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“Moras” and the Sitting Goddess – Mineralogists With Mobile Raman Probe in Museum

The Sitting Goddess.. Picture: Karla Fritze

The Sitting Goddess.. Picture: Karla Fritze

The sitting goddess from Mesopotamia has already some four thousand years under her belt. Secured by an alarm system and guarded by museum attendants in uniform, the statue resides behind glass in the Berlin Pergamon Museum. This is not really the classical working environment for a mineralogist. Even so Martin Ziemann, senior scientist at Institute of Earth and Environmental Science of the University of Potsdam did not come to the museum as a visitor but as a researcher.

 Ziemann wants to examine whether the white alabaster figurine, which is the size of a fist, was once colourfully painted although hints for it cannot be seen with the naked eye. The research scientist works with Raman spectroscopy, a method widely used in mineralogy and archaeometry, to detect even micrometer-sized pigment particles or binders. Since he cannot take such fragile, valuable objects like the little goddess to the laboratory of the institute, not to mention mural paintings or permanently installed large sculptures, he has designed mobile equipment that can actually be used at any location.

“Mobile”, however, is a relative term here. The complete equipment is safely packed in aluminium boxes and cases and weighs about 130 kg. It took the lorry two hours to bring the equipment from Potsdam-Golm to the Museum Island in the centre of Berlin. There it will be used for the first time outside the institute. The instruments are stacked now in a windowless adjacent room of the Vorderasiatisches Museum. The core piece of the installation – the spectrometer – is placed in a flat box not much bigger than a packing case. Ziemann and his student assistant Fritz Falkenau fix the probe head at the tripod so that it extends over the worktable and connect it to the spectrometer with metal-shielded fibre-optic cables. Then they focus the laser beam on the monitor screen.

Raman spectroscopy is one of the few methods to examine ancient finds in a non-destructive and contactless way. A relatively weak laser beam is focussed to approximately ten thousandths of a millimetre and directed at a spot where the experts presume pigment traces. Even if there are only tiny residues, they emit a light response with a pattern characteristic for the respective substance. The jagged line on the computer screen, Raman spectrum, is like a fingerprint. Similar to detectives comparing the culprit with their criminal records, scientists can compare their measured spectra with relevant databases and identify the substance. Artist in ancient Mesopotamia certainly knew black, white, and red pigments from clay mineral, limes, and iron compounds. Other colours were used at a later point. The spectrometer can also identify other products that result from the aging process of these substances and environmental conditions.

First the instrument has to be calibrated with a silicon specimen. Then the scientists can start. The entrances to the room with the sitting goddess are barred and watched by guards. Ralf-Bernhard Wartke, Vice Director of the Vorderasiatisches Museum, opens the glass cabinet with white gloves and takes out the figurine. He carries it carefully to the adjacent room which has been changed into a temporary laboratory. There he beds it into a grey plastic container, on something that looks like sand. “Granulated walnut shells”, Ziemann explains. The figurine rests firmly on this substance and can be still moved quite easily. Sand could possibly wear the surface.

The art historian says goodbye until the evening. The actual work for the mineralogists begins. The goddess gets the sample number VA 4854. Using an optical microscope, Ziemann looks for a slightly worn out spot on its smooth surface. “Such uneven spots enable us to look into the material, so to speak.” He has chosen this spot according to a catalogue of the archaeologist Astrid Nunn of the University of Würzburg that she compiled during her research project “Polychromy of Mesopotamian Stone Statues”. It contains a number of objects that proved to be “suspicious”. Nunn’s research team took photos of them and marked possible places where pigment residues might be found.

How did this quite unusual interdisciplinary collaboration come about? Martin Ziemann smiles, “I always wanted to work in the neighbourhood of the university, too, preferably on something that also interests the people here.” This is how he met restorers of medieval mural paintings in Brandenburg, in Ziesar and Herzberg. They wanted to know how environmental conditions can influence the condition of pigments. For analyses in the lab they had to take tiny samples, however. The experience gained from these analyses Ziemann wanted to use on Buddhist mural paintings from Central Asia in the Berlin Museum of Asian Art. Then he came up with the idea of a spectrometer that could be taken to the objects. Out of different components that are usually meant for other purposes, a base with wheels, a camera tripod, and an extendable beam, he had a kind of crane built in two different sizes. The detector head is tightened to the beam and can be brought to mural paintings or big sculptures in any position, even from below to frescoes on the ceiling. The smaller model can get to a height of about three meters and the bigger one even to six meters with a boom length of eight meters.

Would you like a demonstration? It takes Ziemann and his student assistant about ten minutes to assemble the little crane. It just fits through the door of the adjoining room. The two men roll it carefully, anxious not to knock against anything, to a section of a firmly built-in façade section with a coloured lining. In this section, a number of human figures made of clay are embedded in a relief-like way. It comes from the Mesopotamian metropolis Uruk, from the “Inanna Temple of Kara-Indasch”, as the plate next to it tells us. Construction year: about 1413 BC. Just a few simple adjustments are necessary to place the instrument exactly at the desired position.

But now it is time for the goddess. Despite sophisticated technology the examination is not easy. After all, Ziemann finds the “suspicious” spot marked by the archaeologists quite quickly. “We do not always manage it.” The actual measurement takes several accumulations until you can filter out the “real signals” from the “background noise”. But even these remain strangely unfocussed. Ziemann thinks that the figurine got a very thin, protective coating of resin or wax someday. At least they do not discover any indications of former painting. The assumption of the research team from Würzburg has not been confirmed in this case. Ziemann will not run out of objects for his examinations. There is a great demand. At least the comprehensive construction works on Museum Island had to be interrupted that day, the scientist says happily. “Our system is mobile but during the measurements it reacts extremely sensitive to vibrations.”

Let us finish for today. When bringing back the sitting goddess Ziemann points to the “statuette of a worshipper” made of limestone in the same glass case. This will be his next object. Enthusiastically, he points to the folded hands and the delicate fingers. Meanwhile, the mineralogist enjoys being in the museum. His admiration for the composition of the objects of his investigations has tremendously increased as a result of his dealing with these pieces of art and the detailed knowledge that he has acquired. “Also the students who help us are impressed by their beauty.”

The Scientist

Dr. Martin Ziemann is senior scientist of the research group mineralogy at the Institute of Earth and Environmental Science. Mineralogists do not only deal with questions of geoscience and material science. Their research includes other fields like biology, medicine, astronomy - and also archaeology.

Contact

Universität Potsdam
Institut für Erd- und Umweltwissenschaften
Karl-Liebknecht-Str. 24–25, 14476 Potsdam
ziemann@geo.uni-potsdam.nomorespam.de

Text: Sabine Sütterlin, Online-Editing: Agnes Bressa, Translation: Susanne Voigt