Proceedings 28 4 June 1992 the Warburg Institute, London, following the Society's AGM.
TECHNIQUES OF REMANESQUE CHAMPLEVE ENAMEL: a paper by Mr. Neil Stratford, Keeper of the Department of Medieval and Later Antiquities at the British Museum
The meeting was presided over by the President who expressed his pleasure in introducing the distinguished speaker.
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The study of medieval metalworking is still in its infancy. Archaeology and laboratory analyses were the bases of study for pre-13th century work since there was little literary evidence, apart from the treatise of Theophilus. This could be usefully compared with the objects themselves and modern enamelling practice. The most complete, as well as one of the earliest MSS of Theophilus was Harley 3915; unfortunately the existing translations are variable in their accuracy while published texts of the original MSS are variable in their readings. Dodwell's was the best, but badly needed was a history of Theophilus's compendium and the date of the different Latin recipes. Some dated back to the Carolingian period, others were contemporary with the author, i.e. not much later than 1100, who worked somewhere in the Rhineland or Lower Saxony, possibly at Köln.
Enamelling processes involved firing powdered glass at about 9000, so that it would flow and bond with a base plate, usually of metal. The two basic techniques were 1) cloisonné - in which the base plate was formed as a shallow tray, fine wire being used to delineate the design into different fields in which the powdered glass could be laid for firing; and 2) champleve in which the fields for the powders were cut into the flat surface of the metal plate.
In the earlier middle ages the chief metal was gold, an excellent metal for the enameller; the pure metal melts at a temperature of about 12200, far above that of glass, so that there is little danger of the metal flowing beneath the molten glass. As there is no oxidation at the gold-glass interface, translucent effects could be obtained by firing thin layers of glass over gold. The coefficient of expansion of glass and gold are closely compatible. The fact that there is almost no surviving 12thcentury gold enamel could not be for technical reasons. From 1000 to the mid 13th century, gold was a diminishing asset and the flourishing Romanesque gilt-copper champleve enamel industry was undoubtedly an answer to the desire for a gold-like substitute. It was only in the 14th century that an increase in gold supply encouraged a revival of gold enamel.
Also there was virtually no 12th-century silver enamel, though here the reasons probably were technical, e.g. the melting point of pure silver, about 9600, is dangerously close to that of glass. Two techniques were available for the embellishment of silver: gilding and niello, which looks very like dark enamel. Niello is composed of sulphides of silver and copper and/or lead sulphide, made by burning sulphur with filings of the metals in question.
THE MANUFACTURE OF COPPER CHAMPLEVE ENAMELS, STEP BY STEP
The first stage was the commission. Next came the planning stage. As in other medieval artworks the involvement of the patron in creating the intellectual programme was difficult to decide; it seems that some artists used model books. Certain figure types were repeated over and over again to the extent that even named and well known personages were presented in portrait' form which in fact duplicated a model book stereotype. (The point was illustrated by a Mosan enamel (c.1150) of Henry of Blois, bishop of Winchester, brother of King Stephen. There was a repertoire of stock images that could be used to build up elaborate decorative schemes.
No matter how learned the patrons the craftsmen frequently got it wrong. One of the best goldsmiths of the whole Romanesque period was so unfamiliar with Latin that he misplaced the inscriptions on the British Museum's famous Mosan enamel cross. The scene aimed to depict the account in Genesis of how the patriarch Jacob blessed his grandsons by crossing over his arms so that Ephraim, the younger, received the right hand of blessing. For 12th-century commentators this was a prefiguration of the cross of Christ which was the overriding theme of all the Old Testament scenes on the B.M. cross. But in fact the goldsmith rendered the whole story meaningless by showing the older son Manasseh receiving the blessing: one could only assume a degree of illiteracy. The gap between intention and execution, between the bishop's wishes and what the artist actually produced could not have been greater. And the work in question was, so to speak, at the top of the market.
The design settled, the craftsman assembled his materials: wood where relevant for statuary core, reliquary case or book binding boards, gold for the gilding, gemstones, etc. and copper. In the 12th century, it appeared the principal copper mines were near Goslar, in the western Harz mountains (overtaken post 1200 by mines in Sweden and the eastern Harz). However, it was not certain that Romanesque enamellers in fact used Goslar copper, which had very high lead, antimony and arsenic content while the craft required virtually pure metal - for example, lead in the copper threw up 'white spots' in firing (Theophilus). Analyses at the Museum laboratory had shown that all Romanesque enamels, Limoges, English, etc., were on virtually pure copper.
The Mosan copper enamelling tradition was, perhaps, a by-product, so to speak, of the region’s brass industry. There was no locally mined copper but much was imported for smelting with the local zinc calamine ore to produce the high zinc copper alloys used to cast the great font at Liege. (Analysis of the font confirmed that alloys were not easily controlled, the percentages of zinc, tin, and lead varying considerably from one part of the artefact to another. Ignorance of details of the metal content of the raw materials, e.g. old coin hoards, was perhaps a reason for this.)
The first stage in the making of an enamel was to hammer the pure copper plaque into the shape and thickness required. Next the design was sketched out and the fields cut and cleared to an even depth, normally 1:4 to the thickness of the plaque and requiring the nicest calculation lest the metal warp during the firing thus cracking the glass. X-ray analyses could reveal great subtleties in the working - e.g. depiction of hair could produce virtuoso freehand manipulation of the graving tool. Sometimes the reverse of plaques had been used for trial sketches: strong indication that Romanesque model books were not used as cartoons for the direct transfer of drawings.
There were remarkable refinements of finish. A German plaque of the Apostle Thomas, ~1180 had a shimmering surface comparable to Byzantine mosaics where single isolated gold tesserae were scattered through blue backgrounds to heighten surface vibrancy! At the end of the cutting process, the craftsman of the St.Thomas plaque had dug up little tongues of metal from the floor of the fields which projected as minute studs up to the level of the polished, fired glass. A gold-mercury amalgam was brushed over the surface and gently heated to 3600 to burn off the mercury and leave the gold adhering to the metal studs but not the glass.
Occasionally, details in a champleve piece were done in cloisonné an example being the flower borders on the B.M. cross where quatrefoils and trefoils in the borders were in gilded copper cloisonné - were they perhaps an afterthought, or was it merely easier to execute certain details by dropping ready bent wire into the field?
When the cutting of the fields was complete, the plaque was carefully washed out, ready for the laying of the (well washed) glass powders. The palette of colours was not extensive: white, yellow, green, turquoise, azure blue, opaque red and a deep purple red. Certain mid hues were produced by mixing, while paler tones were also achieved, perhaps by use of a clear flux glass. Thanks to recent laboratory studies by Ian Freestone It seemed that, of the colours, only the opaque red (a potash glass) was manufactured de novo, the others (from analysis of their soda limes)) being either recycled ancient vessel glasses or ancient mosaic tesserae. This confirmed the passage in Theophilus describing the origins of the enameller's raw materials. Only very small quantities were needed - the whole Mosan production for a comparatively restricted luxury market, could have been made from old mosaic tesserae. Whether the amounts of glass required for the large, quasi-industrial output of 13th-century Limoges workshops could have been similarly sourced was another question.
It was essential that the different coloured glass powders flowed at roughly the same temperature and well below the melting point of copper (1106°C). Theophilus prescribed test-firing of powdered glass specimens so that any which did not conform could be rejected. The wet powders were laid into the field with a goose quill. The colours might be single or combined in mixed fields. With these a skilled enameller could achieve marvellous effects of shading and juxtaposition, as in the cross-gartered hose of a Samson figure, turquoise and opaque red and dappled in white (c.1180).
Plaques were fired in charcoal wind ovens, like that of a 12th century Rhineland metal working site recently excavated near the church of Schwarzrheindorf, near Bonn. The same site also yielded crucibles and smelting and alloy debris. The work piece was introduced into the furnace with tongs and a domed cover with projecting vents placed over it to protect it from charring fragments as the charcoal was piled and packed around. With the aid of bellows, the fire is then raised to the most intense possible heat - Theophilus uses the verb candeo to be white hot'. Given that the piece was hidden beneath its cover, only experience could decide the duration of the firing. Cooling had to be very slow and gradual to guard against cracking. As the glass was laid wet it shrank during firing so that several layers had to be fired to bring the colours up to the surface level of the plaque. Very high magnification revealed that the colours, however carefully controlled, did in fact flow into one another. Such magnification could also reveal tell-tale red lines of oxidised copper at the glass interface - chemistry that denied to enamel on copper the much prized translucent effects possible to gold. In fact Mosan enamellers went to extraordinary lengths to achieve such effects. In a marvellous piece now in New York's Metropolitan Museum, the greens and purple reds had been laid on gold foil itself laid on a first firing of glass, thus being sandwiched between two glass layers. Another craftsman used this technique to simulate flames while another even achieved the mottled appearance of exotic grey-blue marble by mixing larger granules which flow differently from the fine-ground colour powder when fired.
Many champleve copper enamel plaques, with other elements such as gemstones might be required for large and complex commissions such as the Shrine of the Three Kings at Koln and mounting marks to aid assemblage were sometimes found. The Mosan great figured champleve enamels were produced from c.1140 to 1190, the ambo of Nicholas of Verdun at Klosterneuberg representing one of the later achievements of the tradition.
The President thanked Mr.Stratford for his informative and beautifully illustrated talk and following questions and discussions from the floor brought the meeting to a close.