Chemical Analyses of Renaissance Enamelled Jewellery

Transcription

1 47 Chemical Analyses of Renaissance Enamelled Jewellery Mark T. Wypyski Department of Scientific Research The Metropolitan Museum of Art 1000 Fifth Avenue New York, NY U.S.A Keywords: jewellery, Renaissance enamels, nineteenth century enamels, X-ray microanalysis, energy dispersive X-ray spectrometry, wavelength dispersive X-ray spectrometry, authenticity Abstract This study presents the analyses of enamel compositions from a group of European enamelled gold jewellery dating from the sixteenth and seventeenth centuries, as well as from a group of enamelled gold objects from the late nineteenth century. Quantitative analyses of the enamel compositions were done using X-ray microanalysis in the scanning electron microscope (SEM- EDS/WDS) to characterise the overall compositions, as well as to identify the opacifiers, colourants and associated elements. These analyses can provide objective evidence to help distinguish between authentic Renaissance period pieces and later pieces done in Renaissance style, confirming or sometimes refuting evaluations based on visual evidence alone. Introduction Jewellery in the Renaissance was valued not only as a visible display of wealth, worldliness and social status, but was also highly valued and appreciated in itself as fine art (Wardropper 2000). This jewellery, usually combining precious stones, pearls and gold, was also commonly decorated with both translucent and opaque enamels (Figure 1). The modern fashion of collecting Renaissance jewellery for its artistic and historical interest apparently began as early as the eighteenth century, although it was during the nineteenth century that large jewellery collections were assembled (Truman 2000), including that of J. P. Morgan, much of which today is in The Metropolitan Museum of Art in New York. Recent stylistic re-evaluations as well as documentary evidence, however, have raised questions as to the authenticity of some of these pieces, as well as many in other collections. The limited availability of authentic Renaissance pieces, coupled with a growing demand by collectors in the nineteenth century, not surprisingly, encouraged the sale of imitation works. One of the largest sources of Renaissance antiquities in the late nineteenth century was, apparently, the Parisian dealer Frederick Spitzer, who employed artists such as the silversmith Reinhold Vasters to supply him with Renaissance style jewellery as well as other types of objects (Truman 1979, Hockenbroch 1986). Surviving documentary evidence and designs for many of the pieces done by Vasters have served to alert art historians of possible fakes in their collections.

2 48 Wypyski Figure 1. Pendant, Hunter with hounds, gold, enamel, pearls, diamonds and rubies, German(?), ca (The Metropolitan Museum of Art, New York; Bequest of Michael Friedsam, 1931, ) However, the presence of a design by Vasters of a particular type of object is not a guarantee that all objects of this type were made in his workshop, as drawings were made of existing pieces, as well as designs for new works (Truman 2000). The analyses presented in this paper summarize the results of work undertaken on a large group of objects to characterize the compositions of well-dated objects in order to provide objective evidence to help sort out the problems of distinguishing authentic Renaissance jewellery from Renaissance style pieces made in the late nineteenth century. Analytical Details Samples of the enamels were removed from the metal substrates with the use of a steel scalpel (samples were only taken from enamels with areas of previous damage or loss, which meant that not all of the colours could be sampled from some objects). The samples were prepared for analysis by embedding them in epoxy, then grinding and polishing the surfaces, and giving them a conductive carbon coating. The compositions were analysed using energy dispersive X- ray spectrometry (EDS), which was used to determine the major and minor elements, while wavelength dispersive X-ray spectrometry (WDS) was used to determine trace amounts below one percent by weight. Weight percentage concentrations of the elements detected were calculated using wellcharacterized reference glasses and glass standards (Verità et al. 1994). The relative variation in the calculated percentages for the major and minor element oxides analysed with EDS has been estimated be less than 2 percent for silicon, under 5 percent for sodium, potassium and calcium, and about 10 percent for magnesium, aluminium, and the metals such as copper and iron. The relative variation for most elements with WDS is also estimated at about 10 percent. Minimum detection limits (MDL) for most elements with WDS under these operating conditions are estimated to be about 0.01 percent, although the MDL of some elements, such as phosphorus, strontium and antimony was found to be somewhat higher, at 0.02 percent, while the MDL for lead and bismuth is estimated at 0.05 percent.

3 Chemical Analyses of Renaissance Enamelled Jewellery 49 Table 1 gives the results of the EDS and WDS analyses, in oxide weight percentages, of the enamel compositions from twelve enamelled gold objects dated to the sixteenth and seventeenth centuries from the collection of The Metropolitan Museum of Art, selected to be representative of the typical compositions found, as well as some examples of exceptions to the more general compositional types. Table 2 provides the EDS/WDS analyses of a selected group of representative late nineteenth century enamelled gold objects. These are listed by the Metropolitan Museum accession number, with a brief description and the enamel colours analysed (the enamels listed in the table are translucent unless otherwise noted). Elements listed in the tables as nd were not detected by WDS analysis. Overall Compositions The most commonly observed enamel colours from the Renaissance pieces were translucent green, blue and red, and opaque white. Somewhat less commonly seen are translucent black, actually a very dark purple or blue, opaque blue and green, and occasional uses of translucent yellow and purple and opaque turquoise and yellow. The sixteenth and seventeenth century translucent enamels can generally be described as having soda-silicate glass compositions, i.e. sodium oxide and silicon oxide based compositions, generally containing about 10 to 16 percent sodium oxide, with relatively large amounts of potassium oxide, typically about 2 to 3 percent, but only relatively small amounts of magnesium and aluminium oxides, generally less than 1 percent each. Calcium oxide is also typically relatively low, generally under 3 percent, much less than usually found in soda-based glasses. Little if any lead is typically found, usually only traces less than 0.5 percent, although small amounts of lead, about 1 to 5 percent, were found in many of the reds. Minor amounts or traces of other elements such as phosphorus, sulfur, chlorine and barium and strontium are also found in these enamels. The translucent red enamels differ from most of the other colours, generally containing less sodium and more potassium than the other enamels, and may usually be described as mixedalkali compositions (having large amounts of both sodium and potassium), rather than sodabased, many with more potassium than sodium (Table 1, no. 3). The reds also contain much more calcium and magnesium than generally found in the other translucent enamels, as well as somewhat more aluminium. Unlike the translucent enamels, the opaque enamels all contain significant amounts of lead oxide, typically about 15 to 20 percent in the white and blue enamels. Opaque green enamels vary considerably, and have been found to range from less than 5 percent to greater than 40 percent, while the few examples of opaque yellow enamels analysed so far all contain about 40 to 50 percent. The presence of lead in these enamels appears to be associated with the opacifers. Aside from the lead and added opacifying compounds, these enamels also have low magnesium and calcium soda-based overall compositions similar to the majority of the translucent enamels. While most of the Renaissance period enamels analysed fit into these generalized descriptions, there are some exceptions. For example, the blue-green and black enamels from MMA (Table 1, no. 3) both contain much more magnesium and calcium than the average, although the translucent red, opaque white and opaque blue enamels from this pendant appear to be typical of the period. A few examples of translucent enamels, including reds and other colours, have also been found, which appear to be typical in most ways, but which contain 5 percent or more lead oxide.

4 50 Wypyski Table 1. SEM-EDS/WDS analyses of sixteenth and seventeenth century enamel compositions (weight %)

5 Chemical Analyses of Renaissance Enamelled Jewellery 51 All colours are translucent unless otherwise indicated.. nd = not detected

6 52 Wypyski In contrast to enamels from the Renaissance period, enamels from the nineteenth century (Table 2) have been observed to have lead-potash-silicate (lead oxide, potassium oxide and silicon oxide) or lead-alkali-silicate compositions (i.e. containing relatively large amounts of both sodium and potassium oxides), generally with much less sodium than in the earlier enamels (Figure 2). Lead oxide values in these enamels generally range between about 20 to 30 percent, although some, particularly the opaque whites, contain as much as about 50 percent. These enamels also typically contain extremely low levels of other common glass-forming elements, such as calcium, magnesium and aluminium typically less than 2 percent calcium oxide, and usually less than 0.1 percent magnesium oxide, and 0.3 percent aluminium oxide. Arsenic is also present, at least in small amounts, in nearly all of these enamels, while in the earlier enamels arsenic is usually only found associated with cobalt containing blues and blacks, although small traces are sometimes also found in other colours. As with the Renaissance period enamels, exceptions to the general compositional types are also occasionally seen in the nineteenth century enamels. While most of the enamels have highlead compositions, some have been found which contain less than 20 percent lead oxide, and a few even have less than 10 percent (Table 2, no. 4). This low-lead green enamel was also found to have relatively high calcium as well, while the other enamels from this object have more typical nineteenth century compositions. Figure 2. Plot of lead oxide and sodium oxide values for Renaissance and late nineteenth century translucent enamels Colourants and Opacifiers Renaissance translucent green enamels were coloured by the addition of relatively large amounts of both copper oxide and iron oxide. The iron oxide adds a yellow colour which serves to shift the bluish colour produced by copper oxide alone towards a truer green colour. Blue-green enamel, not as commonly used, was produced by the addition of copper oxide alone, with only a very small, possibly unintentional, amount of iron (Table 1, no. 3). The amounts of copper oxide in the green enamels generally ranged from about 5 to 9 percent, while iron oxide is typically

7 Chemical Analyses of Renaissance Enamelled Jewellery 53 present from about 4 to 7 percent by weight. A rare example of Renaissance translucent yellow enamel (Table 1, no. 6) contains more than 12 percent iron oxide, with only a trace of copper. The very large amounts of colourants used in translucent enamels, compared to the amounts usually seen in translucent coloured glasses, were apparently required to achieve the desired colour in thin translucent layers over bright metal, and have been observed in late medieval enamels as well (Wypyski and Richter 1997). Traces of tin, arsenic, zinc and nickel were also commonly detected in these enamels, possibly unintentional additions from the copper source used. All of the Renaissance blue enamels were coloured with cobalt oxide, usually present from about 0.5 to 1 percent by weight, although some opaque blues contain considerably more than 1 percent. In comparison, cobalt coloured glass usually contains no more than about 0.2 percent. The cobalt-containing enamels have elevated levels of iron as well, and are also usually associated with nickel, arsenic and bismuth, probably unintentional additions from the cobalt source used. Several blue enamels, however, have been found which do not contain any detectable bismuth, and only small traces of arsenic, although significant amounts of iron and nickel are present. This might indicate that an unusual cobalt source was used in the production of these blue enamels, or that the cobalt ore was processed differently so as to remove most of the arsenic and bismuth. The blue enamels also generally contain additional copper oxide as well, although most of the colour is due to the cobalt. A few examples of opaque light blue or turquoise enamels have been found which are coloured mainly by copper oxide, although traces of cobalt are also sometimes present (Table 1, nos. 5 and 10). So-called black enamel is actually a very dark purple or purple-blue in colour, which appears black in reflected light. These nearly always contain very large amounts of manganese oxide, which produces the purple colour, typically about 4 to more than 10 percent by weight. The black enamels also contain relatively large amounts of cobalt, along with its associated elements, and sometimes contain intentional additions of copper as well. Only a single example was found of low-manganese black enamel, a very dark blue rather than purple colour, containing nearly 4 percent cobalt oxide (Table 2, no. 3). Purple translucent enamel, relatively rarely seen on Renaissance objects, although relatively commonly used during the fourteenth and fifteenth centuries, is also coloured by large amounts of manganese oxide, but without any significant amount of cobalt. The translucent red enamels are also coloured with copper oxide, as are the greens, but with much smaller amounts of copper, about 1 percent or less, present in a reduced oxide form. Small amounts of tin and lead are also commonly found in the red enamels, probably added to help in the reduction of the copper oxide. Small traces of zinc were also noted in some of the reds, possibly from the source of copper used. Opaque white enamels from the sixteenth and seventeenth centuries contain white crystalline tin oxide as an opacifier in the glassy matrix, usually present at about 20 to 30 percent by weight, although some have been found containing close to 40 percent. Opaque blue or turquoise enamels also contain tin oxide, but generally contain less than 20 percent. The tin oxide in these enamels is associated with large amounts of lead oxide. In the white enamels the average ratio of tin oxide to lead oxide is about 1.5 to 1, although some have a ratio greater than 2 to 1. Interestingly, opaque blue enamels have an average ratio of only about 1 to 1, with less tin but about the same levels of lead, while the opaque turquoise contains less tin and lead, but in about the same ratio as in the whites.

8 54 Wypyski Table 2. SEM-EDS/WDS analyses of late nineteenth century enamel compositions (weight %) Renaissance period opaque green and the less common opaque yellow enamels contain yellow crystals consisting mainly of antimony, tin and lead. High magnification analyses of individual crystals revealed the opacifier to be a mixture of lead antimonate and lead stannate (lead-tin yellow). Several of the opaque greens were also found to contain white crystalline tin oxide in addition to the yellow lead antimonate/stannate. Zinc was also noted in most of these enamels as well, and appears to be associated with the source of the yellow opacifier. The green enamels are also coloured with copper oxide, which adds a bluish colour to the yellow to produce a green, and generally contain less lead than the yellows. Along with the overall compositions, differences from the Renaissance enamels can also be seen in the opacifiers, colourants and associated elements found in the nineteenth century enamels. Instead of tin oxide, the main opacifier in the nineteenth century was a lead arsenate compound, observed in the SEM to be present in the form of small round crystallites less than

9 Chemical Analyses of Renaissance Enamelled Jewellery 55 * = Also contains silver at approximately 300 ppm. * *= Also contains approximately three percent fluorine. *** = Also contains approximately fifteen percent uranium oxide. All colours are translucent unless otherwise indicated. nd = not detected 0.5 microns in diameter. Arsenic oxide is usually present from about 3 to 6 percent, with a higher lead content than in most of the translucent enamels. Several examples, though, have been found which contain significant amount of tin oxide in addition to the arsenic, and were observed in the SEM to contain a mixture of crystalline tin oxide and lead arsenate (Figure 3). In contrast to the Renaissance enamels, the nineteenth century translucent green enamels contain only traces of iron, although they also contain large amounts of copper. The most obvious difference in these enamels compared to the earlier ones, though, is the presence of chromium oxide as a colourant, usually present at about 0.5 to 1 percent. Opaque green enamels also contain chromium and copper, but are opacified with lead arsenate (Table 2, no. 1), although one example of an opaque green has been found which contains a fluoride opacifier, probably sodium fluoride, rather than lead arsenate (Table 2, no. 6). Very few examples of opaque yellow from the nineteenth century have been analysed, but, unlike the opaque greens, these appear

10 56 Wypyski Figure 3. SEM micrograph (backscatter detector) showing mixture of relatively large tin oxide crystals and sub-micron lead arsenate crystallites in a late nineteenth century white opaque enamel (Table 2, no. 4). Original magnification 5,000X to be similar overall to the Renaissance yellow and green opaques, and also contain a yellow opacifier consisting of lead, tin and antimony. Another difference found in the nineteenth century enamels is the presence of antimony in the red enamels, apparently replacing copper as the colourant. Antimony oxide is present in these reds from about 2 to 5 percent, usually, though not always, associated with a small amount of tin. Only small traces of copper oxide, if any, are detected in these enamels, generally not enough to have any effect on the colouration. The nineteenth century cobalt-coloured enamels differ from the earlier blue enamels in that none of these were found to contain bismuth. Much smaller amounts of iron, nickel and arsenic were also generally found, except for the opaque blues which contain lead arsenate as the opacifier. As noted earlier, small amounts of arsenic are found in the majority of the nineteenth century enamels, regardless of the colour. While the ratio of cobalt to nickel in the earlier enamels averages about 3 to 1, the ratio in the later enamels averages about 20 to 1. Other less common translucent colours include purple and yellow. While purple enamels are also coloured with manganese oxide as in the earlier enamels, less manganese appears to be generally used. Translucent yellow enamels, however, use different colourants than found in earlier enamels. Several examples have been found which contain large amounts of uranium oxide as the colourant (Table 2, no. 7), while one example has been found which apparently used silver stain to replace the earlier iron oxide colourant (Table 2 no. 2). Discussion European enamels from the sixteenth and seventeenth centuries appear to have been produced much in the same way as late medieval enamels, although the Renaissance enamels use mainly soda-based compositions in contrast to the earlier mixed-alkali ones (Wypyski and Richter 1997). The rediscovery of the yellow opacifier lead antimonate, used in ancient glass through the late Roman period, appears to have been a Venetian innovation of the late fifteenth century, replacing, for the most part, lead-tin yellow in glass and ceramics (Ombelli et al. 2005). The development of cristallo glass was another innovation of fifteenth century Venetian glassmakers (Verità 1985).

11 Chemical Analyses of Renaissance Enamelled Jewellery 57 Cristallo glass was noted for its clarity and homogeneity compared to the more common type of glass and appears to have been made with the use of purified plant ash. Compositional analyses of Venetian glass show reduced amounts of magnesia and lime, as well as increased levels of soda and potash in cristallo in comparison to that found in the more common type of vessel glass, vitrum blanchum. This is consistent with a purification process of the plant-ash which would preferentially remove the less soluble compounds of magnesium and calcium, and concentrate the more soluble sodium and potassium (Verità 1985). In the majority of the sixteenth and seventeenth century translucent enamels, though, the relative proportions of soda and potassium to magnesium and calcium are even higher than that generally found in Venetian cristallo glass (see Figure 4). This probably is due to the use of glass in the production of these enamels which contained an even higher proportion of purified plant ash than that generally used in cristallo, and/or the addition of a high potash source such as ash from ferns or tartar, a deposit that can form from red wine (Verità 1985). Similar ratios of the main glass forming elements have been noted in some sixteenth century Venetian enamels on glass (Wypyski forthcoming). Conclusions Figure 4. Ratios of sodium and calcium oxides versus potassium and magnesium oxides for Venetian cristallo and vitrum blanchum glass and Renaissance period translucent enamels The finding presented here reveal definite chemical differences between enamels from the Renaissance and those typical of the nineteenth century. However, evidence has also been found that some enamellers may have continued to use traditional enamel types well into the nineteenth century, and these compositions appear to be chemically indistinguishable from Renaissance period enamels (Wypyski 2002). Thus, certain compositions discussed here can with a great deal of assurance be described as modern and incompatible with Renaissance production, but the finding of Renaissance-type compositions, while favorable evidence of authenticity, is not proof of a Renaissance date for an object. To complicate matters even further, the possibility of relatively recent re-enamelling of much older objects must be considered for any object found with evidence of modern enamels.

12 58 Wypyski Also, original Renaissance pieces may have had later additions or embellishments, or were remounted to form a pastiche of old and new material (Tait 1992). Technical examination of some Renaissance jewellery has revealed a number of objects which have both Renaissance and nineteenth century type enamels on different parts of the same objects (Drayman- Weisser and Wypyski forthcoming). This is seen here where one side of the Prudence Pendant (Table 1, no. 2) has typical Renaissance enamel, while the reverse face is apparently a later addition or restoration, with typical nineteenth century enamels (Table 2, no. 8). Although current research has not found definite compositional criteria to distinguish between some enamels from the Renaissance and some thought to be later, ongoing research at the Metropolitan Museum and elsewhere may find differences in trace elements, or other elements, such as boron, which were not analysed as part of this study. More work is currently being planned to enlarge the scope of this study, particularly to acquire more data on enamels proven or suspected of being by Vasters from the late nineteenth century, which may aid in the dating and provenance of these objects. Acknowledgements I would like to express my thanks and appreciation to Clare Vincent, Associate Curator in the Department of European Sculpture and Decorative Arts of The Metropolitan Museum of Art, for permission to examine these objects and her encouragement and enthusiasm for this ongoing work. References Drayman-Weisser, T. and M.T. Wypyski, forthcoming, Fabulous, fantasy, or fake? An examination of the Renaissance jewelry collection of the Walters Art Museum, Journal of the Walters Art Museum. Hackenbroch, Y. 1986, Reinhold Vasters, goldsmith, Metropolitan Museum Journal 19/20, Newman, R. 1997, Chromium oxide greens, Artist s Pigments: A Handbook of Their History and Characteristics, 3, Fitzhugh, E.W. (ed.), Oxford, Oxford University Press, Ombelli, M., C. Miliani, and A. Morresi, 2005, Investigations of the decorative techniques and conservation of a majolica altar by Andrea della Robbia, Materials Issues in Art and Archaeology VII, Materials Research Society Symposium Proceedings, 852,Vandiver, P. B., J. L. Mass, and A. Murray, (eds)., Warrendale, PA, Materials Research Society, Truman, C. 1979, Reinhold Vasters the last of the goldsmiths?, The Connoisseur 201, Truman, C. 2000, Nineteenth-Century Renaissance-Revival jewelry, The Art Institute of Chicago Museum Studies 25(2), Tait, H. 1992, Reinhold Vasters: Goldsmith, restorer and prolific faker, Why Fakes Matter: Essays on Problems of Authenticity, Jones, M. (ed.), London, British Museum Press,

13 Chemical Analyses of Renaissance Enamelled Jewellery 59 Verità, M. 1985, L invenzione del cristallo muranese: una verifica analitica delle fonti storiche, Rivista delle Stazione Sperimentale del Vetro 15, Verità, M., R. Basso, M.T. Wypyski, and R.J. Koestler, 1994, X-ray microanalysis of ancient glassy materials: A comparative study of wavelength dispersive and energy dispersive techniques, Archaeometry 36(2), Wardropper, I. 2000, Between art and nature: Jewelry in the Renaissance, The Art Institute of Chicago Museum Studies 25(2), Wypyski, M.T. 2002, Renaissance enameled jewelry and 19th century Renaissance Revival: Characterization of enamel compositions, Materials Issues in Art and Archaeology VI, Materials Research Society Symposium Proceedings, 712, Vandiver, P., M. Goodway and J. L. Mass, (eds.), Warrendale, PA, Materials Research Society, Wypyski, M.T., forthcoming, Technical study of Renaissance Venetian enamelled glass, Annales du 17 e Congrès de l Association Internationale pour l Histoire du Verre, Antwerp Wypyski, M.T. and R.W. Richter, 1997, Preliminary compositional study of 14th and 15th century European enamels, Techne 6,