Mural paintings in a tomb at Luoyang, dated to the Western Han dynasty between 206 BC and 8 AD, carry barium copper silicate pigments that do not occur as minerals. Their artificial nature was never in doubt. One crystal in the samples was another matter.
Three man-made blues and a puzzle. The murals carry , Chinese Blue and Chinese Dark Blue, all barium copper silicates, plus platelet-like crystals of a barium tin silicate whose synthetic or mineral origin had to be worked out. The rare mineral turns out to be involved.
The murals come from a tomb in Luoyang City, Henan Province, dated to the Western Han dynasty, which ran for roughly two centuries either side of the turn of the era. On them are Chinese Purple, Chinese Blue and Chinese Dark Blue, all barium copper silicates, together with platelet-like crystals of barium tin silicate.
Two samples, LY01 and LY02, went through powder , scanning electron microscopy with elemental analysis, and spectroscopy. Each method answers a different question, and together they identify compounds rather than ingredients.
The artificial nature of the ancient barium copper silicates was undoubted. Making them requires deliberate high-temperature chemistry, which is why these pigments are evidence about technology and not only about taste.
The barium tin silicate was the loose end, since it could have been synthesized alongside the rest or collected as a mineral. The authors clarify it through the involvement of the rare mineral pabstite, which puts part of the palette in the ground rather than in the furnace.
wat. The purples and blues on the wall were manufactured, not dug up.
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A synthetic pigment is a chemical industry, so identifying one dates the knowledge rather than the painting.
How do we know?
Zhang and colleagues analyzed the two Luoyang samples by archaeometric methods: powder X-ray diffractometry, scanning electron microscopy with elemental analysis, and micro-Raman spectroscopy.
What is still unknown
Two samples from one tomb, and the question of where the barium tin silicate came from is the part the paper has to argue rather than read off.
Learn something?
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