JAIC 2003, Volume 42, Number 2, Article 7 (pp. 279 to 312)
JAIC online
Journal of the American Institute for Conservation
JAIC 2003, Volume 42, Number 2, Article 7 (pp. 279 to 312)

THE EARLY PAINTED ENAMELS OF LIMOGES IN THE WALTERS ART MUSEUM: HISTORICAL CONTEXT AND OBSERVATIONS ON PAST TREATMENTS

TERRY DRAYMAN-WEISSER



3 THE EARLY PAINTED ENAMELS OF LIMOGES

In the late 15th century Limoges re-emerged as a major center for enameling, specializing in painted enamel plaques on copper. Although these enamels have certain characteristics in common with basse taille, a close examination of the Limoges enamels makes it clear how truly revolutionary they were. The Limoges enamelers broke with the tradition of incorporating the metal support into the surface design. They instead treated the entire surface of the copper support as a “canvas” or panel for painting.

The images for the Limoges painted enamels of this period were mainly religious subjects based on contemporary prints. Recent research supports the belief that many Renaissance prints were handcolored soon after printing (Dackerman 2000). Perhaps these colored prints served as inspiration for the Limoges enamelers. The brilliant colors and overall painterly effects also were akin to those of small panel paintings and manuscript illuminations of the day. In fact, it is possible that the impetus for the rebirth of enameling in Limoges was to compete with the small personal devotional paintings being created at the time.

Louis XI, who reigned from 1461 to 1483, restricted the right to create these painted enamels to a limited group of privileged Limoges families (Verdier et al. 1977). It is not known how many enamelers were active during this early period. Marquet de Vasselot (1921) has identified at least seven workshops judging from stylistic and technical differences. As specific names of enamelers of this period are not known, many enamels have been attributed with designations such as the “Master of the High Foreheads” or the “Master of the Orléans Triptych.”

It is unlikely that these early Limoges enamelers themselves made the glass for enameling. The glass was probably purchased in the form of ready-made cakes. We know that such stocks of enamel colors existed, since the 1326 will of Queen Maria of Hungary lists a bag containing “18 petris vitreis Rubici pro faciendis smaltis et petris vitri albi similiter” (quoted in Campbell 1997, and may be translated as “18 cakes (literally, petrais rock) of Rubicon glass for making enamel and in like manner of cakes of white glass”). Theophilus, writing in the 12th century, explained how cakes of enamel could be broken up. He recommended heating the glass until it was red hot and throwing it into a copper vessel containing water, causing the glass to splinter into small pieces ready for grinding (Dodwell 1961).

The enamels were colored by the addition of metal oxides to the flux. The colorants used on the early Limoges enamels were tin for white; manganese for mulberry; copper, iron, and cobalt for blue and purple; iron and manganese for yellow; iron and copper for green (Smith et al. 1987). Amounts and ratios of colorants, firing temperature, firing time, and reducing or oxidizing conditions in the furnace atmosphere could also affect the color.

Cellini provides one of the few contemporary descriptions of the enamel preparation and application process. He recommends grinding the enamel to a very fine powder in a steel mortar in very clean water and advises the enameler to make a test palette in advance to determine whether the colored enamels selected will melt in the same temperature range. He suggests applying the enamel to the copper with a brush or palette knife and describes the use of a gum (quince seed water made from soaking quince seeds in water overnight) to hold the enamel powder together during the application process. The enamel, he warns, should be heated in the furnace until the enamel just moves, but not runs. For final finishing he recommends polishing with “bits of stone or sand,” then with tripoli, or following the first polishing by putting the enamel back in the furnace and heating slowly until the enamel just begins to run (Ashbee 1967).


3.1 LIMOGES PAINTED ENAMEL TECHNIQUES

A technical study of the early painted Limoges enamel plaques in the Walters collection reveals that the techniques used to create them are more complex than first meets the eye. Michaels (1968) and Smith et al. (1987) describe these techniques, which will be summarized here and supplemented with more recent observations by the author. In all cases a copper plate was used for the support. The earliest painted enamels were done on flat plates, each ranging in thickness from about 18 gauge to 28 gauge at the edges. Later, thinner, slightly domed plates came into use. The introduction of curved plates allowed the craftsman to use less metal while still providing a rigid work surface.

To create the detail and modeling typical of the early Limoges images, several layers of enamel and multiple firings based on melting points were required. The upper layers had to have a lower melting point than those below to prevent remelting and disruption of the lower layers. Generally it took five or six firings. First, the copper plate was cleaned so that the enamel would adhere well. In some examples a layer of silver was applied directly to the front surface of the copper plate before the application of the enamel to create a more brilliant, reflective effect, similar to that achieved in basse taille work. Both sides of the plate had to be covered with enamel during the first firing to equalize differences in expansion and contraction of the metal and the enamel, to prevent warping from uneven heating of the plate, and to seal the metal to prevent oxidation. The enamel on the reverse, referred to as the counterenamel, was made up of the contaminated residues from the preparation of the enamels. The counterenamels of these early works tend to be thick and opaque.

According to the literature, two distinctly different techniques were used to create the initial design layer on the front surface of the plaques (Gauthier and Marcheix 1962; Michaels 1968; Smith et al. 1987). The first method is the following: An initial layer of dark enamel was applied and fired at the same time as the counterenamel. To this surface an opaque white layer was added. Using a method called enlevage, the white layer was scratched away with a tool to reveal the dark enamel beneath, creating the lines of the design. This layer was fired to permanently retain the dark lines. The second method for creating the design entailed the following: An initial layer of opaque white was applied over the entire front surface of the copper plate and fired with the counterenamel. Over this white layer, the design was drawn with a brush or pen in thin dark red enamel. This was fired to make it permanent.

A variation on these techniques, not reported by Gauthier and Marcheix, Michaels, or Smith et al., was observed by the author during examination of a broken corner of an early Limoges enamel plaque (44.145) in the Walters collection. What can be seen in cross section are dark red lines sometimes directly on the copper plate and sometimes separated from the plate by a very thin layer of white enamel. Significantly thicker white enamel can be seen on either side of the red lines. Translucent colored enamels cover these preliminary layers. When the top surface of this enamel is viewed in raking light, one sees undulations with depressions corresponding precisely to the red drawing lines beneath.

One interpretation of these observations is that first a thick layer of white enamel was applied to the front of the copper plate at the same time as the counterenamel. Before firing, the design was laid out using the enlevage technique, scratching through the white enamel and making troughlike depressions. In some cases the tool scratched through to the plate, while in others it did not quite reach the plate, leaving a thin layer of white enamel on the copper surface. The white enamel on both sides of the scratched lines retained the original thickness or perhaps became a little thicker from the displacement of the white enamel from the depressions. After firing, the design was drawn in dark red enamel in the depressions. The fact that under high magnification the red lines appear crisp and sharp and that there is no evidence of blurring or mixing of the red enamel with the white suggests that the white enamel was fired before the red lines were applied.

The red lines were then fired in preparation for the application of the colored translucent enamels. This newly observed technique might represent the methods of a separate workshop or perhaps a transition between the two previously reported techniques, eliminating the need for a dark underlayer.

Regardless of the technique used, the opaque white layer, once fired, provided a luminous “ground” layer, similar to the white gesso layer used in panel paintings of the day. Once the drawing of the design was complete, the plaque must have resembled a woodcut or print. It was now ready for the addition of the translucent colors. The palette was generally limited to dark blue, green, mulberry red, turquoise, and tan (light yellow brown) (Michaels 1968). Some examples also include yellow and violet, as well as the sparingly used rare and expensive clear red enamel.

The powdered, colored translucent enamels were applied using the dark design lines as a guide for placement. The finely ground colored enamel frit was kept in water until ready for use. The wet enamel powder generally held together to form a paste and could be applied with a brush or palette knife. If the powder did not hold together well, a small amount of gum could be mixed in as a binder. In areas where opaque flesh tones would be added in a later step, dark translucent enamel was applied at this time to serve as a preparatory layer. Thus, the entire surface was covered at one time to a uniform level with translucent colored enamels. It is clear from the “spillover” of frit from one translucent color into another at the edges of the design areas that all of the colors were applied and fired at one time. The dark drawing lines below show through the translucent enamel, making the blurred interfaces of the colors less noticeable. An interesting phenomenon is that the color of the overlying enamel affects the appearance of the color of the drawing beneath. For example, dark red lines appear to be black when covered by blue or green translucent enamel.

The surfaces of many of the early Limoges enameled plaques were enhanced with “jewels.” The creation of these embellishments was carried out during the application of the translucent enamel layer. Small squares or discs of silver foil were laid onto the unfired colored enamels, and a small amount of translucent enamel frit, usually in a contrasting color, was added over the foil. If clear red enamel was to be used, gold foil was used instead of silver. Sometimes additional enamel was applied under the foil to provide additional relief for the “jewel.” Thus, after firing, the foil was sealed between layers of translucent enamel, and the “jewel” protruded above the rest of the surface. As in basse taille enameling, the metal foils reflect light back through the enamel, creating a gemlike effect.

The next step was to apply opaque white to selected areas of the now-fired translucent enamel, especially where flesh tones were to be represented, or details such as books, scrolls, and white articles of clothing. According to Michaels (1968), two principal methods were used for flesh tones and garments: The first employed a single layer of opaque white over a dark layer of translucent enamel. By varying the thickness of the white enamel, gradations in shades of gray could be achieved. The thinner areas appeared more gray, while the thicker areas were a purer white. Linear details, such as facial features, fingers, and garment folds, could be defined by the enlevage technique before firing, scratching through the white to the dark translucent layer beneath.

In the second technique an intermediate semiopaque lilac-colored layer (a mixture of opaque white and translucent mulberry) was applied before the application of the final pure white layer. The lilac layer gave a warm tone to the white applied over it. Shading could be achieved by varying the thickness of the white layer. With this method the white areas developed a three-dimensional quality in the thickest areas. In the thinnest areas shadows were created due to the lilac tint beneath. The enlevage technique was used both through the lilac to the dark translucent layer beneath and through the pure white layer to the lilac layer to delineate fine details. It is possible that the two layers were fired separately, or, alternatively, the lilac-colored layer and the pure white could have been applied and fired at the same time. Although there are enlevage lines through the white layer to the lilac, the lines observed are not sharp and therefore could have been done while the lilac layer was dry, but not fired.

After firing these opaque layers, the surface of the enamel was embellished with accents of matte, opaque red ocher for blood and wounds, and red ocher washes for tinting lips and cheeks. Dark brown was sometimes applied over areas of white for inscriptions, or lines on books, or to create other details. Gold lines, designs, and patterns further enriched the surface. These final accents were applied with a low melting point flux so that they could be fired quickly at a low temperature without disturbing the layers beneath.


3.2 DETERIORATION OF THE ENAMELS

The early painted Limoges enamels suffer from deterioration localized to particular colors. This deterioration phenomenon is further complicated by the fact that it is not always consistent, e.g., a particular color may be unstable on one enamel and stable on another, while the latter enamel exhibits instability in a color that is stable on the first. Generally, deterioration appears to be most common and consistent in the blue, mulberry, and purple areas (although other colors also may be unstable), and white areas are always stable. The damage caused by the deterioration of the enamel is significant and progressive, ranging from dulled surfaces in the initial stages to extensive loss and pitting in advanced cases. Active degradation takes the visible form of weeping (droplets of liquid forming on the surface) and crizzling (fine cracking and flaking of the enamel surface), along with the formation of white salts in some cases. The unfortunate result is that the translucency of the enamel is lost and the surfaces intended to be brilliant become dull and unattractive.

It is not known when the deterioration of these enamels was first noted, but it is certainly not a new problem, as photographs taken before 1934 already record surface instability (Walters Art Museum Archives) (fig. 14). As early as 1563 a reference to deterioration in glass appears. Bernard Palissy mentions that the French glaziers blame the sun and moon for decay of their church windows (Oakley 1992). The colors that are unstable on the early Limoges painted enamels are generally stable on the enamels produced after 1530, thus indicating a

Fig. 14. Workshop of Master of the Orléans Triptych, Annunciation, showing the condition of the enamel before 1932. Ca. 1520, enamel on copper, 20.3 × 16.2 cm. Walters Art Museum 44.172. Note the crizzling especially in Virgin's robe and around all “jewels.” Courtesy of Walters Art Museum archives.
Fig. 15. Photomicrograph (x 1.6 on film) of Master, in Workshop of Master of the Orléans Triptych, Virgin and Child with Parrot, showing corrosion from the copper plate on the left edge and crizzling of the enamel. Ca. 1500, enamel on copper, 16.4 × 12.4 cm. Walters Art Museum 44.126
change in the chemical makeup of the glass after that date. Whether this change is intentional because the enamelers realized that there were deterioration problems or is simply coincidental due to a change in the source of the raw materials is unclear.

It has been shown that in the early Limoges painted enamels the blue, mulberry, and purple enamels are chemically unstable due to their original compositions (Michaels 1968; Smith et al. 1987; Biron 1999). In a study by Biron (1999) the compositions of 20 stable and unstable painted Limoges enamels were analyzed using ion beam protoninduced x-ray emission spectroscopy (PIXE) and proton-induced gamma-ray emission spectroscopy (PIGME) methods. Her findings substantially support the accuracy of the recipes for 16th-century Limoges enameling found in the writings of de Vigenere in 1615 Biron (1999). Biron writes:

The enamellers then used sand or pebbles (60–65% SiO2) and as alkaline source the ash of marine plants (salicornia) from the seashores of the Provence or Spain which contained much sodium (13–15 wt.-% Na2O). However, the content of potassium (4–6% K2O) is somewhat too high to be brought into the glass by the soda ash alone. … It is possible but not certain, that potassium-rich ashes of fern or wood (beech, oak … ) were used together with salicornia.” (1999, 166)

Biron found a higher concentration of potassium in the unstable blue, mulberry, and purple enamels (6–8 % potassium oxide [K2O]) than in the stable ones (4–6 % K2O), concluding that the enamelers “used a sodium source of vegetable origin for the unstable enamels as well, but added higher amounts of potassium-rich materials in comparison to the stable glasses” (1999, 167). Her analytical results also point to a different source of potassium for the stable and unstable enamels. The unstable enamels are more enriched in copper, suggesting saltpeter (potassium nitrate) as a possible source. According to Biron, de Vigenere indicated that saltpeter was recommended to create a more brilliant glass (1999). Two recipes for saltpeter appear in Biringuccio's Pirotechnia published in 1540 (Smith and Gnudi 1990) and Georgius Agricola's treatise, De Re Metallica, first published in 1556 (Hoover and Hoover 1950). These recipes specify the use of copper pots or cauldrons for boiling during the production process, and the use of sticks or rods, which may have been made of copper, for precipitating the saltpeter from solution. The use of copper in the production of saltpeter could be the source of copper enrichment in the unstable enamels.

Michaels concluded that excessive amounts of sodium caused the instability of the enamels at the Walters. He also reported the results of x-ray diffraction analysis on the white surface salts, which were identified as sodium formate, possibly from a reaction of the sodium with formic acid off-gassing from display and storage materials (1968, 35). However, for the objects she examined, Biron identified higher levels of potassium as the primary factor causing the depolymerization of the glass structure, making the enamels more prone to be affected by the environment (1999, 168). Smith et al. (1987, 109) also identified potassium as a cause of deterioration.

In addition to Michaels (1968), Smith et al. (1987), and Biron (1999), a number of other articles have appeared on the degradation of early painted Limoges enamels (Philippon et al. 1988; Eveno et al. 1990; Perez y Jorba et al. 1993; Germain-Bonne et al. 1996; Biron and Belassene 1999). A review of literature on crizzling and weeping vessel glass also shows agreement about the causes of deterioration. It is generally accepted that the degradation process involves the interaction of the glass layer with atmospheric moisture, causing hydration of the surface (Brill 1972). In summary, the alkali ions, such as sodium and potassium, tend to migrate to the glass surface where an exchange takes place with water molecules and/or hydronium ions (H3O+). In unstable glass, alkali substitution may occur at the same time as network dissolution, “forming a silica-rich hydrated layer having a more open structure through which ions and molecules can easily diffuse”(Ryan et al. 1996, 842). When the hydrated layer on the surface dehydrates, shrinkage occurs, causing cracks (Hogg et al. 1998). In addition, alkali salt deposits can form. Due to the deliquescent nature of the alkali-enriched surface, a solution with a pH above 9 is created, which can accelerate attack on the silica network of the glass. This attack can open the glass or enamel surface to continued deterioration (Ryan et al. 1996).

The author has noted an additional problem for the enamels that does not affect vessel glass: the alkaline liquid formed during deliquescence can seep through cracks in the enamel to the underlying copper plate, resulting in separation of the enamel from the plate as corrosion products form on the copper (fig. 15, see page 255).

The deterioration of the early painted Limoges enamels can be particularly disheartening to those involved in their care.

Glass deterioration is the phenomenon which has sometimes erroneously been likened to a disease or sickness, mainly due to the insidious nature of the condition. However, unfortunately the similarity ends there, as there is no cure for the deterioration, nor is it infectious. … It is a depressing subject, as there is little that can be done. … The conservator can only hope to temporarily slow down the gradual decay by controlling the surrounding environmental conditions. (Oakley 1992, 18)

Since the cause of the deterioration is inherent in the enamel itself, there may be no way to completely “cure” the problem. It has been suggested that lowering the relative humidity to prevent surface hydration may protect the enamels (Smith et al. 1987); however, lowering the relative humidity tends to increase crizzling and the rate of surface loss from flaking on the glass surface (Brill 1975). Although controlling the environment may slow the reactions involved in the deterioration, it does not present a permanent solution as even minor fluctuations in relative humidity can cause the enamel to cycle between weeping and crizzling. In an experiment in 1986, the author witnessed weeping on enamels above 52% RH and crizzling below 48% at 71°F. It should be noted that the Limoges enamels at the Walters were kept in an air-conditioned, thick-walled, well-insulated building where the environment was always fairly well controlled, usually between 47% and 53% RH and 68° and 72°F.


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