Art restoration and conservation: restoring former glories

“Leonardo was a great fiddler,” explains Professor Martin Kemp, an expert on the artist at Oxford University. “He almost always found it hard to settle on a single composition and would often change his mind.” We know this, in large part thanks to the emergence of highly sophisticated tools that use infrared light to peer beneath the surface of the master’s works.

With today’s technology we can find out what pigments were used in Mona Lisa’s sleeve without physically removing the paint, for instance, or understand how Leonardo’s Salvator Mundi, the most expensive painting ever sold after a 2006-2011 restoration, had been ‘touched up’ by later artists.

Art restorers and conservators now have at their disposal increasingly sophisticated tools, which give them far more data and context about what is going on under the surface of the works they’re looking at. So, while the labour of physically restoring artworks remains a patient, time-consuming and complex craft, restorers and conservators now have access to more information than ever about the works to help inform their decisions.

Until the last couple of decades, conservation and restoration of art remained a relatively low-tech pursuit (although X-ray has been used to explore artworks for over 100 years).

Museum laboratories and conservation teams would normally need to use invasive techniques to investigate the work. With fine art, for instance, a small amount of paint would often have to be removed, which self-​evidently affected the integrity of the piece.

Next up would be a cleaning process, using solvents that could potentially damage the work. Finally, additional painting might be needed to produce what the conservator assessed the artist’s original intentions were, based on historical descriptions and similar works – especially where the work was damaged.

This kind of informed guesswork would often cause controversy. For example, the most recent restorers of Leonardo’s Last Supper – a mural in the refectory of the Convent of Santa Maria delle Grazie in Milan, were heavily criticised in some quarters for the way Jesus’s sleeve drapes beneath the table in the rework, whereas it seemed not to have in the original.

In recent years, art restorers have begun experimenting with a wide range of new tools to help with their works. Dr Eric Nordgren of West Dean College of Arts and Conservation in Sussex explains that these new tools “allow the understanding and documentation of works to a much higher level than was previously possible, often in ways that are either completely non-destructive or that only require micro-sampling”. He adds: “This in turn gives the conservator much more to go on when making decisions of how best to preserve the piece through sympathetic conservation or restoration measures.”

These tools give conservators many more options when it comes to learning about a piece of art. Indeed, the international verification body Authentication in Art recently published a manual describing 50 different technologies that are appropriate for investigating different kinds of work and their constituents.

Take works of fine art. One technique that is widely increasing in use is infrared reflectography (IRR). This method can be useful for understanding the artist’s original intentions – and is especially helpful for understanding the thinking of a ‘fiddler’ who made multiple tweaks to their work, as Leonardo did.

Looking like a cross between a projector and an SLR camera, the IRR camera directs beams of infrared light at a piece of work, explains Kemp. The infrared light can penetrate pigment layers made of animal or vegetable compounds, but is eventually either reflected back, typically by the surface the work was painted on, or absorbed, with carbon-rich material being particularly absorbent.

That means the paint itself is not picked up to any great extent, but sketches with charcoal pencils, made as they are of carbon, do show up by contrast with the substrate. This approach therefore lets the conservation team build a picture of what was going on in the original design.

This kind of technique can reveal some fascinating things. Professor Aviva Burnstock of London’s Courtauld Institute describes a recent research project on Seurat’s ‘Young Woman Powdering Herself’ using an Osiris infrared reflectography camera. To the naked eye the painting shows a woman alone, yet thanks to infrared techniques, the researchers were surprised to discover a self-portrait by the painter which he later decided to cover with a pot plant.

Besides knowing what is under the surface, so to speak, other technologies can tell the conservator what kinds of paint the artist used. Nordgren describes a fourier transform infrared (FT-IR) spectrometer, which can be used to identify chemical compounds present in a sample. In a lab, conservators would place the work or a small sample on a crystal plate. The machine can be used to find out what compounds are present in a sample by measuring the energy absorbed when chemical bonds are stretched, twisted or bent. After a few seconds, this data is fed into software that can tell which chemical bonds are present,  and therefore what a paint is likely to be made from.

In addition to telling us which compounds were used in the pigment, this technique can also help work out if a painting is a fraud. For example, cadmium yellow is a type of paint first introduced in the 1820s. If a painting that is claimed to be from, say, 1550, is shown to contain cadmium yellow, investigators immediately know they’re looking at a fake.

Of course, conservators don’t just restore paintings, but also select other appropriate tools for investigating cultural and historical works made of metal, glass, ceramic and stone – whether that’s statues, pottery or even fabrics. When it comes to conserving this kind of work, the conservation team need to understand what these pieces are made of at an elemental level. If trying to repair damage to a jar made of some kind of brass alloy, for instance, it would be crucial to understand the exact proportions of both copper and zinc in the piece.

Nordgren explains that a portable X-ray fluorescence spectrometer is one of the most useful tools here. The tool uses a self-​contained X-ray tube, which exposes the material to a beam of radiation that bumps some of its constituent atoms up to higher energy levels. These, in turn, emit X-rays with characteristic energies as the electrons return to a lower energy level. The unique patterns emitted by different materials then tell the researcher what the material is made of at an elemental level.

X-ray fluorescence proved its worth in a recent study Nordgren undertook with a student at West Dean college to examine an apparently unremarkable and heavily corroded metal knife. The analysis revealed “the handle to be a complex construction made up of lead, brass and gold, which were revealed more clearly after careful cleaning. What initially appeared to be an unremarkable historic piece was revealed to be an unusual example of bespoke craftsmanship.”

At present, most of the tools conservators use for this kind of research and analysis were originally designed for different purposes. X-ray fluorescence, for instance, was developed for mining and scrap material identification but has been incorporated into conservation labs over time.

For centuries, conservation of important cultural works has been, literally, more of an art than a science. And while the fundamentals of restoration and conservation still involve painstaking manual labour, cleaning and mastery of artistic techniques, the wealth of new technologies now available mean that professionals can more confidently and accurately adjust degraded works to match them as closely as possible to the artist’s original designs.