ANSI
View official documentation (opens in new tab)ANSI Z136.1, Safe Use of Lasers (applies to all laser cleaning regardless of substrate or application)[1]
Black Crust & Stone Soiling Laser Cleaning
Black crust on heritage stone will not come off safely until the crew knows whether they are facing gypsum sulphation, a biofilm, or salt efflorescence beneath the dark layer. Fluence that clears gypsum crust on marble can overheat porous sandstone where the sandstone coupon crust margin at 1064 nm is only about 0.4 J/cm². (Marakis et al. 2003; Siano et al. 2012; Sutter et al. 2023)
Frequently Asked Questions
Why does the safe fluence change depending on the stone type?
Sandstone gives the tightest margin, a small fluence step can remove stone grains with the black crust. Marble and limestone tolerate wider bands before grain damage, though light marble yellows before it fractures. On sandstone coupons Cooper reports crust clearing near 0.85 J/cm² with substrate damage near 1.25 J/cm², about 0.4 J/cm² of separation.
What causes marble yellowing during laser cleaning and how is it fixed?
Light marble can yellow during laser cleaning when infrared pulses vaporize embedded carbon and iron-rich particles, even well below the grain-damage row. The discoloration tracks 1064 nm work, not immediate fracture. Marakis compared first and third harmonic Nd:YAG cleaning; a low-fluence 355 nm finish pass after bulk crust removal targets the organic fraction without the same yellowing pathway. (Marakis et al. 2003; Siano et al. 2012; Sutter et al. 2023; Marakis 2003)
How is the acceptable level of clean determined for heritage stone?
Heritage crews set the acceptable level of clean on stone mock-ups before production passes, not from a universal fluence number. The Met Cloisters protocol on medieval marble capitals required wavelength trials, test patches, and stakeholder sign-off on that mock-up level. Hold fluence at the lowest pass that clears the agreed layer; chasing a beige gypsum veil toward damage fluence breaks self-limiting cleaning.
Sources(4 references)
- Laser cleaning of stone materials — overview lrmh.fr (opens in new tab) — sandstone crust 0.85 J/cm²; damage 1.25 J/cm²; ~0.4 J/cm² separation gap
- The two-wavelength laser cleaning methodology nature.com (opens in new tab) — marble crust 0.75–1.26 J/cm²; clean-stone damage 3.5 J/cm²; 355 nm finish protocol
- Comparative study on 1st and 3rd harmonic Nd:YAG cleaning of black encrustation on marble sciencedirect.com (opens in new tab) — 1064 nm yellowing avoided with 355 nm finish pass
- Laser cleaning protocol on medieval marble capitals — Met Cloisters metmuseum.org (opens in new tab) — wavelength trials and stakeholder sign-off before production cleaning
Laser safety for stone conservation work
Heritage crews still operate under the same laser-safety framework as industrial cleaning, beam enclosure, eyewear, and controlled work zones per the national laser-safety standard. (ANSI Z136.1)
Sources(1 reference)
- ANSI Z136.1 — Safe Use of Lasers webstore.ansi.org (opens in new tab) — national laser safety standard for all laser cleaning applications
When laser cleaning is the wrong first move on stone
Four gates conservators hit before the beam, residual gypsum after IR passes, 1064 nm yellowing, soluble salts, and mock-up protocol on irreplaceable surfaces. ANSI laser-safety zoning still applies before any coupon work. (ANSI Z136.1)
Beige-yellow gypsum veil left after 1064 nm crust removal — Pouli's Acropolis work shows fluence slightly above black-crust threshold can leave a thin gypsum matrix with trapped organic fluorescence. Chasing that veil toward marble damage fluence breaks self-limiting cleaning. Pre-treatment: Finish with a low-fluence 355 nm pass or synchronous two-wavelength protocol rather than raising 1064 nm toward substrate damage..
1064 nm yellowing on white marble after crust removal — Marakis et al. compared first and third harmonic Nd:YAG cleaning — the yellowing tied to 1064 nm pulses is avoided with 355 nm, which targets the organic fraction without the same discoloration pathway. Pre-treatment: Plan a 355 nm finish pass after 1064 nm bulk removal on light marble..
Visible efflorescence or high soluble-salt load in porous stone — Zhang et al. on Yongling Mausoleum identify Na₂SO₄ as the most destructive salt because thenardite–mirabilite transitions drive large volume changes. Localized laser heating can mobilize salts deeper rather than extract them. Pre-treatment: Clay-poultice desalination to measured reduction before any laser pass where efflorescence or drill-core salts are present..
Irreplaceable heritage surface without mock-up validation — The Met Cloisters protocol on medieval marble capitals required wavelength trials and documentation before production cleaning — no universal level-of-clean standard exists for atmospheric soiling on stone. Pre-treatment: Test patches, stakeholder sign-off on acceptable level of clean, and full documentation per ICOM-CC practice..
Sources(1 reference)
- ANSI Z136.1 — Safe Use of Lasers webstore.ansi.org (opens in new tab) — laser-safety zones and controlled work areas before stone coupon work
Black crust forms when acid rain turns stone to gypsum, then traps soot
Atmospheric sulphur dioxide and moisture convert calcite at the stone surface into porous gypsum. That polyporous layer adsorbs elemental carbon and carbonaceous particles from air pollution, which gives the crust its black color. Field measurements on marble sculptures in Beijing report crust thickness from 100–273 μm on sampled surfaces, thicker dendritic crusts in heavily polluted exposures can reach roughly 2 mm and need more cumulative passes than a thin film. (Wang et al. 2022)
Sources(1 reference)
- Black crust formation on ancient marble sculptures in Beijing pmc.ncbi.nlm.nih.gov (opens in new tab) — measured black crust thickness 100–273 μm on sampled marble
Safe energy window shifts roughly threefold across stone types
Substrate absorption, porosity, and crust optical contrast at 1064 nm set how much room the operator has between clearing atmospheric soiling and marking the stone. Brick and concrete thresholds are not characterized in the ns/1064 nm literature, treat those substrates as research gaps, not coupon extrapolations.
| Substrate | Ablation threshold (J/cm²) | Substrate damage (J/cm²) | Process window | Regime |
|---|---|---|---|---|
| Marble (CaCO₃) | 0.75–1.26 | 3.5–3.5 | 2.8–4.7×Widest margin — primary risk below damage is yellowing, not grain fracture | thermal-spallation |
| Limestone | 0.75–1 | 1.2–1.3 | 1.2–1.7×Moderate — color variation and porosity tighten control | thermal-spallation |
| Sandstone | 0.85–0.85 | 1.25–1.25 | 0.4–0.4×Tightest — highest absorption and shallow wet penetration | thermal-spallation |
| Granite | 0.85–1 | 1.1–1.2 | 0.1–0.35×Narrow — biotite grains most vulnerable | thermal-spallation |
Sources(3 references)
- Laser-assisted graffiti removal from granite mdpi.com (opens in new tab) — granite undamaged to 1.1 J/cm²; damage signs above 1.2 J/cm²
- Cooper, M. 'Laser cleaning of stone materials: an overview of current research.' Reviews in Conservation, no. 4, 2003, pp. 1-26. lrmh.fr (opens in new tab) — Source data for atmospheric-soiling-stone — laserDamageThreshold
- Pouli, P. et al. 'The two-wavelength laser cleaning methodology: theoretical background and examples from its application on CH objects and monuments with emphasis to the Athens Acropolis sculptures.' npj Heritage Science, vol. 4, 2016, article 9. nature.com (opens in new tab) — Source data for atmospheric-soiling-stone — laserDamageThreshold
Thresholds that change with stone and soiling type
Cross-substrate stone cleaning numbers for gypsum crust, biological film, and wavelength choice, each row is a different decision, not one crust pass for every black surface.
| Parameter | Value |
|---|---|
| Black crust formation (sulphation) | Calcite converts to gypsum; porous crust adsorbs carbon |
| Gypsum thermal limits | Dehydrates ~128°C; anhydrite melts 1460°C |
| Marble black crust ablation (1064 nm) | 0.75–1.26 J/cm² removes crust without clean-stone damage |
| Sandstone crust vs damage (1064 nm) | Crust 0.85 J/cm²; substrate damage 1.25 J/cm² |
| Granite damage onset (1064 nm) | No visible damage to 1.1 J/cm²; signs above 1.2 J/cm² |
| Marble yellowing at 1064 nm | Avoided with 355 nm finish pass |
| Er:YAG vs Nd:YAG on stone | 2940 nm absorbed by OH groups; 1064 nm for dark crust |
| Nocardia biofilm on limestone (wet) | 0.03 J/cm² complete removal below ablation threshold |
Sources(4 references)
- Black crust formation on marble in Beijing pmc.ncbi.nlm.nih.gov (opens in new tab) — sulphation calcite to gypsum; carbon adsorption blackens crust
- Laser cleaning in conservation of stone, metal, and painted artworks academia.edu (opens in new tab) — 1064 nm yellowing avoided with 355 nm
- Combined Er:YAG and Nd:YAG cleaning at Pisa Cemetery taylorfrancis.com (opens in new tab) — Er:YAG 2940 nm for OH-rich films; Nd:YAG 1064 nm for encrusted stone
- Q-switched Nd:YAG treatment of Nocardia biofilm pmc.ncbi.nlm.nih.gov (opens in new tab) — 0.03 J/cm² optimal on limestone wet; separate from gypsum crust fluence
Pulse energy — Black Crust & Stone Laser Cleaning
Working fluence ~1.13 J/cm² on Marble (CaCO₃) (representative substrate — see table above for others) (window 0.75–3.50 J/cm²). Bars: datasheet max pulse energy; color: process status.
- Wuhan Sintec STPL-V-i1600: 250 mJ — In process window
- Laserax LXQ-UHP 3000W: 150 mJ — In process window
- Laserax LXQ-UHP 2000W: 150 mJ — In process window
- Narran ROD 2000: 100 mJ — In process window
- Narran ROD 2000 Bright+: 100 mJ — In process window
- P-Laser QF-2000: 100 mJ — In process window
- Laserax LXQ-UHP Series (500W–3kW): 100 mJ — In process window
- 4JET JETLASER M1000: 100 mJ — In process window
- Laserax LXQ-UHP 1000W: 100 mJ — In process window
- Narran ROD 1000 Bright+: 100 mJ — In process window
- P-Laser QF-1000: 100 mJ — In process window
- 4JET JETLASER M500: 100 mJ — In process window
- Laserax LXQ-UHP 500W: 100 mJ — In process window
- Narran ROD 500 Bright+: 100 mJ — In process window
- Netalux Jango®: 100 mJ — In process window
- Narran ROD 1000: 50 mJ — In process window
- Narran ROD 500: 50 mJ — In process window
- P-Laser QF-500: 50 mJ — In process window
- Netalux Kamino 300: 50 mJ — In process window
- cleanLASER CL 500: 25 mJ — In process window
- SenFeng SF1000HC: 50 mJ — In process window
- SenFeng SF500HC: 50 mJ — In process window
- Powerlase Vulcan 500c: 40 mJ — In process window
- Narran ROD 300 Air: 15 mJ — In process window
- cleanLASER CL1000iF: 10 mJ — In process window
- 4JET JETLASER M200: 10 mJ — In process window
- Powerlase FL-C100C: 5.0 mJ — In process window
- Narran ROD 100 Air: 1.5 mJ — In process window
- PULSAR Laser SHARK P CL 1000A: 1.5 mJ — In process window
- P-Laser ECO-C 500: 1.5 mJ — In process window
- PULSAR Laser SHARK P CL 500A: 1.5 mJ — In process window
- PULSAR Laser SHARK P CL 300M: 1.5 mJ — In process window
- Han's Laser HC-PD 200W: 1.5 mJ — In process window
- PULSAR Laser SHARK P CL 200M: 1.5 mJ — In process window
- Han's Laser HC-PD: 1.5 mJ — In process window
- Han's Laser HC-PD 100W: 1.5 mJ — In process window
- PULSAR Laser SHARK P CL 100M: 1.5 mJ — In process window
- Han's Laser HC-PD 50W: 1.1 mJ — In process window
- P-Laser QFC-300: 1.0 mJ — In process window
- P-Laser ECO-C 200: 1.0 mJ — In process window
- In window
- Below threshold
- Near damage
- Damage risk
Top questions about Black Crust & Stone Laser Cleaning
What is black crust formation?
Sulphation reaction: calcite (CaCO₃) in limestone/marble reacts with atmospheric SO₂ and water to form gypsum (CaSO₄·2H₂O). The porous gypsum crust then adsorbs elemental carbon and carbonaceous particles from air pollution, giving it the characteristic black color (Wang 2022).
Sources(1 reference)
- Wang, X. et al. 'Study on the mechanism of the black crust formation on the ancient marble sculptures and the effect of pollution in Beijing area.' Heliyon, vol. 8, no. 9, 2022, e10417. pmc.ncbi.nlm.nih.gov (opens in new tab) — Sulphation between the carbonate and sulfur compounds induces the transformation of calcite (CaCO₃) into gypsum (CaSO₄·2H₂O); polyporous gyp