Blogs » "A MEETING OF THE MINDS " Experts Meeting on Enamel on Metal Conservation- Paper #2

"A MEETING OF THE MINDS " Experts Meeting on Enamel on Metal Conservation- Paper #2






  • Let's get academic....

     

     

                                A Meeting of the Minds – Paper #2

                                                        At the Frick

    To refresh everyone’s mind, 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 U.S. and the third meeting 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.

     

    For those members just joining, Part 1 of this series is printed below.

     

    The second paper presented in this day- long event was on the use of a Raman spectrometer to analyze enamels on metal from the l5th to the l9th century. Here is a condensed version of the abstract and an in- depth scientific discussion of glass structure and the characteristics of the Raman spectrum.

     

    “ON –SITE ANALYSIS OF ENAMELS FROM THE FIFTEENTH TO THE NINETEENTH CENTURY: AN ATTEMPT TOWARDS DIFFERENTIATION BETWEEN GENUINE ARTIFACTS AND COPIES” - Burcu Kirmizi and Philippe Colomban

     

    ABSTRACT: A selection of 22 Chinese cloisonné and 13 Limoges painted enamels from the 15th to the 19th century were studied on-site in the storage rooms of the Musée des Arts Décoratifs in Paris using a portable Raman Spectrometer .

     

    The aim of this study was to identify the composition of amorphous (glass matrix) and crystalline (pigments, signatures of processing) phases by means of Raman spectrometry which might also serve as a tool for discriminating between different production periods as demonstrated . 

     

    INTRODUCTION: The earliest examples of enameling on metal appear to be the rings from the Mycenaean tombs dating to the second millennium BC in Cyprus. Different types of enameling techniques such as champlevé and cloisonné were being used in the Celtic productions dating to as early as the 3rd century BC. During the 11th century, the Byzantines developed the art of cloisonné enameling to a very high level and this technique had reached China by the 14th century. By the Ming period (1368-1644), the cloisonné technique had been well established with the use of a palette of six to eight colors and further enlarged by mixing colors and the addition of opacifiers.

     

    At the time of the development of cloisonné enameled objects in China, this technique had disappeared in Europe with the emergence of a new technique called “painted enameling” toward the end of the 15th century in Limoges, France. Using this technique, Limoges artisans created sophisticated paintings as multi-layered enamels on copper supports. During the 17th century the quality of the enamels had started to decrease until a remarkable revival in the 19th century. The workshops produced modern Limoges School pieces and also replicas of Renaissance enamels, which were often being sold as originals.

     

    Raman Spectrometry has been used extensively in the last decades for a wide range of archaeological and historical artifacts and remains. One of the most significant reasons for the increasing value of Raman spectrometry is that it is a non-destructive technique and does not require sample preparation. In the case of archaeological and historical objects, where moving them from the museum is not always possible due to their size and preciousness, the portable Raman spectrometers make it possible to analyze them in situ.

     

    Now let’s get down to pure science talk The formulas presented in this article are not in their correct  scientific format as the site does not print them correctly. If any member would like a copy of these formulas, I will be glad to email you my copy in Word.)

    Vitreous materials, such as glazes and enamels, are all silicate-based and have a network of SiO4 tetrahedra. The  SiO4 tetrahedron is a covalent entity and has a well-defined vibrational signature which gives a characteristic Raman spectrum. The connectivity of the tetrahedra is based on the bridging oxygen atoms located at the corners of the molecule. These SiO4 tetrahedral connections are modified by the incorporation of alkali, or alkaline earth metal ions, or both, in the glass structure, resulting in changes in the properties of the glass, such as melting temperature, viscosity, color, etc. The presence of these modifiers in the glass structure leads to the formation of different types of tetrahedral connections which are expressed by the Qn notation where “n” is the number of bridging oxygen atoms varying between 0 to 4. Consequently, the depolymerization of the SiO4 network by these modifying species is reflected in the intensity, line width and spectral position of the Raman bands. 

     

    The characteristic Raman spectrum of glass consists of two broad peaks as bending and stretching modes at around 500 and 1000 cm-1 respectively. Five spectral components as  Q0 Q1 Q2 Q3 and Q4     are postulated from the Si-O stretching region according to the Qn  notation.

     

    Different parameters that are extracted from the Raman signature of the enamels such as the wave number maxima of the Si-O stretching and bending components and their intensity ratio (Ip, the polymerization index = A500 /A1000 ) are used for the identification of different glass compositions. The polymerization index values directly correlate to the glass composition and firing temperature. 

     

    EXPERIMENTAL: The measurements were performed on-site in the storage rooms of the Musée des Arts Décoratifs in Paris using a Raman spectrometer (HE532 Horiba Jobin-Yvon) with an Nd:YAG laser light source providing a 100 mW, 532 nm line  in conjunction with a Nikon 50x ultra long working distance objective (Kirmizi et al. 2009, 2010. The raw spectra collected were then subjected to the process of baseline subtraction by using LabSpec (Dilor) software in order to see the Raman signature of the molecule Si-O better and eliminate the contribution of the Boson peak (Colomban 2008). The deconvolution of the different spectral components, as Qn for stretching and Qn  for bending regions is done by Origin software as described in Ricciardi et al. (2009).

     

    RESULTS and DISCUSSION

     

    The characterization of Chinese cloisonné and Limoges painted enamels as a function of their composition and color was carried out by means of Raman spectrometry for the first time in this study.

     

    Additionally, an attempt was made for the differentiation betweengenuine artifacts and 19th century restorations or fakes for Limoges examples. Most of the Chinese cloisonné enamels fall into the lead-potash-lime group (15th-16th and 18th-19th centuries) while the soda-lime group is the most common for Limoges enamels. 

     

    One of the 16th century Chinese cloisonné objects was found to be in the soda-rich group, suggesting the production of a different workshop or its assignment to a later date. The polymerization index (Ip) for the soda-lime group varies between 2-3 according to the calcium content and a medium firing temperature, whereas the Ip values for the lead-potash –lime groups are lower, between 1-2 due to the lead content.

     

    In some cases, the identification of crystalline and amorphous phases by non-destructive Raman analysis can be used as post quem date markers, such as the confirmation of lead arsenate as an opacifier in the 19th century Limoges works. On the other hand, fluorite was only found in Chinese cloisonné productions. Cassiterite was also identified as an opacifier in Limoges samples dating from the 16th – 19th century.

    Hematite red and Naples yellow pigment variations were also detected which give characteristic Raman signatures in both types of enamels. Among the different pigments detected by this method, cobalt silicate was used for blue and green enamels and chromate-based compounds  for pink enamels. The very strong light absorption of some red enamels seems consistent with the use of copper nanoparticles dispersed the glassy phase. If one of these markers is not sufficient to reach and assignment for dating the combination of many of them is generally a proof of embellishments, undocumented restoration procedures, or fakes.

     

    A good knowledge of Raman spectroscopy and of the ancient and modern technologies is mandatory to have reliable conclusions. It should be noted that for old artifacts of the same origin or that have been in the same place for long periods, the intensity of the Raman signal recorded with standard conditions is an additional piece of information to compare the age of different glasses.

     

    And now you know a little bit more about the great scientific minds at work in the labs of our grand museums.

     

    AUTHORS

     

    BURCU KIRMIZI, Scientist, corresponding author 

    Middle East Technical University Graduate School of Natural and Applied Sciences Archaeometry Program, Ankara, Turkey. Ms. Kirmizi holds an M.S. degree in Archaeometry from the Middle East Technical University in Ankara, Turkey, where she was a research assistant from 2006-2008. In 2008-2009 she worked in the Laboratoire de Dynamique, Interactions et Réactivité (LADIR) at the Université Pierre-et-Marie Curie in Paris as a fellow of the Scientific and Technological Research Council of Turkey. At LADIR, she studied the application of Raman spectrometry for the characterization of vitreous materials. Currently, she is a Ph.D. candidate in Archaeometry at METU and she is working on the characterization of Byzantine and Early Ottoman ceramics from the western Anatolia region of Turkey.

     

    PHILIPPE COLOMBAN, Scientist

    Laboratoire de Dynamique Interactions et Réactivité (LADIR) UMR 7075, CNRS, UPMC, 2 rue Henri Dunant, 94320, Thiais, France