August 20, 2012
Lasers ID ancient artists' intent
A new laser system aids art conservation and restoration.
By Ashley Yeager
Shooting a laser at a priceless 14th century painting may seem problematic. But, precisely tuned and timed, the laser system may be the only non-destructive way to get into the mind of long-dead artists like Puccio Capanna and determine his materials, techniques and intent for painting the Crucifixion around 1330 A.D.
Duke chemist Warren Warren originally designed the laser system, which uses less power than a laser pointer, to detect changes in the chemicals that give skin cells their color. But one day, while walking through the National Gallery in England, Warren wondered whether his laser system would be able to identify pigments in art the same way it does for chemicals in human skin. He says the new laser system is a tool that can identify both the three-dimensional structure of layers in the art piece as well as the chemical signatures of the pigments in it.
In art, the pigment is the material, a powder of salt or stone, which makes up the color of paint. Warren and his team tested the laser system with small samples of the aquamarine pigment lapis lazuli, the red pigment vermillion and several other synthetic and naturally made pigments. The scientists discovered that the laser system, which uses infrared light, could identify distinct chemical fingerprints for each of the pigments and that it could even distinguish between the man-made and natural versions. The tests showed it also could penetrate through discolored or deteriorated surfaces.
William Brown, a conservator at the North Carolina Museum of Art, says the results are promising. He was so impressed by the first set of experiments that he now feels confident enough to volunteer Capanna's the Crucifixion as the first full piece of art for scientists to analyze. The laser system could identify the precise pigments and layering techniques the artist used and possibly where Capanna's pigments came from geographically. It might also verify whether this piece came from the same altarpiece as a panel on display at the Vatican, Brown says.
Warren's group is not the only one at Duke to work on scientific methods for imaging art. Chemist Tuan vo-Dinh is developing a system based on nanoparticles, biochemical engineer Joe Izatt is using optical coherence tomography and computer scientist Robert Calderbank is using a form of mathematical analysis, called wavelets, to study brush strokes. The collaboration with the NC Museum of Art is not a first either. Brown has spent many years working with fine arts conservator Adele De Cruz, an adjunct associate professor of chemistry at the university.
About 15 years ago, De Cruz designed a laser technique to remove waxes, dirt and other incrustations from the surfacesof ancient paintings, sculptures and other works of art. She says Warren's laser system offers conservators a "new way to analyze works of art and use the information to organize our approach to cleaning it." The new laser system by itself cannot be used for cleaning an artwork, but it can provide "valuable information" about the raw materials an artist used and possibly who painted the piece, if the artist's identity is unknown, she says.
Conservators currently use x-ray and ultraviolet wavelengths of light to study paintings and also look at how light scatters off of the materials. Each technique, however, has limitations. Some can’t probe through layers of pigments. Others can't determine the chemical characteristics of the materials that make up a pigment.
The laser system is based on pump-probe spectroscopy. It is an imaging technique that shoots two laser pulses through a series of lenses and mirrors and into a custom-built microscope. A tiny sample of a painting, or in the case of The Crucifixion, the entire piece, sits on the stage of the microscope. The pulses are timed so that the pump pulse excites the molecules at the focal point of the microscope's lens. Then, after a billionth of a millisecond delay, the probe pulse hits the same pigment. The intensity of the probe pulse will change depending on its interaction with the excited pigment molecules. The change in intensity over time of the probe pulse gives each pigment a unique pump-probe signature, says Tana Villafaña, a Duke graduate student who works on the project. She says one of the clearest signatures the team has identified is from the aquamarine pigment lapis lazuli. Artists originally made this pigment from a relatively rare, semi-precious stone. The stone is ground, then the lapis pigment extracted through a process of mashing the coarse grind under water in a ball of wax and resin. The purified pigment is then tempered with egg yolk and water to the right right consistency and gloss for brushing it onto a canvas.
Originally the stone for lapis lazuli was found only in Afghanistan. When explorers discovered the new world, they found a similar stone in Chile to make the pigment. Now, paint companies manufacture it synthetically. Villafaña and Prathyush Samineni, who received his doctoral degree partially based on this work in July 2012, studied natural and synthetic samples of the pigment using the pump-probe laser system. To their surprise, each type – the synthetic, Chilean and Afghani version, each had a different chemical signature. The laser system can also recognize the chemical signatures of these pigments, even if they are layered beneath other colors.
"We're really excited about this tool, especially the 3-D and chemical analysis and that it's right down the road," Brown says. He's hoping that if the results from the analysis of lapis and vermillion on the Crucifixion come back with enough detail, perhaps the team could get Vatican conservators to send their Capanna piece to Durham. Studied together, the pieces might reveal more about Capanna, how he structured the altarpiece that encapsulated the works and possibly what story he was trying to tell.
The team is still testing and standardizing the laser system. If the research and development continues, Warren says it is possible that the laser system could be turned into a portable device, making this type of analysis easier for conservation scientists and art conservators around the world.
Pump-probe imaging of historical pigments used in paintings. Samineni, P. et. al. (2012). Optics Letters 37, 8: 1310-1312.