The decline of decaying paintings could be reversed thanks to a new technique from German scientists.

Aging paintings on glass were the target of the team from the Technical University of Berlin and the Federal Institute for Materials Research and Testing in Berlin. They studied reverse-glass paintings, which are made by painting onto the back of a sheet of glass. Unlike stained glass, the paintings are not fired and the contact between glass and paint is fragile, leaving them prone to corrosion which can have devastating effects.

Led by Birgit Kanngiesser, the team used a technique called 3D micro X-ray fluorescence (3D micro XRF) to find out what elements were in the corroded areas of the glass of a corroded 19th century painting and compare them to healthy areas, all without damaging the painting any further.

XRF involves firing an X-ray beam at the sample to ionise its atoms. Electrons fall down to fill the gap left by the removed electron, and emit the spare energy as photons. Each element produces a different characteristic pattern of photons. In micro XRF the beam can be focussed onto a very small area to collect information about only that area. By using a powerful X-ray source and applying their recently-discovered calibration improvements to the technique, Kanngiesser and co-workers were able to study the composition of layers buried under the surface without causing any more destruction.

It has long been suspected that ingredients of some of the paints used can cause the painting to self-destruct, and Kanngiesser's evidence backs this explanation. The team identified the paints used and found that lead and mercury from the paints had diffused into the glass. The information they found should help conservators understand the way these paintings corrode and find the best way to protect and restore them.

Kanngiesser hopes the technique will also be useful in other fields 'such as environmental and material science, archaeometry, and life science. In contrast to other elemental imaging methods, there is no principal size restriction of the objects investigated,' she explained. 'The non-destructive character may be indispensable for some applications, like in archaeometry and life science. For the latter the possibility of obtaining elemental distribution in virtual cross sections without having to cut the sample might open up new ways of in vivo measurements.'

Clare Boothby