Laser cleaning applications in the preservation of cultural heritage have been known since the 70’s but have had a significant increase in the last ten years, thanks to studies on the ablation process and the development of laser systems tailored for this application. The growing evidence for efficacy of this approach, the increase of case studies and the wide dissemination of its use in restoration fields around the world have also been joined by a large number of conferences and scientific publications on the topic. At the same time, laser systems for conservation purposes appear more and more often in fairs and exhibitions.
In addition, some important key studies have raised the interest of the mass media that gave a high resonance to this innovative technique extending its dissemination and knowledge.
At least 400 laser systems are currently working in conservation and restoration laboratories both in Europe and in the rest of the world. All this shows how laser technology has moved from science labs to commercial production and finally to the restoration sites. The scientific contribution of leading research institutes in fact plays a fundamental role in the acceptance of laser cleaning technique in daily conservation practice.
In the restoration of artworks you can meet a lot of different conservation issues: the complete or partial removal of black crusts, of corrosion products, of retouching and repainting of damaged coatings and biodeteriogens from various substrates such as stone, wall paintings, metal, wood, paper. The variety of combinations between the substrate and the decay to be removed and the need for high selectivity make the optimization of the laser ablation process difficult: the versatility and flexibility of the laser system is therefore of fundamental importance to safely deal with the largest number of cleaning problems.
The study of the optimization of the process of removing unwanted incrustation takes place through two main channels: the variation of the wavelength or the variation of the pulse duration.
The Nd: YAG solid state lasers in their fundamental wavelength at 1064 nm in the near infrared have proven immediately the most suitable for the application in question. It has also been shown that an intermediate pulse duration between that of the Q-switch laser (<10ns) and of the free running (<200μs) lasers allows to avoid both the photomechanical damage induced by very short pulses, and the photothermal damages produced on the surface by irradiation with very long pulses. The Nd: YAG laser systems called Short Free Running (SFR) and Long Q-switching (LQS) have therefore been proposed to overcome the aggressiveness of the Q-switching lasers (QS) on extremely brittle materials and to reduce both the photothermal and photomechanical damages possibly caused by too long and too short pulse durations.
In recent years, there have appeared on the market and in restoration and research laboratories laser systems for cleaning with wavelengths different from the “traditional” Nd: YAG: in particular, are proving to be particularly effective the Er: YAG (Erbium systems, with 2940nm wavelength) and the Ho: YAG lasers (Holmium systems, with 2100 nm wavelength).
These two new systems are today present in the Laboratory of Diagnostic for Conservation and Restoration of the Vatican Museums that is at the forefront in Italy and Europe in the restoration and in the experimental application of the most sophisticated technologies for the conservation of cultural heritage. The Director of the Laboratory, Prof. Ulderico Santamaria, is in fact always staunch supporter of the indispensable union between art and technology for the conservative restoration, following the well-known principles of the theory of the restoration of the famous historian Cesare Brandi.
Within the Laboratories, the laser technologies have long been studied and applied successfully. The restorers can count on a large number of different lasers that is enriched year after year. Among the artworks restored in the Vatican Museums and returned to their original beauty thanks to the laser we remember the wall paintings by Michelangelo in the Pauline Chapel, a painted sandstone Egyptian sarcophagus, several marble sarcophagi from the Roman period, the frescoes of the Necropolis of Santa Rosa, the basement of Antonio Pio column.
It was also tested to restore stone artefacts exhibited outdoor, to remove lime deposits and iron oxides, as well as to passivate ancient metal objects (e.g. Vatican Gardens, Roman sarcophagi, etc.).
The Holmium laser, which comes from the medical field and just landed in the world of restoration. The strength of this system lies in its extreme ability to pulverize hard aggregates (and is in fact used in medicine for the treatment of kidney stones) combined with a simultaneous biocide action capable of eliminating mosses, fungi, lichens, and any of organic and biological nature material deposited over time on the works, often left to the elements, such as those of the Vatican Gardens.
Within the Vatican Laboratories After a diagnostic survey campaign, a controlled fluence laser type Nd:YAG Qsw 1064 nm was successfully used to remove selectively the natural resins found on two paintings on canvas by Lorenzo Lotto (“Presentation of Jesus in the Temple” and “Jesus Christ’s Baptism”, both the property of the Museum of the Holy House of Loreto.
More recently has been tested the Erbium system, which has been successfully applied for the removal of layers of ancient restorations on the board painting of “Madonna della Cintola” by Vincenzo Pagani. Thanks to these successful trials the same system is actually applied in ISCR institute in Rome to remove hard deposit from a board painting of St. Bartholomew, from a Tuscan school of the fifteenth century: the layers of this deposit appeared very unique and tenacious, almost chalky, and had proved resistant to removal with conventional chemicals.
In conclusion, the intense trials conducted in recent years by accredited Restoration and Research Laboratories in Italy and around the world has led the expansion of laser applications in various conservation problems. With the advent of new systems and new laser wavelengths even more interesting fields of application will be explored.
[by Alessandro Zanini]