A color that induces euphoria in the brain also has a sinister side.
A painting by Giotto titled “The Kiss of Judas” hangs in the Scrovegni Chapel in Padua, Italy. It depicts a crowd of intertwined figures in multi-colored cloaks—but one stands out from the rest, brightly distinguished by a patch of dense gold. It is Judas, draped in a bright yellow cloak, placing his treacherous kiss on Jesus’s cheek. Nothing else in this painting matters more than this almost neon-like robe and the color yellow—used so dramatically that Giotto must have had important reasons for doing so.
If we wish to tangibly convey an impression of warmth in a pigment, in just one color, many will immediately choose yellow. For most of us, this is a color that delights our minds. Resulting from the stimulation of the two most numerous photoreceptors in our eyes—those that detect red and yellowish-green—yellow gives us a feeling and impression of warmth, radiance, comfort, and coziness, crystallized in a single color. Although in the middle of the day the light of our nearest star—the sun—is almost white, in children’s drawings the sun is always a golden orb, often painted very carefully so as not to muddy its perfect yellow hue, its fiery heat. This sensory brilliance of yellow is scrupulously exploited.
Every lighting technician is familiar with the value of 589 nanometers. It’s the measurement for a particular narrow section of the rainbow—the very section that contains golden yellow. This light is emitted by common sodium-vapour lamps, in which heated sodium glows with a dazzling, perfect hue of yellow (you can easily confirm this by throwing a pinch of table salt, i.e., sodium chloride, into the bluish flame of a gas cooker; the sodium released from it and heated to luminosity will instantly tinge the flame with golden streaks). We all know sodium-vapour lamps very well, for most city streets and motorways are illuminated by them. Thanks to the stimulation of the most numerous photoreceptor cells in our eyes, we see the most clearly in the light of such lamps, even when the road is shrouded in fog that obscures the contours of objects.
But yellow is also the hue of people’s skin when they suffer from liver ailments. And it’s the irritating tint of photographs in which digital cameras try to fix the bluish light of fluorescent lamps and, by filtering out this blue, sometimes go a step too far, coating everything with a drab yellowish tinge. We also dislike how, over time, the pristine whiteness of paper becomes yellowed: if, in addition to cellulose, a sheet of paper contains large quantities of lignin (an integral element of the type of wood used in paper manufacturing), over the years it oxidizes, producing yellow-tinted chromophores.
When Giotto painted his Judas, did he intentionally clad him in that sickly, rotten yellow signifying decay and weakness? There is something in this, for mediaeval and Renaissance depictions of Judas often presented him cloaked in folds of yellow and gold, as if singling him out with the brightest color stigmatized his treachery even more. But in connection with Judas’s golden cloak depicted by Giotto and many other painters of the epoch, there is another quite sinister and long-forgotten aspect of the color yellow. Painters of that era had access to several types of yellow pigment, differing in the chemical details of how they filtered colors from light. One version seemed particularly ominous—and it’s the one that still, even today, glows with a neon radiance on Judas’s cloak. The pigment known as “Indian yellow” was adored by artists; it was wonderfully bright and translucent, and very easy to blend with water and oil. In daylight, this color seemed to shine with its own inner radiance—all thanks to the natural fluorescence of the derivatives of euxanthic acid contained in the pigment. Painters loved it, despite its repulsive and suffocating odor. Indian yellow reached the peak of its popularity in the nineteenth century only to suddenly disappear from the market in the 1920s.
The reason why Indian yellow was withdrawn from use was by no means its smell, although the stench was a direct result of the method used to produce the pigment. The details of this process are known to us thanks to only a few documents that have survived to the present day. Scientific research has quite clearly confirmed the accuracy and reliability of these old descriptions.
The people of India mastered the art of obtaining Indian yellow, using cows for this purpose. In order to force the animals to produce large quantities of euxanthic acid, they fed them mango leaves for their entire lives—and nothing else. This draconian diet brought the animals to the brink of starvation and forced their livers to produce by-products that ended up in their urine. After the urine was collected and cooled, it was evaporated, and the resulting bright yellow powder, called purree, was shaped into small balls and packed for shipment. The painters received it either in this raw form or after additional processing to remove the greenish pigments present in cow urine.
In 1908, the Indian government banned the production of this pigment, declaring the whole process profoundly inhumane. Nowadays, fortunately, we don’t need Indian yellow. Its synthetic alternatives aren’t only just as intense, but also more resistant to sunlight.
But whenever you admire paintings by Giotto, Turner, or Van Gogh, know that an infamous pigment—the product of selfishly manipulated cow physiology and a testament to the wildest human fantasies and the quest for a perfect color—is responsible for every golden glimmer, such as the stars and galaxies rippling across the sky in Van Gogh’s frenzied The Starry Night.
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