Introduction to laser cleaning in cultural heritage

Abstract

Laser cleaning is a precise, ‘touch-free’ technique that uses focused laser radiation to remove contaminants from surfaces. It has become increasingly popular in a cultural heritage context due to its ability to target contaminants with minimal damage to underlying materials, particularly where traditional mechanical or chemical cleaning may pose risks to delicate surfaces. However, every cleaning intervention requires a degree of assessment and monitoring, and lasers are no different. This Technical Brief will provide an overview of the physical phenomena behind laser cleaning, give examples of successful cultural heritage applications and list the main pros and cons of the technique.

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Principles of operation

The removal of material from a surface using a laser is commonly called laser ablation (Fig. 1). Ablation is based on differences in optical absorption, thermal properties and adhesion between the unwanted surface layers and the substrate.¹ The most common ablation mechanism is thermal in nature. In a normal sequence of events, laser energy is absorbed by surface contaminants (Fig. 2a). The laser pulses are typically in the nanosecond range, but this duration is long enough for the absorbed energy to spread through the material. This leads to an increase in temperature at the surface (Fig. 2b), followed by the thermal evaporation of the surface layer and by the dissipation of the energy as heat to the underlying bulk (Fig. 2c).

Fig. 1 Removal of spray paint from stone with a compact laser system.

Fig. 2 Mechanism of laser ablation with short-pulse lasers (typically in the nanosecond pulse duration range): (a) absorption of the laser energy by the material; (b) rapid localised heating of the unwanted dirt layer, sometimes extending to the surface; and (c) vaporisation and ejection of fragments of dirt.

Other mechanisms include photomechanical effects, where shock waves or pressure gradients can detach loosely bound layers, or photochemical ablation, where bonds are broken via photon energy (usually in UV laser systems). Note that photochemical changes can occur over extended time periods and be complex in nature.

Various factors influence the ablation mechanism, such as the type of laser, the laser parameters and the physical and chemical properties of the materials. Some key laser parameters include:

– Wavelength: chosen based on the absorption characteristics of the contaminant and substrate materials. Common choices include Nd:YAG lasers (neodymium-doped yttrium aluminium garnet), either at the fundamental wavelength of 1064 nm or at harmonic wavelengths such as 532 nm, which offer a general-purpose, deep-penetration solution generally suitable for metals and stones. Other types of lasers can also be used (Fig. 3).

Fig. 3 A selection of lasers and their wavelengths. The most commonly used laser in conservation is the Nd:YAG laser operating at 1064 nm or at harmonic wavelengths (e.g., with a KTP crystal: potassium titanyl phosphate, emitting at 532 nm). Er:YAG lasers (erbium-doped yttrium aluminium garnet, emitting at 2940 nm) are becoming increasingly popular.

– Pulse duration: laser cleaning is usually performed using pulsed lasers, with pulse durations typically in the nanosecond or microsecond range. Shorter pulse durations, such as those on the picosecond or femtosecond scale, are known to minimise heat-related effects and the risk of damage to the substrates.

– Fluence: this is the energy delivered per unit area (J cm⁻²). The fluence used for cleaning must typically exceed the ablation threshold of the contaminant while remaining below that of the substrate. Thresholds vary widely with material properties.

– Repetition rate and scanning speed: can be adjusted to control the cleaning rate and prevent substrate overheating or overcleaning. In practice, repetition rates can vary from 1 Hz to several hundred kHz depending on the experimental setup and the operator's need to balance process efficiency with control and substrate safety.

The penetration depth during laser cleaning varies widely depending on the laser wavelength, pulse duration and material properties, typically from a few nanometres to several micrometres per pulse. Thicker or more adherent layers may require multiple shots per spot/passes of the laser, or higher energy, while very thin or sensitive substrates require low penetration and minimal interaction with the laser to avoid damage.

Applications in heritage conservation

Laser cleaning has been widely applied in heritage conservation over the last 20 years, and has often proven to be effective for the treatment of built heritage and various types of objects when other methods are unsuitable. The initial trials were conducted on stone, due to the self-limiting nature of the process – dark surface contaminants absorb laser energy while the underlying light-coloured stone remains undisturbed. Various types of materials have been studied subsequently, and many successful examples can be found in the literature, ranging from historic facades to paintings.^2–4

Stone and masonry

Laser cleaning can be highly effective on stone and masonry, especially for the removal of black crusts (gypsum-based, pollution-derived, disfiguring encrustations) and past conservation treatments on limestone, marble and sandstone, but also for biological colonisation or graffiti on historic buildings and monuments (Fig. 4). The use of lasers does not typically result in erosion, which is common with mechanical abrasion or acid treatments. However, special care must be taken on polymineralic stones such as granite due to the differences in susceptibility of the individual minerals to laser irradiation.

Fig. 4 Laser cleaning of graffiti from stone: (a) before cleaning; and (b) after cleaning. Images from Brand et al., see ref. 5 (published under CC-BY-4.0 [https://creativecommons.org/licenses/by/4.0/]).

Metal objects

Laser cleaning has been widely used for the removal of corrosion products, surface dirt and other unwanted coatings from historical and archaeological metal objects, including those made of bronze, copper, iron and silver. Corrosion layers can often be selectively ablated, while fragile surface features such as engravings, fine details, patinas and gilding can be preserved.

Painted surfaces

Lasers have also been used on painted surfaces to remove overpaint, varnish or surface dirt with minimal effect on the original pigments or binders. The selective interaction is based on the optical contrast between the layers. Er:YAG lasers (emitting at 2940 nm) are particularly effective in these cases.

Organic substrates

Gentle surface cleaning has been reported for organic materials such as wood, textiles, parchment, leather, bone and ivory (Fig. 5), where traditional wet or abrasive methods may cause irreversible damage. Laser interaction with organic materials must be finely tuned, as many of these substrates absorb laser energy and are sensitive to heat. While still considered an emerging application, advances in mid-infrared lasers and ultrashort-pulse systems have expanded the potential for safe and effective cleaning of organic substrates under tightly controlled conditions.

Fig. 5 Laser cleaning of surface dirt on a fragment of carved ivory with a Nd:YAG laser. © Victoria and Albert Museum.

Plastics and modern materials

The use of lasers on modern materials is an emerging area of research in heritage conservation. These materials present unique challenges due to their diverse composition, variable optical properties and sensitivity to heat. Some lasers hold promise for the safe removal of contaminants, coatings or degradation products, and as the understanding of polymer–laser interactions improves, this method may become a valuable tool for conserving modern heritage and design objects.

Regardless of the material treated, laser cleaning requires a close assessment of the surfaces to verify the cleaning efficiency and effects on the materials. Post-treatment analyses are typically used to confirm the selective removal of unwanted layers, identify residues or evaluate changes in surface morphology, chemistry or other properties. This usually involves a combination of visual and microscopic inspection, spectroscopy and surface topography measurements (e.g., optical microscopy, scanning electron microscopy, Fourier-transform infrared spectroscopy or laser-induced breakdown spectroscopy).

Advantages

Laser cleaning offers various benefits compared with more traditional cleaning methods (see Fig. 6). As it does not require contact with the surfaces and, in favourable circumstances, can be highly selective due to the differences in absorption, laser cleaning minimises physical stress on fragile surfaces and allows for precise removal of contaminants, making it ideal for the conservation of delicate objects.

Fig. 6 Potential benefits of using lasers for cleaning in conservation.

The technique does not usually require chemicals or solvents, although lasers such as Er:YAG systems are often used in combination with other cleaning methods to improve the process.

The laser parameters such as wavelength, pulse duration and fluence can be finely adjusted to suit different materials, providing consistent and reproducible outcomes. The process is highly controlled, making it possible for conservators to remove surface deposits gradually.

Portable and compact machines are readily available on the market at reasonable prices and are designed for field deployment, making them suitable for use directly at heritage sites or in museums. Additionally, laser cleaning can be integrated with in situ monitoring tools (e.g., laser-induced breakdown spectroscopy, LIBS⁶), supporting informed decision-making during treatment.

Limitations

It is, however, important to keep in mind some of the limitations of the technique.

The use of lasers is generally challenging, for example, when the contaminant to be removed and the substrate have similar compositions. They can also be risky for heat-sensitive or thermally unstable materials, for which even low fluence may cause damage. Transparent or reflective surfaces may scatter or reflect the laser beam, limiting its effectiveness. If the laser affects not only the unwanted surface layer but also the layers underneath, damage will occur. Discoloration or burning has been reported, especially on sensitive materials such as paper, textiles or paint, but also on stone (yellowing, darkening or loss of colour). Note that these changes sometime do not occur immediately. Surface melting is also possible, and fine details or inscriptions can be lost during ablation. Finally, microcracking or thermal stresses can be generated, weakening the structure of the object or causing its long-term deterioration. Such damage is often irreversible, highlighting the importance of careful parameter selection during laser cleaning. In this context, analytical methods are also essential to assess the effects of lasers on the surfaces. Techniques such as microscopy and scanning electron microscopy are typically used to help monitor changes in texture, composition or colour during and after treatment.

In practice, other parameters usually add complexity to the laser cleaning process. For example, non-flat surfaces, such as 3D objects with irregular shapes, angles and curves, can be challenging to treat because of the short depth of focus of lasers (typically a few hundred micrometres to a few millimetres). The laser beam may not remain properly focused across the surface, leading to inconsistent energy delivery. This can cause some areas to be overexposed, potentially leading to damage, while other areas may be underexposed, resulting in inadequate cleaning.

Purchasing a laser cleaning system, usually comprising the laser, a delivery system, some additional optical elements, safety goggles, and fume extraction system, and setting it up in a dedicated space is a high initial investment. Laser systems costs vary, typically from 15 000 GBP to 70 000 GBP depending on the type of laser. While they are not yet a standard fixture in most laboratories, laser cleaning systems are increasingly present in specialised heritage science environments.

The use of the high-power lasers required to achieve cleaning introduces important risks to operators, such as eye and skin damage. Inhalation of hazardous fumes and accidental laser reflections should also be considered. As a consequence, strict safety measures and specialised training are required, which may limit the widespread adoption of the technique.

While careful parameter optimisation and specialist knowledge are required, ongoing advancements in portability, automation and safety are expanding this technique's accessibility and ease of use. As laser technology continues to evolve, the role of lasers in scientifically informed conservation practices is likely to grow, making them an essential component of the modern conservation toolkit.

This Technical Brief was prepared by Dr Julia Brand (University of Canberra, Australia) on behalf of the Heritage Science Expert Working Group of the Analytical Methods Committee and approved by the AMC on 17 December 2025.

References

1. C. Fotakis, D. Anglos, V. Zafiropoulos, S. Georgiou and V. Tornari, Lasers in the Preservation of Cultural Heritage, Principles and Applications, CRC Press, Boca Raton, 1st edn., 2006, p. 336.
2. S. Siano and R. Salimbeni, Advances in laser cleaning of artwork and objects of historical interest: The optimized pulse duration approach, Acc. Chem. Res., 2010, 43(6), 739–750,  DOI:10.1021/ar900190f.
3. M. Bertasa and C. Korenberg, Successes and challenges in laser cleaning metal artefacts: A review, J. Cult. Herit., 2022, 53, 100–117,  DOI:10.1016/j.culher.2021.10.010.
4. P. Pouli, M. Oujja and M. Castillejo, Practical issues in laser cleaning of stone and painted artefacts: optimisation procedures and side effects, Appl. Phys. A, 2012, 106, 447–464,  DOI:10.1007/s00339-011-6696-2.
5. J. Brand, A. Wain, A. V. Rode, S. Madden and L. Rapp, Towards safe and effective femtosecond laser cleaning for the preservation of historic monuments, Appl. Phys. A, 2023, 129, 246,  DOI:10.1007/s00339-023-06455-x.
6. Analytical Methods Committee AMCTB No. 91, Laser-induced breakdown spectroscopy (LIBS) in cultural heritage, Anal. Methods, 2019, 11, 5833–5836,  10.1039/C9AY90147G.

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