A soft touch with nanoscience
Another major innovation in the last decade is the use of colloid science and nanotechnology in conservation. The starting point was a problem presented to another University of Florence scientist, colloid chemist Piero Baglioni, in the mid-1980s: how to remove beeswax spots from Renaissance frescoes in the Brancacci Chapel in Florence. Traditional solvent cleaning was not working because of surface porosity, as Baglioni recalls. ‘If you solubilise the wax, [it] penetrates even more deeply inside the walls.’
Baglioni came up with a microemulsion: a clear mixture of organic solvent and water, stabilised with a surfactant that sits at the interface between the water and organic phases. The organic phase in the microemulsion forms small 5–15nm sized droplets, creating a huge surface area to act like a detergent, dissolving in this case wax. The dissolved wax is prevented from spreading, because the organic phase is held within the microemulsion, The organic solvent is less than 2% of the microemulsion, which decreases the overall toxicity dramatically.
Baglioni was again called upon for an answer when the removal of polymer coatings applied to frescoes in the 1960s became a priority. Ageing rendered the polymers insoluble to most organic solvents because of oxidation and cross-linking reactions, but Baglioni created a series of microemulsions that did the trick. In Mexico, polymer coatings were applied to frescoes as late as the 1990s; within ten years it was clear this was a mistake. In 2008, Baglioni started to work with the Mexican National Archaeological Institute to come up with a removal treatment for murals at the Mayan site Mayapan.
The system they came up with consisted of a number of solvents including pentanol, propylene carbonate and ethyl acetate. It had a structure known as a ‘swollen micelle’, which is on the borderline between a classical microemulsion and a micellar solution, where surfactant molecules aggregate into micelles – nano-sized spherical clusters. In a swollen micelle, a number of the solvent molecules are found in both the water phase and organic droplets and this proved to be the best and most versatile system, able to remove a range of polymers. Baglioni suggests the solvent molecules dissolved in water are first able to diffuse into the polymer coating to swell and break down the polymer, followed by more solvent diffusing from the nanodroplets into the water allowing the continued breakdown and detachment of the polymer.
Baglioni has also pioneered the use of nanoparticles for repairing deteriorating frescoes. Artists generally painted directly onto wet calcium hydroxide plaster, which reacts with atmospheric carbon dioxide to form calcium carbonate (calcite). Over centuries, pollution and humidity causes the carbonate layer to break down and sulfate, nitrate and chloride salts within the walls recrystallize, leading to deterioration of the painted surface. Baglioni was sure that nanoparticles would improve on conventional conservation methods. ‘We thought that if we put the calcium hydroxide back into the paintings, we can reform the carbonate pieces in situ.’ His treatment injects calcium hydroxide nanoparticles dispersed in alcohol and their small size, just 10–100nm, allows them to penetrate several centimetres into the frescoes and slowly reform the depleted calcite.
Since developing the nanoparticles in the late 1990s, Baglioni realised they would also work for paper and canvas conservation. The breakdown of cellulose fibres in paper and fabric occurs through acid-catalysed hydrolysis and leads to the characteristic yellowing and embrittlement. Using hydroxide nanoparticles, paper and canvas is deacidified with the excess particles quickly converted into carbonate, providing a protective neutral pH.
Baglioni explains size is the crucial criteria in using nanoparticles. ‘Depending on how fast or slow you want the formation of calcite to be, you play with the size of the particle.’ Smaller particles will have faster reaction times. For the consolidation of frescoes, larger 100nm particles might be selected. Their size balances the ability to penetrate into a fresco with a reaction rate slow enough to produce large mechanically strong calcite crystals. Whereas for the conservation of paper, where a high pH can be damaging, smaller 50nm particles are preferable so that excess particles form calcite within hours.
The metabolic power of bacteria
An unusual approach to cleaning came from the Polytechnic University of Valencia, Spain. We are familiar with enzymes in washing powders but Pilar Bosch’s team has taken things a step further, using bacteria to remove inorganic crusts and animal glues from frescoes. Because bacteria can produce a whole host of enzymes they can deal with complex cleaning problems, metabolising organic and inorganic matter into hydrogen sulfide, molecular nitrogen or carbon dioxide. Over the last 10 years, the method has been successfully used at several important sites, including the Camposanto in Pisa, Italy, a 13th century burial ground supposedly constructed around soil brought back from Jerusalem during the Crusades. It was decorated with extensive frescoes that have been badly damaged.
Biologist Bosch worked with a team of microbiologists, chemists and conservators to develop the treatment. She says they started in the microbiology lab: ‘We have to find the micro-organisms that are able to metabolise the substances we want to clean without damaging the artwork.’ All bacteria tested are harmless and do not form spores. Over several projects, Bosch tested different strains of the nitrate-reducing bacteria Pseudomonas stutzeri, commonly found in soil and water. The strains chosen were able to remove nitrate crusts and animal glues when applied with agar gel and left for between several hours and several days.
Bosch says that there are no signs of any problems with the frescoes treated ten years ago and, although there is sometimes a knee-jerk fear of using bacteria, interest in the method is growing, with several companies investigating making commercial kits. She is looking at other applications, for example removing glue from paper.
An acrylic time-bomb
The introduction of organic dyes in the 19th century, followed by paints based on acrylic polymers in the 20th has set a conservation time-bomb ticking. The survival of vivid colours in Renaissance art is largely due to the chemical stability of the inorganic pigments used, but this is not the case with modern acrylic paints. While the scale of the problem is not yet clear, it is likely that within several hundred years synthetic dyes will have faded and plastic binders depolymerised, becoming powdery. ‘Being provocative, we can say that if you looked at the Museum of Modern Art in New York in 100 years it will be an empty museum, because what’s inside will be completely degraded,’ Baglioni says.
Baglioni is starting to bring his knowledge of microemulsions to bear on this unsolved problem. He says that while the chemistry is complex, he has some ideas of how to reverse the chemical degradation and hopes in the future to develop cleaning gels able to reformulate original compositions. The Tate Gallery also has a significant stake in the conservation of modern acrylic paints and Ormsby is involved in ongoing research on their conservation.
A number of the innovative methods described have become standard conservation practices. Laser cleaning is well established and Baglioni’s nanoparticles are commercially available, but as the name suggests, conservation is a conservative field and the adoption of new methods is rarely fast. Siano describes a ‘natural inertia’ in conservators which he thinks is responsible for holding back the early uptake of laser ablation methods, but Baglioni concedes that ‘in some cases it’s easy’, where there is an urgent need, such as for polymer removal.
In the long term, we may be grateful for a measured, scientific mindset in conservation. ‘Works of art are unique and valuable, so we need to ensure that what we do – whether it’s removing something or adding something – is guided by conservation ethics and stands the test of time,’ Ormsby says. With an ethical and innovative approach, there is hope that modern acrylic art will last as long as the Sistine Chapel frescoes.
This article is reproduced with permission from Chemistry World. The article was first published on June 23, 2014.