THE IDENTIFICATION OF BLUE PIGMENTS IN EARLY SIENESE PAINTINGS BY COLOR INFRARED PHOTOGRAPHY
WELL-PRESERVED or recently cleaned early Sienese paintings often stun modern viewers because of the brightness and variety of their color. It was the Sienese artist's aim to dazzle the beholder with the splendor of color just as with the shimmer of tooled gold surfaces. The most important of these colors, often reserved for the mantles of the Virgin and of Christ as Redeemer and Judge, were the blues.
Azurite and ultramarine were the two blues commonly used in early Sienese panel painting. Indigo, a very dark blue vegetable dye, although widely used in the 13th century, is generally found only in minor details of 14th-century paintings. In contrast, azurite—then commonly called azzuro d'Alemagna—was often used for draperies, particularly in works by Duccio and his circle. Azurite is a basic copper carbonate; the pigment is made by grinding and purifying samples of the naturally occurring mineral azurite (Gettens and FitzHugh 1966). To obtain an intensely colored pigment the particles must be left quite large; if they are ground too finely azurite looks gray.
As a blue, ultramarine was considered far superior to azurite. Because it was a tonally rich color, in contrast to the relatively flat and grainy azurite, ultramarine was usually preferred by panel painters. In addition, the hue of ultramarine—a slightly purplish-tinged blue—was held to be the ideal complement to vermilion and gold on panels. As Cennino Cennini's eulogy to ultramarine in his late 14th-century treatise on the arts reveals, a mystique surrounded this color:
Ultramarine is a color illustrious, beautiful, and most perfect, beyond all other colors; one could not say anything about it, or do anything with it, that its quality would not still surpass. … Let some of that color, combined with gold, which adorns all the works of our profession, whether on wall or on panel, shine forth in every object.
*(Cennini 1960, 36)
Aside from its tonally rich color, ultramarine was also considered an exotic, high-quality pigment because of its origins and price. Extracted from the semiprecious mineral lazurite (lapis lazuli), mined in the ancient quarries of Badakshan in an area now part of Afghanistan, it was imported from the Middle East via Venice (Plesters 1966). Furthermore, the process for making the pigment was laborious and time-consuming, and, as a result, the color was extremely expensive, sometimes even as costly as gold leaf. Originally the pigment was produced using a simple washing and grinding method that yielded a pale grayish-blue color. However, by about 1200, a new, more complex, but infinitely more effective method for extracting the blue color was developed. As Cennino Cennini explains, finely ground, high-quality lazurite is mixed with wax and resin to form a paste. Then this paste is immersed in a bowl filled with a weak lye solution (potassium carbonate) and poked with sticks to free particles of the blue mineral, which settle out. This process is carried out several times, releasing the most intense blue particles first and lower grades in subsequent extractions. The last extraction produces a gray pigment known as ultramarine ash. All grades were used (Cennini 1960, 36–39).
Although ultramarine was preferred for its coloristic qualities and associations, it was used much more sparingly than azurite in most early Italian workshops because of its great expense. In the mid-15th century in Florence, a period for which documentation exists, high-grade ultramarine cost 10 to 15 times as much as azurite and even low-grade ultramarine was about two-thirds the price of the finer quality pigment. These ratios are based on the prices paid by Filippo Lippi in the 1450s and by Neri di Bicci in the 1460s (Borsook 1975, 40–41; Bicci 1976, 270–71). It seems reasonable to assume that the price ratios in Siena in the late 13th and 14th centuries were not markedly different. Another indication of the exclusive nature of ultramarine is the fact that in early Renaissance painting contracts this pigment, like equally expensive gold leaf, was often specially supplied by the patron and considered as a separate payment. The amount of ultramarine used could determine, at least in part, the value and price of the painting (Merrifield 1849, 1:ccxi-ccxii; Lerner-Lehmkuhl 1936, 36).
The informative descriptions of painting projects in the work-diary of Neri di Bicci reveal how ultramarine was treasured and used in relation to azurite by this mid-15th-century but highly traditional Florentine artist. It is significant that Neri di Bicci describes not only where he put gold on his panels and what quality of gold he used, but often which blue pigment he used and where. Other colors are rarely mentioned and then only generally. For example, in the entry of July 5, 1466, devoted to the altarpiece of the Madonna and Child with Four Saints (to this day in San Michele, Arezzo) Neri says that he painted the mantle of the Virgin with ultramarine but used azurite for all the other blue areas (Bicci 1976, 273). Similarly, for the Trinity (San Niccolò Oltrarno, Florence, 1463) Neri used high-grade ultramarine of a value of 128 lire for the blue draperies but azurite for portions of the frame (Bicci 1976, 204). Apparently Neri saved ultramarine for the most important parts of his compositions, the blue draperies of the figures highest up on the religious hierarchy, usually the mantle of the Virgin.