Laser cleaning of stone and sandstone — what the literature says and what is not yet known

RC-Laser Team · 2 November 2026

How a laser tells dirt from stone, when sandstone yellows or loses colour, why a test on a small area is done, and what is known about brick.

Why a laser can tell dirt from stone

Laser cleaning of stone exploits a difference in light absorption: dark contaminants absorb the energy, while pale stone largely does not. The process is called self-limiting, provided the settings are safe ^[3]^[4]. At the Metropolitan Museum a laser was used to remove a thick black crust from two marble capitals; after tests with three wavelengths, 1064 nm was chosen because the black crust absorbs that light and the marble largely does not ^[1]. The author of a text from the 1990s notes that once the dirt is gone, the following pulses deliver too little energy to damage the surface, but only at safe parameters; above a certain threshold plasma and shock waves form, which are less selective and able to damage the substrate ^[4].

What soiling the literature describes

Among the soiling a laser can remove, the sources list black crusts, residues of earlier treatments, biological growth and graffiti ^[3]. Facades may also carry soot, oil and metallic stains, paints and residues of earlier water-repellent treatments, and different soiling may need different agents ^[5]. Staining from iron and manganese in the sandstone itself is a separate matter: cleaning can speed up their leaching and bleach the stone ^[6].

Sandstone and other stones: each reacts differently

In a study of a black crust on siliceous sandstone from Montjuïc, an Nd:YAG laser caused loss of binder that could not be tied to fluence alone; the authors attribute it to uneven micro-roughness and grain orientation and propose reporting, besides fluence, the total energy delivered per spot ^[2]. In a laboratory test with a femtosecond laser, sandstone had a lower ablation threshold than granite and marble, and on a clean sandstone sample every setting tested caused discolouration; the authors concluded that the process is not yet ready for this stone ^[7]. That is a laboratory laser, not standard service equipment. Polymineralic stones such as granite call for particular care because their minerals react differently ^[3].

Yellowing and discolouration: what has been observed

Stone can yellow, darken or lose colour after laser cleaning, and these changes sometimes do not appear immediately; surface melting, loss of fine detail and micro-cracking are also possible, often irreversible ^[3]. At the Metropolitan Museum the marble had a yellow tint after cleaning; analysis linked it to organic material that had penetrated the stone earlier, but the author stresses that the cause is not fully established ^[1]. For sandstone with a black crust, Nd:YAG lasers performed better than excimer lasers, and a slight colour change was noted after cleaning at 1064 nm ^[8]. On granite, infrared radiation intensified a yellowish tone with black paint, and exceeding the fluence thresholds caused grains to be torn out ^[9]. The conclusion is cautious: discolouration happens, its causes are not always clear, so we do not promise there will be none.

Brick: what is known and what is missing

We found no openly available studies on laser cleaning of fired brick or clinker, so we do not transfer results from marble or sandstone to brick. What is known is the risks of other methods: according to US National Park Service guidance, high-pressure water can abrade brick, blasting strips the glazed surface, and hydrochloric acid can damage brick and dissolve lime mortar ^[5]. That justifies looking for gentler methods but does not prove a laser will work on a given brick. The result is decided by a test on a small area.

The test on a small area

Conservation guidance agrees: cleaning starts with the gentlest method, with a test in an inconspicuous place, separately for each material and finish, and harsher agents are used only if needed ^[5]. The Scottish guide for sandstone requires a programme of trials: inspection, trial cleaning and a report, with test areas representative of the variation in the surface and with photographic records ^[6]. The sandstone study recommended spot tests first and then area tests with manual movement of the beam ^[2]. After treatment, an analysis confirming selectivity and detecting residues is recommended ^[3]. It is also worth agreeing an acceptable level of cleanliness: complete removal of the soiling may not be necessary, and cleaning previously soiled stone can leave a visible outline of the clean area ^[10].

Laser versus other methods

The author of the 1990s text describes air abrasive blasting as removing material indiscriminately, especially from a decaying surface, and chemicals as able to leave residues in the stone; a laser works without mechanical contact, but cleaning large building surfaces is slower than blasting ^[4]. The Scottish guide adds that blasting can roughen and pit sandstone and that chemicals in porous stone can leave stains and salt efflorescence ^[6]. The same guide cites a study according to which cleaned buildings may deteriorate faster than uncleaned ones, so cleaning by any method has to be justified ^[6].

How we work

We describe facade and stone cleaning on the service pages. Before work on a larger area we run a test on a sample. We give an initial estimate from a photo, and the final price after viewing the object on site.

This text reports study results and conservation guidance and does not guarantee an outcome on a specific object. Many of the works cited concern the laboratory or single objects.

Sources

Sources as of 2 November 2026.