The Weird History and Chemistry of the Pigment: Indian Yellow

Introduction:

Indian yellow was a popular paint that is no longer a well known with an origin shrouded in mystery. While you might not be aware of it, you have almost 100% seen it before.  

    It is a very luminous yellow and one of the first to have the purity of yellow that it can display. Earlier yellows (orpiment and yellow ochre, for example) are rather muddy and border on orange or brown and can be dull. Indian yellow can be very bright and saturated, as shown below:

The History:

As previously mentioned, the origin and use of Indian yellow is inconclusive. Some stories have been told as to how it is made. The most popular and widely-reported story is that cows are separated from the rest of the herd and put in an enclosed area of sand. The cows are given nothing but mango leaves to eat, and little to no water to drink. This makes them very dehydrated, concentrating their pee into a vibrant yellow. The pee is absorbed by the sand, which is left to dry. The clumps of dried sand are eventually dug out and refined into a more pure substance.

    The veracity of this story has been called into question as there are variations, such as the urine being directly collected from the cow and then boiled. Others claim it is derived from plants entirely with no animal involvement. 

    The pigment was, as the name suggests, created in India. Europeans gave it that name once it was imported started in the 1400s. When the stories of the production method for the paint were circulating in the mid-to-late 1800s the popularity and use of the pigment started to wane. It continued in use until 1921 when it officially became commercially unavailable. There were some reports/claims that the practice of creating Indian yellow was outlawed in 1908 due to concerns of animal cruelty, but these reports cannot be confirmed, so take that with a grain of salt.

    The color was quite popular for a time because of the properties noted earlier. It first saw its use as a dye for clothing, wall coloring, Rajput miniatures (Mughal era Hindu court style of painting), and frescoes. One example can be seen above this section. Once it was imported to Europe it saw use in oil and water-based paints. There have been many paintings found to contain this pigment. One of which is Joseph Mallord William Turner's The Angel, Standing in the Sun (1846):

You may ask yourself, "You said that I had seen it before but I have never seen any of these examples before. Did he lie when he said that I have seen it?" And I would respond with no, I just haven't revealed it yet. The most famous example of Indian yellow's use is Vincent van Gogh's Starry Night from 1889.

Lots of the yellows of this painting were at the very least mixed with Indian yellow. The most prominent yellow feature that is sure to have the most Indian yellow is the moon in the upper right corner of the painting.

The Chemistry:

While there is no definitive answer as to how to pigment was produced, chemical analysis has been performed on samples of the pigment. This can potentially give a good idea as to how it was created through the chemicals found.

Formula: C₁₉H₁₆O₁₁Mg · 5 H₂O

    The actual chemical makeup of this pigment is also hard to determine as it has been noted to change from sample to sample or source to source. What is commonly found among the samples of pigment clumps is magnesium euxanthate. This chemical is the magnesium salt of euxanthic acid. Salts come from neutralized acids or bases (base in this case) and the formula shows a magnesium atom bonded to the euxanthic acid. This acid is classified as a xanthonoid, or a phenolic organic compound with a heterotricyclic shape.  

To break it down to more understandable terms, a phenol group (picture above on top) is a ring of carbon (pictured below the phenol) that has an -OH (hydroxyl group) bonded to it. Heterotricyclic seems tricky, but it is quite simple when broken down to root words. Hetero = different, tri = three, and cyclic = ring or circle. Put them all together and it is just a fancy term for three rings that are together and aren't entirely made up of the same atom (the rings aren't all carbon).

Now that you know what a xanthonoid is, you can clearly see it in this diagram of a euxanthic acid. The three rings of mostly carbon (notice the oxygen in the ring) bonded with -OH are on the left side of the diagram. But what is the rest of that stuff? Well, euxanthic acid is also a glycoside, which means a sugar bonded to another functional group. Plants produce glycosides to make certain chemicals inactive to store them for later use. 

So, we have established what this chemical is and that it comes from plants. The question now is, do mango trees produce this chemical. According to Science Direct and Frontiers, the answer is yes. Mango trees actually produce multiple xanthones. Mangoes also contain magnesium. SO the chemical can definitely come from mango leaves. 

Modern Applications:

As mentioned earlier, this pigment has been unavailable since at least 1921 due to ethical concerns. However, there are modern methods of recreating it. Either through synthetic recreation of its compounds, or recreating it through mixing modern paints that are commercially available. The latter is much more cost effective and viable for the normal person that isn't a chemist. The modern mix for Indian yellow consists of nickel azo, Hansa yellow, and quinacridone burnt orange.