The ability to see through walls and other objects Superman-style is surely high on the wish-list for many children.
Sadly, with the purchase of a child’s first pair of novelty X-ray glasses, such dreams rapidly fade – plastic glasses do not create superpowers or enhance vision.
Until recently, we scientists felt a similar disappointment when we applied X-ray technology to paintings.
But thanks to technological advances and the analysis of an Arthur Streeton self-portrait we’ve managed to bring art and science together in a fascinating way.
Historically, artists have been known to recycle canvases by painting over their artwork to save money. Up until the early part of the 20th century, this was often done with white lead paint.
What we’re trying to do is use X-ray technology to look through the lead paint and see the artwork that was first laid on to a canvas.
In doing this, we might find hidden images or changes in the development of the image by the artist.
But it’s not that simple.
One of the key challenges posed by historical paintings is the composition of the paint, which contains heavy elements – including lead and mercury – that obscure traditional X-ray images. (Think of it as an unintentional version of those lead gowns you are asked to wear during medical X-rays to protect your insides).
It’s only in the last couple of years with the advent of more advanced X-ray fluorescence (XRF) spectroscopy techniques that we have been able to generate a much clearer picture of what lies beneath some of the world’s most significant works of art.
In 2010, the use of XRF spectroscopy hit the headlines when the French National Museum used it to perform an analysis of Da Vinci’s Mona Lisa.
In that instance experts were able to probe the 500-year-old masterpiece without even taking it down from the wall, providing unique insight into the mind of the artist.
One of the key strengths of XRF spectroscopy (which can be performed at synchrotrons and is thereby known as Synchrotron XRF) imaging is its ability to reveal the distribution of the pigments of underlying brushstrokes.
The technique characterises a wide range of pigments, and because different pigments were favoured, or came into use as artists’ colours at specific times in history, it can ultimately indicate provenance and attribution.
Synchrotron XRF uses a beam of high-energy X-rays to penetrate each layer of paint, creating a more complete picture of the strokes, chemicals and materials on a canvas.
This information is analysed and translated into elemental data sets which are used to reveal images and other elements previously invisible.
Our analysis of the painting, which is in the National Gallery of Victoria’s care, was not just concerned with unearthing the image hidden underneath thick layers of lead-based white paint, but also perfecting the technique which would allow us to perform an in-depth analysis of the painting without damaging it.
Before performing any work on the Streeton painting, we applied historic paints actually used by Streeton to canvas swatches and irradiated them with a dose 1,000-times greater than the actual scanner would expose the painting to.
At no point was damage observed. This is important because if art galleries are going to give us access to paintings, they want to know we aren’t going to damage them, and rightly so.
The next challenge was to ensure a stable environment to house the painting in – similar to the carefully controlled conditions and climate at the National Gallery of Victoria.
To perform a scan of the painting we used the CSIRO-developed Maia scanner, one of the most advanced in the world.
One of the main advantages of the Maia scanner is that it allows paintings to be scanned quickly, minimising the time the artwork needs to be away from the gallery. It also allows the entire painting to be scanned, not just a cross-section.
(Cross-sections from paintings are in the order of a quarter of a millimetre or smaller, and are taken judiciously to understand the materials and paint layer structure. They are painstaking and time-consuming to prepare for analysis.)
Our scan of the Streeton painting using the synchrotron with the Maia detector took 22 hours – about a fifth of the time it would take us to perform using traditional XRF techniques.
Using the tunability (ability to select the exact energy of the X-ray beam used) and monochromatic (very narrow frequency) properties of the synchrotron beam we were able to obtain the maximum amount of information about elements that lay under the surface of the painting.
This despite the thick layer of white lead-based paint that covered it.
The images we captured were at tens of megapixels and with a spatial resolution (the size of the smallest detectable element) of tens of microns, where one micron is a millionth of a metre.
The result was a very clear image of Arthur Streeton’s self-portrait (see main image above) and the development of a new process for analysing paintings quickly and without impact on the integrity of the painting.
Given the ubiquitous use of lead white paint in historic paintings, our hope is that this technique will be applied to other artworks in the future.
This is an exciting prospect for science, but could be quite a confronting concept for the art world. The complexity of the science involved will be a new realm for art historians and curators and may even result in some theories about particular artists being upturned.
There is no doubt there are dozens of paintings out there with questions against their authenticity or origin. As this technique is gradually refined there will be fewer places for the secrets of art to hide.
Of course, others might argue that those secrets are part of what makes art attractive.