After the surprising result of the cold-painted black enamel on all investigated late medieval reverse painting glass objects, the question came up why a painting medium from a genre of stained glass found entry to the palette of reverse painting on glass, which usually takes recourse solely to pigments of panel paintings.
For the “Meditation Panel” (Fig. 6)—the earliest piece examined in this study—a pure contour painting in black enamel can be found without the use of any glaze for shading. Schlie (1895) wrote as early as 1895 about the technique of the “Meditation Panel”: “… the surface on which the drawing with black enamel is executed is subsequently gilded, and individual parts of the depiction, for example the garments, are also overlain with a translucent green and red lacquer.”
On other glass works painted in reverse that were produced from the mid-fifteenth century onwards, the black paint was technically more elaborate than just contouring or cross-hatching. Figure 7—a detail from the “St. Magdalena”—initially shows a contour of black enamel. A thin, semitransparent black glaze was then painted onto the contour. Additional modeling was carried out with fine and broad engravings. The contour was in part retraced with an additional application of black enamel.
Such painting or drawing technique can be carried out only with a special, rapidly drying painting medium. Black enamel, a lead glass colored black with iron oxides (and usually containing copper compounds), fulfills this requirement when for example gum is added to it as a binding medium. A pigment like lamp black requires a great amount of binding material and is less suitable for progressing the work of art. The black paint needs to be dry before continuing with the next layer to avoid destroying the underlying work.
After the identification of cold-painted black enamel on reverse paintings on glass, we have to ask whether cold-painted black enamel was also used for the decoration of panel paintings and stained glass.
Many examples of panel painting have been preserved in which the gilded parts were decorated using fine brushes and deep-black paint (Luckenbach 2002). In this context, Jensen (2007) speaks of black enamel drawing in analogy to stained glass. Examples include the external sides of the “Three Kings Altar” (Cathedral of Cologne), the “Bartholomew Altar” (Wallraf-Richartz Museum, Cologne), the “Altar of the Holy Family”, and the “Ortenberg Altar” (Hessian State Museum, Darmstadt). But so far, natural-scientific examinations have turned up evidence only of lamp black pigments, not of black enamel (Kühn 1993; Kühnen 2000; Schäfer et al. 2001). The use of carbon pigments in oil as binding medium in a panel painting is comprehendible. The black contour is the “last” paint layer applied, so a capacity for rapid drying is not essential. Up to now, all these investigations were conducted by taking samples. However, the question of further analyses has to be posed. Carbon-based pigments and black enamels cannot be distinguished by near infrared reflectography, so the only method for nondestructive investigation seems to be X-ray fluorescence analysis.
Analytical results on cold black painting in medieval stained glass artworks seem to be rare. Few references are known. As an example, the Royal Institute for the Study and Conservation of Belgium’s Artistic Heritage houses exhibits some thirteenth and fifteenth century stained glass artworks that have passages of black and green cold painting. The analysis of the pigments and binders was carried out with a scanning electron microscope and an infrared spectroscope. However, the black cold painting is not original but considered a repair applied at a later date (Fontaine et al. 1996).
Our result presents a contribution to the discussion whether reverse painting derives from stained glass artwork or panel painting. Reverse painting on glass is first related to stained glass artwork through the painting technique. Both the substrate material glass and black enamel used for contouring, glazing, and engraving come from stained glass. The application of the black paint is, however, different: in the case of stained glass, it is always fired, while in medieval reverse painting on glass, it was always applied cold, as our investigation shows. Starting with pure contour painting in the early pieces (about 1330), it developed into contour together with the application of a glaze that was partially engraved. At the end of the investigated period (about 1550 onwards), the black enamel contour disappeared in favor of pure oil paint.
If the black enamel in reverse painted glass derives from stained glass, all the other materials used for decoration, like oil paint or gold and silver leaf, are derived from panel painting. Thus, we have here an art genre that combines materials and techniques from other genres. Nonetheless, reverse painting on glass can be regarded as an independent art genre due to the challenging technique (painted in reverse) and the general appreciation of splendid artwork.