FOREIGN COLOR: Discovering an organic pigment, such as the red lac used on this 1150-1200 Morgan Madonna, helps scholars trace the trade of dyes across continents. Image: NATIONAL ACADEMY OF SCIENCES/PNAS
The chemistry skills and know-how needed to extract pigment from a plant or insect and create a solid dye date back some 4,000 years to ancient Egypt, new research concludes. Previous analysis had only confirmed such techniques to about 1200 B.C.
Analyzing pigment has been an important way of identifying and studying artwork for decades. Chemical profiles of colored material can tell a detailed story about when, where, how, and sometimes even by whom a piece of art was made.
But the older the remnant, the fewer fragments of color typically remain. And organic dyes—such as those derived from plants and insects—have required larger samples for analysis than inorganic colors extracted from the most ancient, mineral-based pigments. Taking a sizable sample of pigment from a painting or sculpture for analysis is "absolutely out of the question," says Marco Leona, a scientist at The Metropolitan Museum of Art (the Met) in New York City; doing so would destroy too much of the work. So, until now, many ancient works have kept their secrets locked away in natural pigments.
But by improving on a tried-and-true laser-based microanalysis technique, Leona, head of the Met's Scientific Research Department, has been able to analyze priceless pieces of pigment from extremely old cultural products without damaging their integrity.
The process—a version of surface-enhanced resonance Raman scattering (SERRS)—uses silver nanoparticles to absorb miniscule amounts of dye molecules. In doing so, it overcomes two key obstacles that had prevented the compounds from being studied in the past: The silver nanoparticles enhance the reading of the otherwise dilute dyes; they also prevent the otherwise fluorescent substances from reflecting too much light when a laser is shined on them.
The findings, published online today in the Proceedings of the National Academy of Sciences, reveal some of the oldest known uses of various natural dyes, including the discovery of madder (red, plant-based) dye on a leather Egyptian quiver remnant (2124 to 1981 B.C.)—thought to be the earliest example of such pigment use.
The results were obtained by analyzing microscopic samples, some less than 25 micrometers across. "We were surprised that it worked with…samples [that small]," Leona says. With previously available methods, he notes, "we would have had to remove a visible chunk."
The importance of studying organic dyes in addition to traditional mineral-based pigment is clear to Leona. "They correspond to major industries, commerce and technology," he says of the dye advances. The organic pigments can also be studied to identify forgeries and match similar works of art.