What do they have in common for science, the study of Ötzi, the so-called ice man, the prehistoric paintings in the Altamira caves, the delicate and detailed miniatures found in the illuminated manuscripts of the Middle Ages, the sculptures in the Cathedral Burgos, the enormous crystals of the Pulpí geode, in Almería, or the meteorites of Martian origin? The answer is a non-invasive technique known as Raman spectroscopy.
It is not the first time that we have talked about Raman spectroscopy in Talking with Scientists. A few months ago, the Professor of Crystallography and Mineralogy at the University of Valladolid, Fernando Rull, explained to us how this technology helps to identify the minerals that exist on Mars, but, as usually happens in science, before developing instruments that help the space research, the technologies used in them have been developed for other purposes.
Fernando Rull and his team have been using this technology for years to study the mineral components and chemical processes that take place in the extensive geological and cultural heritage of the Earth. They have been pioneers in the development of portable Raman instruments, which have been used to try to determine, above all, what materials were used in the execution of works of art and the way in which they were applied.
When a very intense laser light is illuminated on any material sample, it returns light in two different ways. One reflects most of the incident light, without altering its frequency, or, in other words, without altering its color. Another part, much more tenuous, so tenuous that it has an intensity of the order of one billionth of the incident light, explains Fernando Rull during the interview, is altered light that contains frequencies (colors) different from the original illumination. This light, which has been modified by the atoms and molecules of the material, contains information about the substances and minerals in the material being observed.
If, for example, a rock is illuminated with the laser, the reflected light makes it possible to identify, by analyzing the composition of frequencies, whether the mineral that composes it is quartz, calcite or any other component it contains.
The great advantage of Raman spectroscopy is that it is a non-invasive technique, something of fundamental value when analyzing artistic heritage, which, due to its extraordinary value, cannot be modified by extracting samples that damage it. Another of the advantages of Raman spectroscopy is that it has been possible to develop portable instruments that allow it to be applied “in situ”. This allows researchers to go to the place, be it a cave or a building of historical value, for example, and carry out the study without having to come into direct contact with the material, simply by illuminating it with laser light and analyzing the response.
Fernando Rull and his team have been pioneers in the development of portable Raman instruments, which have been used in many and varied investigations, some of which we mention below.
Surely you have heard of Ótzi, also known as the “snow man”. He was a human being who died more than 5,000 years ago trapped in the ice of a glacier in a border region between Austria and Italy. He was discovered in 1991, mummified naturally and keeping his clothing, weapons and other utensils. His study offers unique information about the way of life in the Copper Age. The analysis of a sample of his skin with Raman technology has made it possible to compare it with that of current people and provide information about the mummification process he underwent.
With this technology, the frescoes found in the Church of Basconcillos del Tozo, Burgos, built in the 18th century, have been studied. XIV. When analyzing the mural paintings, Fernando Rull and his team discovered that the materials used by the artists were more expensive for the most important figures, such as Jesus Christ or the Virgin, while poorer materials were used for the less relevant characters.
Burgos Cathedral, an impressive architectural work declared a World Heritage Site by Unesco, contains countless jewels of art, including the five that make up the trasaltar, whose construction began on July 5, 1497. During the 20th century, the The deterioration of the stone has worsened, mineral efflorescences appearing and the original limestone fracturing. The study by means of Raman spectroscopy allowed to know the mechanism of degradation and crystallization of the salts coming from the burials that exist under the floor of the cathedral.
Many studies carried out with Raman spectroscopy have made it possible to provide information on the material used in medieval manuscripts such as “Los Beatos” del Beato de Liébana, made in the 17th century. VIII and later centuries. The analyzes of the miniatures revealed the use of pigments of mineral origin, very poisonous, with a high content of lead, mercury and arsenic, among others.
Fernando Rull also talks about the study of the material used in the Cuevas de Altamira; the analysis of a recently discovered painting that could have been painted by Gustav Klimt and the study using Raman spectroscopy of meteorites of Martian origin.
I invite you to listen to Fernando Rull, Professor of Crystallography and Mineralogy at the University of Valladolid, principal investigator of the ExoMars Raman instrument, head of the SuperCam calibration system at Mars 2020 and director of the group ERICA.