Oil is used in paint to help fix the dye and help it adhere to a surface. Oil also changes a paint's drying time and viscosity.
More complex than the standard mineral pigments and animal glue previously favored, the technique hints of Indian, West Chinese, and Mediterranean influences, Taniguchi said.
The murals were likely completed by teams of artisans, as was common in Asia until recent times.
The mixed layers of organic material such as oil and resins blended with pigments is quite a "sophisticated manner" of painting, she noted.
The Bamian murals might also be the first confirmed use of resins in paintings, Taniguchi added.
Drying oils have been identified in a number of medieval European and Byzantine paintings. Such substances, for instance, were being used throughout Europe by around A.D. 800.
But the Afghan research shows that the chemical properties of oils were known long before then.
"The use of drying oils in painting clearly shows an understanding of the properties of this material," said Ioanna Kakoulli, a materials archaeologist at the UCLA/Getty Conservation Program in Los Angeles, California, who was not involved in the analysis project.
Kakoulli and fellow researchers will soon announce the discovery of oil-based paint in a Byzantine mural in Cyprus dating to the 12th century A.D.
"... To date, the [Afghan] murals are among the earliest examples where drying oils have been identified as binding media in painting," Kakoulli said.
Sharon Cather is a wall-painting expert from the Courtauld Institute of Art in London.
"The discovery of the use of oil [in Afghanistan] is important, because it shows that these undervalued paintings are far more important and far more sophisticated than anyone might have thought," Cather said.
The method also "very probably reflects [the] prevailing painting practice in the region," she added.
Other paintings of similar technical complexity and age can be found along the Silk Road, which was "the avenue for the diffusion of Buddhist religion and Buddhist art," Cather said.
(Related photo: "'Stunning' Buddha Art Found in Nepal Cliff" [May 7, 2007].)
Not all of the cave murals contained oil-based paints, though, or used them in the same way.
"Some paintings from other caves were depicted with different materials and techniques," Japanese researcher Taniguchi said.
"This shows how different painting techniques were introduced in Bamian from different regions in different periods of time."
Further analysis of Central Asian sites might provide even older examples of oil-based paintings, Taniguchi said.
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