The application of laser technology in the world of conservation dates back to the sixties where laser was first used to remove black encrustations from white marble by exploiting the different optical properties of both materials. Following on from these early uses of the technology, the technique was further researched by physicists, restorers and custodians of cultural artefacts so that to date the technology takes on a highly relevant and very precise function in the restoration process.
In the last few years, the use of laser cleaning on metal artefacts has undergone further developments; also thanks to the work of Dr. Salvatore Siano (researcher at Florence’s National Research Centre in the Institute of Applied Physics). The technology has enabled the development of new lasers which are more closely aligned to the needs of the restorers.
Some advantages in the use of this technology
- Non-invasiveness: the use of chemical substances and abrasive materials is not required. Even before any strengthening is undertaken it allows for the treatment of extremely delicate or significantly altered surfaces without any physical contact. In this way, invasiveness is minimal.
- Greater level of control: the deteriorated layer is removed by only a few microns per pulse, therefore the depth of the procedure can be defined with a high degree of precision.
- Greater precision: the cleaning process only occurs in the area on which the laser beam is directed. This area can be set very accurately, depending on the repair requirements. Moreover, fibre optic lasers allow moulded surfaces to be treated, including very complex ones.
- Selectivity: with solid state lasers (Nd:YAG) the different radiation absorption capacity of the materials due to their colour, means the laser’s effect is even more selective in terms of its result. In fact, the modified layer to be removed, usually very dark in colour, completely absorbs the laser’s light. The substrate, usually of a brighter colour, reflects this light, thus limiting or completely impeding the effect of the laser.
- There is no use whatsoever of chemical substances.
There are various pieces of equipment used in the world of restoration for cleaning works of art, and all of this equipment conforms to certain criteria: laser beam sources, duration of the laser pulse and light diffusion method.
The following table outlines some of the characteristics
- Wavelength 1064 -532nm
- Pulse duration Around 8 ns
- energy 1J
- Energy 1064 nm = 1J
- Energy 532 nm = 500mJ
- Maximum Frequency 20 Hz
- Maximum spot diameter 10mm
- Beam delivery articulated arm
The implementation stage:
The implementation of the laser cleaning process is based on the concepts of ablation thresholds. Today these have been supplanted by the concepts of cleaning threshold and damage threshold. The former refers to the Fluence (J/cm2) needed to remove the layer to undergo ablation (spallation). The latter refers to the fluence that the layer needing conservation can withstand, without sustaining any damage. There are two types of damage which can occur: thermic and mechanical damage; which appear as variations in colour, micro-explosions, and scratch damage to the colour, etc.
The job of the laser restorer is to research these thresholds and find the middle ground between these thresholds limits, so he can treat the artefact with the greatest care. The ideal conditions are created when the damage threshold has much higher fluence values compared to the ablation threshold. It is here that the laser excels. When this is not possible due to the inherent nature of the layers, a number of strategies can be adopted, which include soaking, alternating the cleaning process with traditional processes, and the variation of laser irradiation (thus a variation in the duration of the laser light pulses).
This latter strategy, today is of great interest in terms of research. Current scientific literature has already proven quite robustly, that the variation in wavelength, and more specifically the duration of the laser pulse produce very different effects on the irradiated material.
In many cases the use of the laser technology has proven highly successful in maintaining some types of significantly aged veneers and it has been used to save parts made from fabric, which would have deteriorated if chemical agents had been used.