Venetian grinds: the secret behind Italian Renaissance painters' brilliant palettes.

While sifting through 15th- and-16th century documents at the state archives in Venice, Louisa Matthew came across an ancient inventory from a Venetian seller of artist's pigments. The dusty sheet of paper, dated 1534, was buried in a volume of inventories of deceased persons' estates.

As Matthew, an art historian at Union College in Schenectady, N.Y., scanned the more-than-l00 items on the list, she realized that it was exactly what she had dreamed of finding. "I remember thinking, 'Did someone plant this here?'" she says. 'And why hadn't anyone noticed this before?" This inventory of artists' materials could hold the answer to a question that had long vexed conservation scientists: How did Venetian Renaissance painters create the strong, clear, and bright colors that make objects and figures in their paintings appear to glow?

The diversity of items on the list amazed Matthew. It included not only painters' pigments such as azurite, vermilion, and orpiment, but also raw materials used in a variety of crafts. "So, it wasn't just the painters who were buying from the color seller," she says. Glassmakers and dye-makers were also frequenting the shop. If the color shop was a nexus for all these different craftspersons, she reasoned, "maybe they were sharing ideas"--and materials too.

That last speculation spurred Matthew's colleague Barbara Berrie, a conservation scientist at the National Gallery of Art in Washington, D.C., to reexamine the Venetian paintings that she had been studying for the past few years. Previous analyses of microscopic paint samples taken from a handful of works had revealed many aspects of the artists' techniques, such as their process of layering colors, but art historians had found few recipes detailing how Venetian artists made their colors. "The materials on this inventory list suggested that we needed to look more widely," says Berrie.

Sure enough, when she re-analyzed her paint samples, she found a variety of types of glass particles mixed with the paint.

"It's a very exciting finding," says Jennifer Mass, who heads the conservation-science lab at Winterthur Museum in Delaware, adding that the optical properties of glass might explain the clarity and translucency of Venetian paints, which capture and reflect light in distinctive ways.

These findings and reports by several other conservation scientists imply that Italian Renaissance artists weren't merely painters. They were also experimental chemists who mixed and matched unconventional ingredients. Says Berrie, "They used new materials to create an art for their time."

SILICA GALLERY The presence of glass in Venetian paintings makes sense historically. "During the Renaissance, Venice was the glassmaking capital of the world," says Mass. By the late 15th century, the glassmaking industry was burgeoning, and with it came the creation of high-quality colorless glass, called cristallo, that was prized throughout Europe for its transparency and clarity.

Venetian glassmakers also made a wide variety of brilliantly colored glass objects.

Since Venice was a major port, it received textiles, dyes, ceramics, gems, and other goods from all over the East. These imports inspired the local craftspeople. For instance, to satisfy the Venetians' growing taste for ancient and expensive artifacts, glassmakers figured out how to make fake precious and semiprecious stones out of colored glass. "It was a time of tremendous innovation," says Matthew.

In an age of new ideas, it's no coincidence that Venetian painters exploited novel materials to expand their art. Before Berrie's discovery of glass particles in paintings at the National Gallery, she had some evidence suggesting that Venetian painters experimented with other unconventional materials.

The artist Lorenzo Lotto kept a painting notebook in which he listed materials that he bought. Among them were mercury and sal ammoniac (ammonium chloride), the latter being a white or colorless crystalline salt found in volcanic regions. Although these substances were common in alchemy, Berrie suspects that Lotto used them to adjust his colors.

Using scanning electron microscopy, energy-dispersive spectrometry, and other sophisticated analytical techniques, Berrie began to explore the glass-in-paint idea. She started with Lotto's 1522 painting "Saint Catherine" (see cover).

Berrie's microscopic sample came from the sleeve of Saint Catherine's red dress. Viewed in cross-section, the paint can be seen to contain round silica particles 4 microns to 8 microns in diameter that were mixed with red pigments known as lakes.

Upon closer examination, Berrie saw that the silica represented a high-quality form routinely used by Venetian glassmakers. During the Renaissance, they obtained it from quartzite pebbles along the Ticino River in northern Italy. They would then grind the quartzite into a fine powder, says Berrie, who presented her findings at the Materials Research Society meeting in Boston last December.

"For the Venetians to be able to use this ultrapure source of silica was a real technological innovation" says Mass. Traditionally, glass was made from sand, which is loaded with impurities such as iron. The iron gives glass a green tint. Using pure silica, helped Venetian glassmakers to create their colorless cristallo.

Perhaps Lotto was trying to achieve the same clarity in his paintings. "He was layering these paints so thinly, he must have been taking advantage of glass' optical properties;' says Berrie.

The glass particles made his paint somewhat transparent, so light shining through one layer would accentuate the color in an underlying layer more so than would paint devoid of such particles.

What's more, varying the amount of glass in each layer created a more-or-less transparent paint. Therefore, instead of a uniform tone, the glass enabled Lotto to create a more vibrant surface, says Berrie.

SPLASH OF GLASS Lotto and other Venetian artists may have used glass to do more than just add vibrancy to their paintings. Glass particles literally expanded the artists' palette, creating colors that were not otherwise possible.

Before Berrie's investigation, researchers knew of only two types of colored glasses used by Italian Renaissance painters. One was a yellow glass called lead-tin yellow, and the other was blue smalt, a potassium silicate that derived its color from small amounts of cobalt. Artists frequently used blue smalt as an alternative to the more exotic and expensive blue pigment lapis lazuli which was imported from the Middle East.

Now, it appears that painters used glass to expand their choice of colors. For example, in Lotto's 1523 "The Nativity," Berrie found yellow glass particles in a sample taken from Joseph's orange robe. Unlike lead-tin yellow, the particles included antimony and potassium, as well as lead. The antimony gave the glass a hint of orange that would have enabled Lotto to achieve a warm tone, says Berrie.

In the 1575 painting "Christ at the Sea of Galilee" by Jacopo Rubosti, known also as Tintoretto, Berrie found a green-colored glass. Under high magnification with a scanning electron microscope, Tintoretto's green particles are seen to contain areas of blue from cobalt and areas of yellow from iron. The mix of colors summed into a green hue.

Because iron made up a relatively large proportion of the glass, Berrie reasoned that it could not have been a natural impurity. She says that the green smalt is "definitely something that would have been manufactured specially." Glassmakers probably made green smalt and then peddled it to color sellers.

A second sample taken from the same Tintoretto painting contains a mysterious yellow glass particle. This pigment appears in an intense yellow layer under the green paint illustrating parts of the stormy sea. Initially, the material appeared to be ordinary lead-tin yellow. Upon closer examination, however, Berrie discovered that it has a slightly different chemical composition, suggesting that artists such as Tintoretto used a range of yellow glass pigments, each variant perhaps created by glassmakers who tweaked its chemical makeup.

POWER PAINT Mixing glass with paint may have served a non-aesthetic purpose as well. Marika Spring, a conservation scientist at the National Gallery of London, has found glass in several works by two other Italian Renaissance painters--Perugino and Raphael-who mixed powdered colorless glass with red lake pigments and many others.

Rather than enhancing the colors, the glass likely functioned as a desiccant, or drying agent, Spring says. Before the 15th century, Italian painters mixed their pigments with tempera, an egg yolk solution that dries quickly on wood or canvas. However, when painters switched to oil-based paints, they encountered a problem: Oil dries very slowly. Because artists applied several layers of paint to their panels to create certain effects, 'having a way of making the paint dry quickly was fundamental" says Spring.

The transparency of the glass may also have been important, she notes. The red-lake pigments were already quite transparent. It would have made sense for painters to choose a transparent drying agent, such as colorless glass, to preserve the transparent quality of the paint, says Spring.

In Perugia, Italy, at a workshop in 2003 on Perugino's painting technique, Spring ran into several other conservation scientists who have found powdered colorless glass in 15th- and 16th-century Italian paintings.

Claudio Seccaroni of the Italian National Agency for New Technologies, Energy, and the Environment (ENEA) in Rome analyzed dozens of Perugino's works using a technique called X-ray fluorescence analysis. With this method, which does not require the removal of paint samples, Seecaroni and his colleagues detected significant amounts of manganese associated with layers of red lake.

Normally, lake pigments do not include manganese, but that element was a standard ingredient in common colorless glass formulations.

Coincidentally, manganese is a drying agent. Spring suspects that painters chose colorless glass to mix with their paint because of its quick drying effects. These studies, combined with Berrie's findings, suggest that different artists may have used glass for different reasons.

Now researchers plan to re-create glass paints in the lab. "This would be an opportunity to understand better how these artists worked, to reproduce the effects they were creating,' says Mass.

As the story unfolds, Berrie says, she appreciates more than ever the ingenuity of Venetian artists and the beauty of their work. "Part of their greatness was to step beyond the established practice and adopt materials from other crafts and trades," she says. Not only did the artists use glass to fill gaps in their palettes, but the materials' optical effects--intentionally or not--also brought their paintings to life in unprecedented ways.