The use of laser technology for the restoration of historical objects dates back to the 1970s, and since then, it has been associated with spot cleaning with a diameter of up to several millimetres. Recent years of rapid technological development have made possible the partial automation of the process of build-up removal and the elimination of imperfections, but also the reduction of the size of equipment. Consequently, mobile, hand-held cleaning systems began to emerge, which ensure the high quality of the cleaned surface and unprecedented performance. The main advantage of the current solutions is the automated cleaning of surfaces up to several centimetres in width and height.
Each of these devices’ operating principles is based on the process of laser ablation, i.e. the generation of a highly concentrated but very short laser pulse to vaporise the contaminants from the surface of the cleaned object.
The essence of the entire cleaning process is the selection of the appropriate wavelength of light and parameters of the laser beam for both the cleaned surface and the contaminants. This is quite a challenge for people with minimal experience with lasers. There is a growing selection of equipment on the market that can be found under the heading "laser cleaning" or "laser ablation". The vast majority of these are industrial devices that only enable work on metal surfaces. This equipment can be dangerous to historical objects (despite the technology being described as non-invasive) and the outcome of the process depends on both the operator and the equipment itself.
Choice of technology
Modern laser sources allow for the design of very versatile cleaning devices, whose parameter range enables cleaning almost any surface of historical objects. However, this is not possible without additional solutions, which are applied simultaneously at the level of preparation of the energy distribution of the laser pulse, automation of its movement in the X and Y axes, and calibration in the appropriate software. To obtain the required quality in the field of historical objects conservation, it is necessary to correlate the operator’s experience with the latest technological solutions.
One of the first steps in automation was the use of a head with a 1D galvanometer scanner on the axis X. This solution allows the laser spot to be automatically moved along one axis (left/right). The result of this solution is the possibility of “line” cleaning. However, it has been observed that an excessive cleaning effect is created at the edges of the scan. This is due to the nature of the scanner – as it moves the spot to the end of the working field, it must decelerate and start moving it in the other direction. The deceleration of the spot at the edges of the scan area results in the surface being exposed to the spot for longer than it was across the width of the scan. This leads to the excessive cleaning of the edges of the scan and visible marks on the cleaned surface.
This is not the only problem with 1D scanners in conservation applications. To clean the working field in the X and Y axis, the head must be manually moved on the Y axis (up/down). This causes not only the analogous problem of uneven surface exposure to the laser and surface cleaning of the entire working field. Holding the laser in one place for too long creates the risk of surface damage. Depending on the material, this degradation can manifest itself in different ways, e.g. the face of a red brick will turn grey, visible cavities can form in sandstone and wood can be burnt.
The application of a second galvanometric motor in the laser head has enabled scanner operation in the X and Y axes. This significantly reduced the impact of operator hand movements, the risks associated with material damage and improves the uniformity and aesthetics of the cleaning effect. Depending on the lens, these scanners can automatically clean an area as small as 18 × 18 cm. All you have to do is press the light emission button and hold the machine head still.
This type of scan provides the highest cleaning efficiency and uniform surface coverage, but despite significant improvements in surface safety and cleaning quality, there is still the problem of the cleaning effect. Finding a solution has proven to be more difficult than expected. Consequently, some manufacturers have started using less efficient scans in the form of circular, sinusoidal or similar paths. Their geometry (e.g. circle) allows the spot to move at a constant speed without slowing down in individual parts. With this solution, the excessive cleaning effect disappeared, but the problems of uneven surface coverage and the possibility of surface damage returned.
A few cleaning machine manufacturers have managed to develop their X and Y scans to near perfection. Devices using such a scanning method are distinguished by their significant capacities so that they can be used successfully both for cleaning sculptures or entrance portals and for large-scale works such as facades.
The future of laser cleaning
The process of laser restoration of historical objects has always met the highest quality standards, but it has never been so efficient and easily achievable. Every year, we see more and more advances in technology and we invest in the latest solutions. We are convinced that the laser cleaning market is starting to reinvent itself and that it is the future-proof method of removing layers in historical object conservation.
Autor: IRLaser Sp. z o.o., www.irlaser.pl