paint on pre-1978 buildings requires containment and proper disposal per EPA RRP rules.
• Structural concerns. Cracked lintels, bulging walls, or deteriorated courses that might be affected by even gentle blasting.

The Acid Alternative (And Why We Avoid It)

Muriatic acid (hydrochloric acid) is the old-school method for cleaning brick. It works, but:

• It can burn lime mortar and soft brick
• It leaves salt deposits (efflorescence) as it dries
• It's hazardous to handle and generates toxic fumes
• Runoff is an environmental liability

For new construction cleanup (removing mortar smears from fresh brickwork), dilute acid has its place. For historic restoration, it's a last resort.

Get In Touch

If you've got a historic building, monument, or masonry structure that needs cleaning or graffiti removal, send us some photos and we'll tell you what approach makes sense. Every building is different, and the right method depends on the materials, condition, and what you're trying to remove.