Blogs » "A MEETING OF THE MINDS" - Scientific Analysis of Medieval Limoges Enamels - Paper #1

"A MEETING OF THE MINDS" - Scientific Analysis of Medieval Limoges Enamels - Paper #1

  • A MEETING OF THE MINDS

                                                        Paper #1

     

     

    On October 8, 2010, I had the opportunity of attending an Experts’ Meeting on Enamel on Metal Conservation, which was held at The Frick Collection in New York City.

     

    It was the first meeting in the United States and the third meeting internationally, for the “International Council of Museums – Committee for Conservation”, better known as ICOM-CC Enamel Group of the Glass & Ceramics and Metals Working Groups.  The conference chair and enamel group coordinator was Agnés Gall Ortlik, Conservator in Private Practice and the local organizer was Julia Day, Editor and Assistant Conservator, The Frick Collection. 

     

    The day long event began with introductory remarks by Director

    Anne L. Poulet, The Frick Collection, and was followed by Gerhard Eggert, ICOM-CC Glass and Ceramic WG coordinator and Director, Objects Conservation Program, State Academy of Art and Design, Stuttgart.

     

    While the first paper presented was extremely scientific and filled with charts, graphs and chemical analyses of enamels, it provided an insight into the process used to identify chemical composition of enamels and the true identity of enameled treasures.  Here is a condensed version of the abstract.

     

    “SCIENTIFIC ANALYSIS  Authentication of Limoges Enamels by Noninvasive Techniques: The Larcade” - Angelo Agostino, Co-Authors:  Maurizio Aceto and Simonetta Castronovo.

     

    ABSTRACT: Medieval Limoges enameled jewelry are valuable objects to collect, and have led to a proliferation of copies in more recent times which are now widely dispersed in collections all over the world.

    It is sometimes difficult to identify originals through a stylistic approach; therefore a systematic methodology based on noninvasive

    analytical techniques was proposed to distinguish between medieval and modern enamels. This work focused on the medallions in the

    Larcade collection at the Musée du Louvre in Paris. Analysis was carried out using ultraviolet spectroscopy to obtain elemental composition. Multivariate analysis was also used to compare this data with that obtained on enamels from the Piedmont area.

     

    Until recently, X-ray fluorescence spectroscopy (XRF) was used as a preliminary technique for the study of glass, giving qualitative information only, which was often approximate and incomplete. This was because of the well-known problems concerning light matrix (glass) examination and the impossibility of detecting important elements such as sodium and magnesium. Recent developments in this technique and the effective noninvasive approach allowed quantitative results to be obtained on a number of Limoges jewelry pieces in the Piedmont area.

     

    Byzantine Limoges jewelry, developed between the 12th and 14th centuries, is characterized by Champlevé on metal and is usually mercury gilded. The historical debate on dating these artworks has been carried out by many scholars and was the inspiration to define a noninvasive analytical methodology using XRF to objectively identify the origin of production and to create a reliable quantitative database for future evaluations. The database now includes the analysis of over a hundred Limoges artworks including medallions and decorated caskets.

     

    Among them is the Scrinium Cardinalis from Piedmont, a traveling chest from the so-called treasure of Cardinal Guala Bicchieri (ca. 1150 – 1227) found in 1823 during the restoration of Sant’Andrea basilica in Vercelli. This object is now at the Museo Civico di Arte Antica, Turin, Italy, along with other similar items probably belonging to the same collection. It is decorated with 15 medallions, 10 cantonali, and 24 brackets, producing a rich palette of colored enamels.

     

    The Bicchieri collection originally consisted of at least 5 scrini or cophini de opere lemovicensi – wooden boxes richly decorated with Limoges enamels – as well as illuminated manuscripts and other precious objects. It now largely remains in the Piedmont region and is partly dispersed in several museums. It was therefore evident that the main role of Cardinal Bicchieri’s original collection was to define the technology used for the medieval enamel production in Limoges. Because many collections may contain imitations that are not always easy to recognize, a result of the l9th Century revival period in enamels, this study offers a solution for the identification of earlier and later enamel production due to chemical differences in the glass.

     

    From a chemical point of view, there are several indicators that can help differentiate the composition of enamels produced at different periods. Medieval enamels have a similar composition to that of the  Roman period – soda-lime glass opacified with tin or antimony oxides.

    However, l8th and 19th Century enamels are characterized by raw materials coming from Germany, essentially lead-potash glass opacified with arsenic oxides. Coloring agents determined by metal

    cations are almost the same except that in the Middle Ages impurities were present in some minerals, while in more recent times they were added intentionally. 

     

    As an example, the blue obtained with cobalt salts is present in very low concentrations in Medieval works and is usually related to

    nickel, bismuth, and arsenic impurities.  In Modern works the blue of cobalt salts is present in greater quantities and usually lacks impurities.

     

    Another striking example is the presence, as coloring agents, of elements only known from the l9th Century: chromium (green, blue);

    uranium (yellow, brown, green); or vanadium (green, blue, gray).

    These differences can be highlighted by chemical analysis as long as there is a database which contains enamels dated with certainty for

    reference.

     

    What was also discovered was that in Medieval enamels the flux is

    mainly based on sodium and for Modern enamels mainly on potash.

    The stabilizer is a calcium compound in Medieval enamels and a lead compound in Modern enamels.  And finally, the opacifier is a tin or antimony compound in Medieval enamels and arsenic in Modern enamels.

     

    AUTHORS

     

    Angelo Agostino, Senior Scientist, corresponding author

    Dipartimento di Chimica Generale e Chimica Organica,

    Universitá di Torino, corso M. d’Azeglio, 48, 10125, Torino;

    Centro di Eccellenza NIS (Nanostructured Interfaces and Surfaces),

    Universitá di Torino, tel. 011 6707585, fax 011 6707591,

     

    Mr. Agostino has a Ph.D. in Chemistry and is a Senior Scientist at

    The University of Turin, Italy, in the Department of General Chemistry.

    He has worked in international large-scale research facilities such as the European Synchrotron Radiation Facility, Grenoble, and spent over ten years as a scientist in an academic environment. He is an expert

    in X-ray Fluorescence and X-ray Powder Diffraction and has developed sophisticated research techniques involving materials analysis and surface technology. He has spent time at the Getty Conservation Institute as visiting scientist involved in research on XRF applications. 

     

    He has spent the last five years researching noninvasive techniques for authentication of artworks.

     

    Maurizio Aceto, Senior Scientist

    Dipartimento di Scienze dell”Ambiente e della Vita, Universitá del Piemonte Orientale, via Teresa Michel, 11, 15100, Alessandria;

    Centro Interdisciplinare per lo Studio e la Conservazione dei Beni Culturali (CenISCo), Universitá del Piemonte Orientale, via Manzoni, 8,

    13100, Vercelli

     

    Simonetta Castronovo, Curator

    Museo Civico di Arte Antica – Palozzo Madama, p.zza Castello, 10123, Torino