On September 28th, 2017 we learned about

Test authenticating paintings’ particular yellow pigment turns up with problems of its own

By the time you graduate from that first batch of eight crayons, you start to learn that just how much variability there is between one hue and it’s closest cousins. Despite similar naming conventions, you wouldn’t arbitrarily substitute Strawberry for Raspberry Red. These differences can even persist in what is supposedly the same color, depending on its formulation. A pigment known as Indian yellow is so particular it’s origins have become the stuff of slightly unbelievable legend. More importantly, the pigment has also become the center of controversy in identifying the dates and possible authenticity of many famous paintings from the early 1900s.

One of the best places to see Indian yellow is in the sunsets painted by Joseph Mallord William Turner. The impressive but not overly-saturated yellow of the late afternoon sun in Caernarvon Castle, for instance, is an example of a pigment that originally became popular with European painters back in the 14th century. Supposedly, it was produced only in Bihar province in India, where cows were fed only mango leaves. The cows’ urine was then collected and dried so that the remaining concentrate could be mixed with oil, creating the subtle hue that many artists desired. This method of production then continued until the early 1900s when it was halted out of concern for the cows, although that also happens to be around the same time when synthetic dyes and pigments were becoming more widely available.

Testing paint with the wrong test

Whether or not it was ever in a cow’s bladder, true Indian yellow is a magnesium salt of euxanthic acid. Its synthetic competitors were generally azo-based dyes, including tartrazine, a compound found in food coloring today. The synthetic, “fake” Indian yellow may have looked the part to most observers, but it was quite different on a molecular scale. This has proved useful to historians and curators, as the type of yellow could be used to prove when a particular painting was created (or at least altered or repaired.) By comparing the chemical signature of a euxanthic acid against an unknown painting, a researcher could tell if it was painted before or after the early 1900s. Theoretically.

This system was recently discovered to have a major flaw. People’s understanding of the two different paints was correct, but the “real” Indian yellow that they were often comparing new samples against was discovered to respond to measurements just like “fake” tartrazine pigments did. Essentially, people were checking for forged or altered paintings by comparing them to another “fake.” While figuring out how this mix-up occurred in the first place will take some time, curators are now looking to double check the paintings already in their collections. Fortunately, only Indian yellow made with euxanthic acid will fluoresce under ultraviolet blacklights. This low-cost test promises a quick alternative method of authentication, at least until a new spectral analysis standard can be established.

Source: The hunt for Indian yellow by Raychelle Burks, Chemistry World

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