Conservators fine-tune optical methods for art

17 Jun 2002

Laser-based techniques, once viewed with scepticism by art conservators, are finally gaining acceptance in a range of applications. Michael Hatcher reports.

From Opto & Laser Europe April 2002

Art conservators tend - as you might expect - to be a conservative bunch. They have been relatively slow to take up laser technology, preferring to stick with tried-and-trusted tools such as scalpels and chemical solvents. And who can blame them? After all, nobody wants to be known for attempting to tidy up the Mona Lisa, only to burn a hole in da Vinci's canvas.

However, lasers have much to offer art conservators, and are gradually gaining acceptance. They have uses both in cleaning and non-cleaning applications. In the former category, Q-switched Nd:YAG lasers have long been used on buildings blackened by centuries of smoke and, more recently, car exhaust emissions. Some of the world's most famous buildings, including parts of the cathedral of Notre-Dame in Paris, have been cleaned in this way.

Parts of the precious Chinese terracotta warriors of the Qin dynasty have also been laser cleaned. In fact, the use of laser-induced breakdown spectroscopy (LIBS) to monitor the effects of laser cleaning in real time was pioneered on these ancient figures. This is one example of the increasing use of online control techniques - a non-cleaning use of lasers in art conservation.Laser techniques are traditionally used in the field of stone-cleaning, but this is being extended to metal, glass, textiles and paper.

One example of a laser-treated metal artwork is Lorenzo Ghiberti's Porta del Paradiso (The Gate of Paradise), part of the Baptistry in Florence, Italy. The Porta is regarded as the most celebrated gilded bronze relief of the Florentine renaissance. Conservators first cleaned the darkened areas with nitric acid in 1949, only for a flood to deposit sludge all over Florence in 1966. Coated with black encrustations of gypsum, copper oxide and organic material, the Porta had lost much of its appeal.

Salvatore Siano, a physicist from the Istituto di Elletronica Quantistica (IEQ) in Florence, worked with chemists and curators from the Opificio delle Pietre Dure, which is responsible for the [Porta's] preservation programme. Using a Q-switched Nd:YAG laser operating at the fundamental 1064 nm frequency and emitting 110 mJ 28 ns pulses, the team first removed encrustations from six small areas of Ghiberti's masterpiece. With restoration now in full swing, Siano says that using the laser will significantly reduce the cleaning time because the sculptural elements of the Porta can now be treated in situ, instead of having to be dismantled as before.

Siano and his colleagues at the IEQ have also developed a novel Q-switched Nd:YAG system, in which they say the pulse length can be easily tuned from hundreds of nanoseconds to several microseconds. They achieved this by positioning optical fibres of different lengths within the resonator. The longer the fibre, the longer the pulse emitted. This is important for applications involving the ablation of material, because there is a trade-off between the high mechanical stress induced by short pulses and the increased temperature effects induced by longer pulse-lengths.

Siano described his team's efforts at the fourth Lasers in the Conservation of Artwork (LACONA) conference in Paris. The biannual meeting has grown rapidly since the inaugural event hosted in Crete by the Foundation for Research and Technology Hellas (FORTH) in 1995. The associated LACONA organization now has about 300 members. As one might expect from their historical inheritance, speakers from Italy, Greece and France make up the bulk of conference attendees.

This is not to say that the field is the preserve of these countries - the UK and Germany are also well represented. Martin Cooper and colleagues at the UK's National Museums and Galleries on Merseyside, Liverpool, have recently used an Nd:YAG source to treat a wooden Native American totem pole.

An emerging field is paper cleaning. Wolfgang Kautek of Germany's Bundesanstalt für Materialforschung und -prüfung has been working on a project to clean a sheet of music found in the church of St Michael in Munich. The church was bombed in the Second World War and the parchment was damaged by fire. Further damage was caused when the fire was extinguished with water.

Kautek describes the technique used to clean parchment as similar to that used for tattoo removal - both materials are essentially collagen fibres, the difference being that parchment does not regrow. "Skin is basically parchment," said Kautek. The ideal irradiation wavelength for cleaning is 500 nm, and Kautek used Innovative Berlin Laser's doubled Nd:YAG source at 532 nm - a new system that features a completely enclosed class 1 laser source.Paper cleaning is now the subject of a new EU project. Collaborators include FORTH spin-off Art Innovation, based in the Netherlands, and Kautek. Art Innovation distributes Nd:YAG systems for sculpture-cleaning from Lynton Lasers. It also specializes in multispectral imaging and sells an online LIBS monitoring station.

Using multiple fibres to collect light from the laser-ablated plume, LIBS spectra are collected for each fibre per laser-pulse. The spectral information is fed back into the laser system, controlling the cleaning process. The instrument, which can identify different layers in a painting, is also suitable for other industrial applications.

Back at FORTH, where the LIBS instrument was originally developed, Demetrios Anglos and colleagues are also turning their attention to paper. They analysed an ecclesiastical Byzantine manuscript thought to date from the late 12th century (see over) and found that the pigments on which the coloured writing was based were white lead, vermillion red and lead-tin yellow. This was consistent with the date estimate.

Art Innovation's Rianne Teule says that the company supplies its technology as part of a package including equipment, accessories, training, information and workshops, to convince conservators to invest in its products. "We find that conservators need to have hands-on experience to feel comfortable with the technique," said Teule. She and her colleagues are now looking to develop a portable excimer-laser system for cleaning mural paintings - for which conventional YAG systems are not suitable - and a multispectral imaging system for paper treatment.

According to Costas Fotakis of FORTH, conservators are now beginning to optimize laser parameters for individual applications. This is exemplified by the solution found to the "yellowing" effect often seen when materials such as limestone are laser cleaned. "You get yellowing if you don't optimize the laser parameters," said Fotakis. "However, this can be avoided if you use different irradiation wavelengths." In general, Q-switched YAG lasers are used for cleaning, so the problem can be overcome by employing second or third harmonics.

Fotakis agrees that there is now a trend towards the use of lasers on unconventional substrates, such as ivory and fabrics. He has been involved in an elephant-ivory cleaning project that involves Meg Abraham and Odile Madden from the Los Angeles County Museum of Art, US. They found that the most succesful method for ink removal was to employ laser radiation in the visible range.

John Asmus is regarded as the grandfather of laser-based art conservation. He pioneered laser-cleaning and holographic recording techniques in the early 1970s, working on the terracotta army and Mona Lisa, among others. Asmus sees no limits to the types of material that can be laser-treated: "In 1972-1973 I successfully treated every material that the curators could find in their collections. Much of the work done in the last decade has been towards understanding, optimizing and improving upon those initial efforts. Stone cleaning was the first application that caught on. I'm impressed with the emergence of online control using LIBS and Raman imaging."

Asmus has always likened the laser to a superior, albeit more expensive, scalpel. He and Fotakis both point out that in another field in which the scalpel is omnipresent - medicine - there was some initial resistance to adopting the laser. And just as laser treatment in medicine has now seen widespread acceptance, they expect the same thing to happen in the art world.