Modern Chemistry Techniques Save Ancient Art

Nanoparticles, laser cleaning, and glue-eating bacteria restore valuable frescoes and paintings
lasers restore Gates of Paradise

Low-wavelength lasers removed centuries of damaging grime from the bronzed Gates of Paradise in Florence, Italy.
Credit: Wikimedia Commons

The history of art conservation is almost as long as the history of art itself. Michelangelo’s Sistine Chapel frescoes were first restored in the mid-16th century, only decades after being painted. Water damage had caused a white potassium nitrate scale, which was treated with linseed and walnut oil. But such cosmetic solutions were sticking plasters and lacked the scientific understanding that has now revolutionised the field. Today, conservation scientists are finding innovative ways to clean and protect our cultural heritage.

Joyce Townsend, a conservation scientist at theTate Gallery in London, UK, explains that all conservation projects start with a detailed assessment of the work. ‘There is always a debate between conservators, curators and scientists thinking what is absolutely necessary to be done. Is there an option right now to do nothing at all, or is that unsafe because the object is deteriorating?’ As well as providing for an artwork’s survival, Townsend describes the basic ethical principles of any conservation process as ‘ensuring that you do no harm to the work, and that wherever possible anything you do could be reversed by a later generation’.

Cleaning aims to restore artworks to how the artist intended them to look, but this is rarely simple and sometimes controversial. For example, the most recent Sistine Chapel restoration in 1994 had a small number of critics expressing horror and labelling the now bright frescoes ‘Disney-esque’. Conservation scientist Bronwyn Ormsby, also at the Tate Gallery, believes a more nuanced view is required. ‘The concept of taking a work back to its original appearance is essentially flawed, as materials change with time regardless of the level of protection.’

How an artwork is cleaned will depend on the nature of the material to be removed. With paintings, a variety of organic solvents are used, but the most common solvent is water, often with chelating agents, surfactants or salts to control pH. Applying solutions through tissues, gels and sponges is becoming the norm, due to the level of control offered by holding the cleaning system at the upper surface of the art. Such gels, introduced in the late 1980s, are usually water-based emulsions thickened with cellulose or synthetic polymers. By slowly releasing the solvent, they prevent some of the swelling damage that free solvents cause to paint layers. 

Cleaning up past mistakes
The problems that conservators face are sometimes caused by past conservation treatments. Historical conservation techniques seem bizarre to us today – for example, the Sistine Chapel was restored in the seventeenth and early eighteenth centuries using, among other things, wine and sponges made of bread. The common practice of ‘lining’ deteriorating canvases is a good example of a process that now causes problems. ‘In the nineteenth century, they did this by hand ironing on the surface and actually pressing the painting. That has led to a lot of changes,’ Townsend explains. ‘At the same time, if they hadn’t done it, these paintings might not be with us today.’

The problem is not confined to the distant past: during the 1960s, for example, it became popular to use synthetic polymers to consolidate and stabilise frescoes – plaster-based wall paintings. They seemed like the perfect replacement for the wax coatings previously used, but over time it became clear that this was not the case. Their presence drastically changed the paintings’ surface properties, causing mechanical stresses and crystallisation of salts beneath the painting leading to accelerated disintegration. In addition, the polymers themselves became discoloured and brittle, particularly in hot countries such as Mexico, where they were extensively used on murals at pre-Colombian archaeological sites.

These problems have driven innovations, but so has the need to create treatments that do not expose conservators to harmful solvents and chemicals. Conservation does not always take place in a lab – a conservator might be treating a fresco in a small, badly ventilated space. Health and safety concerns have motivated new methods, as well as a desire to use green processes that minimize environmental harm.

Destroying dirt with lasers
One method that ticks these boxes is laser ablation. Surface deposits can be removed from artworks with short bursts from a laser beam. The technique was first used in the 1970s for cleaning ‘black crust’ from marble buildings and sculptures. This is a deposit often found in polluted environments and can be difficult to clean. It is largely gypsum (calcium sulfate dehydrate) mixed with airborne particulates. The method was slow to catch on as lasers were very expensive, and there was also scepticism about the cleaning control it offered.

By the mid-1990s, laser cleaning was established for stone and started to be used for other materials such as gilded bronzes and frescoes. A major breakthrough came when an Italian physicist at the National Research Council Institute of Applied Physics in Florence, Salvatore Siano, developed a method that used even shorter pulses, of only micro to nanosecond duration. The neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers commonly used in conservation emit light of 1064nm wavelength, in the near infrared. They are hand-held, portable devices and in most cases treatments will be carried out on a wetted surface to prevent over-heating. Siano’s work investigated a number of pulsing methods, including Q-switching which uses an optical switch to create a maximum energy pulse as short as 6ns.

The laser cleans by heating and expanding the surface, causing pressure waves that act like a rapid vacuum cleaner burst and detach surface material. If water is present at the interface between layers, its vaporisation also produces fractures. Shorter high-intensity laser pulses favour pressure effects while thermal effects dominate with longer pulses. By carefully selecting the pulse energy criteria, the laser will remove encrustations and dirt while leaving the material underneath unharmed.

Siano and his team first used laser treatments on metals in 2001 with a major restoration of the Porta del Paradiso (Gates of Paradise), the gilded-bronze doors of Florence’s Baptistery of San Giovanni, designed in 1401 by early Renaissance artist Lorenzo Ghiberti. The masterpiece, often claimed as the starting point of the Renaissance itself, contains 40 gilded reliefs of Old Testament scenes, which were heavily encrusted. ‘The use of an infrared wavelength, which is highly reflected by gold, makes the laser removal very safe without any risk of damaging the gold film,’ explains Siano.

His team also began using lasers for treating frescoes and one of the most interesting examples is the 2009–10 restoration of the Santa Tecla catacombs beneath Rome. The early Christian paintings in this damp underground environment were covered with a hard-to-remove calcareous crust. Siano says this was ‘a conservation problem that was absolutely insoluble with the traditional approaches’. The laser cleaning uncovered the earliest known images of the apostles Paul, Peter, John and Andrew on the ceiling of a fourth century AD tomb.

Siano is now turning his laser to removing old varnishes and restorations from easel paintings. He is still using a Nd:YAG laser, but passing the 1064nm beam through a nonlinear optical crystal to produce a visible green second harmonic wavelength of 532nm. At this lower wavelength, the laser cleans by causing photomechanical surface expansion. Siano used this method to restore a female portrait from the studio of the Italian Futurist artist Giacomo Balla (1871–1958). While not a standard option for paintings, Siano estimates that laser systems are now regularly employed for sculptures, bronzes and frescoes by more than 400 European conservation institutions.

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