The use of Raman Spectroscopy in identifying minerals on Mars and in historical paintings was highlighted during a visit to the Open University. This technique, named after Sir Chandrasekhara Venkata Raman, allows for non-invasive examination of pigments, aiding in authenticity cases and art restoration. The visit confirmed the presence of modern titanium dioxide white pigment on a 17th-century painting.
The answer in this case lies in Raman Spectroscopy, a technique used for identifying minerals, whether they are on the surface of the red planet or in the paint of a picture from the mid-1600s.
Last week we were at the Open University in Milton Keynes where they specialise in planetary and space science, building miniaturised instruments that end up on missions to the moon and Mars. Raman is one of the techniques used to characterise the minerals on the planet’s surface; as an approach it also works remarkably well for identifying paint pigments, many of which are also minerals.
We needed access to their terrestrial Raman microscope because it can examine the whole of a small painting, thus removing the need to take samples which would be examined individually. As our project involved hunting widely for small fragments embedded in the paint, this piece of kit was ideal. Using the OU’s system allowed us to search and analyse many different locations safely and non-invasively.
Raman spectroscopy is named after Sir Chandrasekhara Venkata Raman (1888-1970), who was one of the early observers (in the late 1920s) of the effect on which the technique is based. Technically, the Raman effect is due to what is called ‘inelastic’ scattering of photons of light off of molecules; chemical bonds in the molecules interact with the light, shifting the frequency (colour) slightly up and down. These shifts, their position, number, and strength, are characteristic of the specific molecule being analysed, so can be used to identify the compound. Because the technique is precise, sensitive, and can operate on a microscopic scale, it has become a go-to method in the art science community for identifying pigments in paint. It also perfectly complements other forms of analysis, such as those that give the elemental composition, so that we can be very specific about which pigments are present, and where. Raman microscopy is consequently one of the standard set of instruments we routinely use when we are working on authenticity cases.
So, to the OU. Because Raman spectrometers use laser light they are normally enclosed in a light-proof shroud; we needed a model that had an open stage that we could mount the painting on – theirs is a slightly older model without the enclosure. At the same time, since they work on exotic materials, the instrument is in a special clean room, so we had to dress up in full clean-room gear for the measurements…
We were following up on some previous work on the painting that had argued that the modern pigment titanium dioxide white was present and therefore the painting was of equally modern origin. Elemental and Raman analyses had reportedly detected titanium dioxide white, so our job was to check this, especially whether it is of the modern (20th century) form of the pigment, plus also gather other information such as exactly where in the paint structure it was. The reasons for this were that titanium dioxide occurs in, for instance, earth pigments, and can therefore legitimately appear in a 17th century painting; also, even if the pigment is of modern origin, if it is on the surface of the paint it could have been applied during modern restoration treatment.
And what did we find? After hunting around, we located a white clump of particles that gave us a spectrum like this:
The peak positions and intensities fit very nicely with the mineral anatase, one of the two principal crystal forms of titanium dioxide. (We’ve shown a reference spectrum inset for comparison.) From the context and appearance under the microscope we were also confident that we were dealing with the modern synthetic form, not the naturally occurring variety. However, since we were working under a microscope we could also see that the pigment clump was quite clearly lying over the top of the main paint layers of the picture, not mixed into the body. We hadn’t found any evidence for titanium dioxide white in the underlying layers elsewhere, so this clearly supported the idea that this modern pigment is most likely a modern addition.
All-in-all a very successful visit – perhaps next time we’ll go mobile with the Mars Rover!