Todd Dunning Author Bio

Acrylic (PMMA) is one of the most optically demanding polymers we clean — the damage threshold sits at just 0.8 J/cm², and even a small drift above that causes surface melting at the 160°C thermal degradation point. The safe working window is tight: 0.5–0.7 J/cm² gets contaminants off without clouding the surface, while anything below 0.4 J/cm² leaves residue behind.

Ash presents a combination that's unusual in hardwoods: high laser light absorption (82% at 1064 nm) paired with a porosity fraction of 0.6 — roughly twice that of oak — which means contaminants don't just sit on the surface, they work into the wood structure. That depth of penetration is why slower cleaning speed matter here; two passes at 100 W, 50 kHz, and 500 mm/s with 50% overlap reach embedded grime without burning the open grain.

Brick's porosity is what makes it both easy and tricky to clean by laser. At 15–25%, pores are large enough for soot, biological growth, and mineral deposits to migrate well below the surface, which is why a single fast pass rarely gets the job done. The saving grace is strong energy coupling — 92% light absorption at 1064 nm — and a usable working window of 1.15–2.5 J/cm² between cleaning onset and structural damage.

Cedar's combination of high porosity (0.79 fraction) and low density (350 kg/m³) makes mildew and soot removal genuinely tricky — contaminants penetrate deeply into the open grain structure rather than sitting on the surface. A slow 500 mm/s scan with 50% overlap and 2 passes at 100 W clears biological growth without raising the grain or scorching the natural oils that give cedar its distinctive character.

Concrete's 15–25% porosity means contaminants soak past the surface layer — but that same porous structure confines the cleaning problem to the cement paste, which fails at 4.5 J/cm² while the aggregate holds to 5–6 J/cm², leaving only a 1.0 J/cm² process window before visible paste erosion begins. Density is 2400 kg/m³. Compressive strength is 25 MPa (typical structural concrete). Tensile strength is only 3.2 MPa – concrete cracks when pulled. Fracture toughness is 0.7 MPa√m – very low.

Granite's 5–15% mica fraction darkens irreversibly above ~1.2 J/cm², and the feldspar majority melts at 3.0 J/cm² A coarse-grained trait absent in fine-grained igneous stone like [Basalt](/materials/basalt-laser-cleaning). — so the ~0.3 J/cm² window between cleaning onset (1.2 J/cm²) and the 1.5 J/cm² substrate-damage ceiling is tight on cleaning parameters, and it varies by quarry. Density is 2700 kg/m³. Porosity is only 0.006 (0.6%) – very low, which means contaminants sit on the surface. Compressive strength is 211 MPa – very strong.

Bay Area overhaul shops stripping cadmium primer or chromate stacks cannot run that work through an acid methylene-chloride bath without failing ASTM F519 within 48 hours on cadmium-plated 4340 steel. Rule 8-29-305 also caps aerospace strippers at 400 g/L precursor organics or 10 mmHg vapor pressure, so the tank is not a generic solvent job. Pulsed 1064 nm lift of a YSZ topcoat has to stop before the MCrAlY: 70 percent overlap left ceramic, while the two-step bond-coat scan still leaves a 4 µm recast. Complete removal of a 200 µm four-layer coupon on A357 needed 5.09 J/cm² at 700 mm/s. Treat that coupon as a coating-stack window, not a generic aluminum number.

A Bay Area fabrication shop can shed its solvent permit burden and cut weld rejects with the same single change. Solvent degreasing pulls a shop into Bay Area Air Quality Management District (BAAQMD) Regulation 8 Rule 4 obligations for evaporating solvent vapor, and into California Proposition 65 liability from chlorinated solvents, while inconsistent [weld prep](/applications/weld-prep) drives rework nobody budgets for. Pulsed laser cleaning is a dry process that raised tensile strength 30% and elongation 200% on high-strength low-alloy (HSLA) steel over solvent-degreased surfaces in butt-weld testing (Zhu et al. 2020) — fewer rejects on the code-critical joints where weld qualification drives the project schedule. The surface is ready for welding right after cleaning. Because it evaporates no solvent, that single change also lifts the [BAAQMD Regulation 8 Rule 4 and Proposition 65 obligations these solvents carry](/compliance).