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Art history

Glittering ceramics

In a laboratory located beneath the Musée du Louvre, scientists and conservators are using an exceptional array of equipment to reveal the manufacturing secrets of luster-decorated ceramics. An expert in materials physics, Marc Aucouturier sheds light on some of these methods, the optical properties of which have crossed centuries without losing their brilliance.

aucouturier

© A. Chéziere/CNRS Photothèque

Marc Aucouturier with Aglaé. Since it was first set up, other accelerators dedicated to heritage studies have appeared throughout the world.


This is a professional’s nightmare!” exclaims Dominique Bagault, a photographer at the LC2RMF.1 Golden reflections, colors that change under diffused or reflected light...  Luster-decorated ceramics have survived the centuries without losing their magnificence. The oldest pieces date back to the 9th century in Mesopotamia. The technique then spread throughout the Arab world, then to Europe, brought by the Moors to Spain, and has been used practically without interruption until today. However, the manufacture of these exceptional objects, from both historical and technical points of view, is still poorly understood.

luster decoration

© C2RMF, D.Bagault

A diffused (left) or reflected (right) light beam produces a very different appearance of the same luster decoration.



“Luster is obtained by applying a vitreous layer2 to the ceramic object, followed by a clay paste that contains metallic salts, mainly silver and copper,” explains Marc Aucouturier, who has put his experience in materials physics at the service of cultural heritage. “Firing in an atmosphere generated by the combustion of broom3 then transforms these salts into metal and gives the decoration its special appearance. Old recipes still exist, but the authors were always rather vague about key manufacturing steps: firing times and temperatures, or the moment at which the atmosphere changes.” Already at the time, industrial secrets were well protected.
To uncover these mysteries, the Louvre is allotting major resources: X-ray diffraction techniques to analyze the structure of lusters, two electronic microscopes to determine their composition and topography... and above all Aglaé, the Louvre’s elemental analysis particle accelerator4–entirely dedicated to cultural heritage. These major technological resources provide far more than the overall composition of the lusters, they are able to analyze their detailed structure, layer by layer. “This is the only instrument that can produce good quantitative data without damaging the material,” adds Aucouturier. The principle? Aglaé generates an ion beam of particles under vacuum which is then transfered to the air where it hits the ceramic. When a particle reaches the material, it is deflected with a loss of energy. By measuring this loss, scientists can calculate not only the thickness of the layers, but also their atomic composition.

assiettes

© C2RMF, D.Bagault

Ultraviolet rays that illuminate the plate (right) detect any fracture zones so that restored parts can be distinguished from the original ones.



Initial results, obtained in collaboration with Anne Bouquillon, a ceramics expert, Délhia Chabanne, a CNRS post-doctoral fellow and Évelyne Darque-Ceretti, from the CEMEF in Sophia-Antipolis5 have revealed the different manufacturing processes that correspond to each period. It appears that Egyptian potters in the 10th and 11th centuries abandoned the luster coloration technique developed under the Abbasid dynasty (750-1258) in Baghdad (Iraq). Instead, they colored the vitreous layers using various metal salts: purple was obtained using manganese oxide, blue using cobalt, and green using copper oxides. During subsequent centuries, the technique spread to Iran and much of the variety was lost. As for the Spanish Renaissance potters, they used all the earlier techniques, as dictated by fashion and commercial demands.

vase

© C2RMF, D.Bagault

An 18th century vase from Iran. Its appearance has changed according to fashion, but the technique has crossed the centuries.



Yet there’s much to learn. Scientists are hoping that they will soon identify the technological leaps linking one type of production to another. From all the Islamic lusters available to them, they have already obtained data that will require more than a year to analyze. And this influx of data is continuing: “Since we work for the Ministry of Culture, we have access to all French national museums, and can borrow pieces quite easily. In most cases, actually, museum conservators contact us directly,” adds Aucouturier. This is the case for the Musée de Cluny in Paris, which holds an impressive collection that covers the 14th to the 18th centuries. The Gubbio Museum in Italy has also requested assistance from the team in characterizing the masterpieces produced by Mastro Giorgio, one of the most eminent potters of the 16th century. The differences in composition should provide historians with new clues to the development of fashions, trade, and ceramic firing processes.
Isabelle Bauthian


Notes :

1. Laboratoire du Centre de recherche et de restauration des musées de France (CNRS / Ministère de la Culture et de la Communication).
2. At this stage, the ceramics are referred to as being glazed.
3. Mediterranean shrubs of the genus Cytisus. Today, however, burning sugar is the preferred method.
4. Accélérateur Grand Louvre d'Analyse Elémentaire.
5. Centre de mise en forme des matériaux (CNRS / École des mines de Paris).


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