Laser technology for monument restoration has its roots in the 1970s and is still associated with spot cleaning using a beam with a diameter ranging from a few to several millimeters. Recent years of rapid technological advancement have partially automated the process of removing layers and eliminated its imperfections, while also reducing the size of the equipment. As a result, mobile, handheld cleaning systems have emerged, ensuring high-quality surface cleaning and achieving unprecedented efficiency. The main advantage of current solutions is the automated cleaning of surfaces with dimensions of up to several centimeters in width and height.
Each of these devices relies on the process of laser ablation, which involves generating a highly concentrated yet very short laser pulse to vaporize contaminants from the surface of the cleaned object into a gaseous state.
The essence of the entire monument cleaning process lies in selecting the appropriate light wavelength and laser beam parameters for both the cleaned surface and the removed impurities. This poses a significant challenge for those starting their experience with lasers. There is a growing variety of equipment available in the market, often found under the terms "laser cleaning" or "laser ablation." The vast majority of these are industrial devices designed solely for working with metal surfaces. While this technology is often described as non-invasive, this equipment can be hazardous for historic objects, and the outcome of the process depends on both the operator and the device itself.
Modern laser sources allow the creation of highly versatile cleaning devices, whose parameter ranges enable the cleaning of almost any surface of historic objects. However, this is not achievable without additional solutions simultaneously applied at the level of preparing the laser pulse energy distribution, automating its movement in the X and Y axes, and calibrating it using appropriately prepared software. To achieve the required quality in the conservation of monuments, correlating the operator's experience with the latest technological advancements is essential.
One of the initial steps in automation was the use of a galvanometric scanner 1D in the X axis. This solution allows the automatic movement of the laser beam along one axis (left/right). However, it was noticed that this generates a so-called over-cleaning effect at the scan edges. This results from the scanner's operation, which, while moving the beam to the end of the working area, has to decelerate it and start moving it in the opposite direction. Decelerating the beam at the scan edges leads to prolonged exposure of the surface to its action, compared to the entire width of the scan. Consequently, this causes over-cleaning at the scan edges and visible traces on the cleaned surface.
This isn't the only issue with 1D scanners in conservation applications. To clean the working area in the X and Y axes, the operator must manually move the head in the Y axis (up/down). This results not only in a similar problem of uneven surface exposure to the laser's action but also increases the risk of surface damage. Depending on the material, its degradation may manifest differently, such as discoloration of red bricks, visible indentations in sandstone, or scorching of wood.
The use of a second galvanometric motor in the laser head allowed the creation of a scanner operating in both the X and Y axes. This significantly reduced the influence of the operator's hand movement, decreased the risk of material damage, and improved the evenness and aesthetics of the cleaning effect. Depending on the lens used, such scanners can automatically clean surfaces up to 18 × 18 cm. All that's needed is to press the radiation emission button and hold the device head still.
This type of scan ensures the highest cleaning efficiency and uniform surface coverage, but the problem of over-cleaning persists despite significant safety improvements and cleaning quality. Finding a solution has proven more challenging than anticipated. Consequently, some manufacturers have started using less efficient scans in the form of circular or sinusoidal-shaped paths. Their geometry (e.g., circular) enables moving the beam at a constant speed without deceleration in specific sections. While this solution eliminates the over-cleaning effect, it reintroduces issues of uneven surface coverage and the potential for damage.
Only a few manufacturers of cleaning devices have managed to develop X and Y scans almost to perfection. Devices with such scans boast significant cleaning efficiencies, making them suitable for cleaning sculptures, entrance portals, as well as large-area works such as facades.
The process of laser monument restoration has always met the highest quality standards, but it has never been as efficient and easily accessible as it is now. Year by year, we notice significant technological advancements and invest in the latest solutions. We are convinced that the laser cleaning market is undergoing a complete transformation, representing the future method of removing layers in monument conservation.