Bright and Poisonous Colors

Part Four of Hugh Aldersey-Williams’ Periodic Tales begins with a section titled Chromatic Revolution. He starts off with a story about finding his father’s old artists’ paints while clearing out some boxes. Chrome Yellow, Viridian, Vermillion, and Greens “rich in arsenic” are mentioned. It reminded me of my own childhood, reading the paint tubes of an art set, although I’m guessing my paints were less toxic? (Painting was a required course in my elementary school. I was terrible at it.) What I did not know is that the one element that “provided more and brighter artists’ pigments than any other” was cadmium.

Cadmium was discovered by Friedrich Strohmeyer, professor of chemistry and pharmacy at the University of Gottingen. In 1817, he also did double duty as the “inspector of apothecaries”, and it was during one of his inspections that “a preparation of medicinal zinc oxide was clearly not what it was purported to be.” When heated, the substance turned yellow, then orange, but no lead was found upon further analysis. Strohmeyer went to the source (a chemical factory), obtained a sample, and chemically removed the zinc. This left him with a “pea-sized lump of a bluish grey material, rather like zinc in appearance, but shinier”. Cadmium is named after calamine, the zinc ore where it is found. Many new metallic elemental discoveries were made in this way because elements closely related to each other (sometimes by ionic charge, sometimes by ionic size) could substitute each other in an ore. Cadmium is just below zinc in the periodic table. Poisonous mercury is just below cadmium. Zinc, cadmium and mercury are in the same column of our periodic table.

Cadmium sulfide was a hit among painter-artists. By varying the level of different impurities in the salt, many vivid colors became accessible. These ranged from red, orange, yellow and green, although not blue. (Blue has always been a difficult color!) The author writes: “As each new tint became available, it powered in turn the yellow sunsets of Monet, the orange-soaked Arles interiors of Van Gogh…” There was speculation that Van Gogh’s mental state was affected by noxious paint pigments. As recently as 1989, the Rhode Island senator attempted to ban cadmium in pigments to prevent leaching of toxins into the water supply. Artists vocally protested what they saw as ‘chemical censorship’. In fact, paint on canvas doesn’t cycle back into the environment unless the paintings go into the landfill (a horror to artists!). Hence artists’ paint is a drop in the bucket where the commercial use of colored pigments is concerned. Industrial exposure to cadmium, which has many other uses, turns out to be the much greater problem.

Aldersey-Williams writes: “It seems beyond sad – almost a moralistic affront to our capacity for sensuous delight – that so many of the highly coloured chemicals should also be poisonous. This true not only of the salts of cadmium but also of many long-known pigments such as yellow lead-chromate and the vermillion of mercuric sulphide. Poisons in fairy tales often come in coloured bottles, or are coloured themselves. Christian Dior’s counter-intuitively marketed perfume Poison exploits this mythology in a purple glass bottle shaped like an apple.” It’s no wonder that in the TV series Once Upon A Time, the evil queen (of poison apple fame) has purple as her color of magic. Although interestingly, the bright colors of edible apples or poisonous berries are not due to metals but to organic compounds with extended pi-delocalized systems such that electronic transitions are in the visible range of the electromagnetic (EM) spectrum.

While leaves and fruit come in red, orange, yellow and green, you don’t see blue. It’s a tricky color for a variety of reasons. (Incoming brainwave: I think I’ll pose this conundrum to my chemistry students!) There aren’t many blue salts. I learned from Periodic Tales that the great chemist Berzelius prepared various vanadium salts including ones that are bright turquoise and pale sky blue, preserved in test tubes at the Berzelius museum in Stockholm. Most familiar to us, though, are salts of cobalt and copper. Copper(II) sulfate is the standard blue solution found in introductory chemistry labs.

I remember Cobalt Blue being one of the names in an art set paint tube. I did not know that its name comes from an evil spirit associated with one of the four elements of the ancients. Apparently the popularity of the blue hue along with the discovery that smaltite (the cobalt ore) was found in silver mines, led to the Saxon miners blaming their difficult work on the Kobold – a small Earth demon from Germanic middle ages mythology. Aldersey-Williams writes: “The labor was hard and involved exposure to harmful fumes, released when the main ore’s other main ingredient, arsenic was roasted off.” Kobold blues, you might call them, in analogy to ‘cadmium blues’, the symptoms of being poisoned by cadmium vapor.

Octarine is the color of magic in Terry Pratchett’s Discworld series. Only magic users and cats(!) can see it. Eight is the magic number of the Discworld, and Octarine is the eighth color of the EM spectrum. How fitting that magic is associated with the EM spectrum! In that world, magicians would recognize the signs of magic anywhere, and muggles would not. Early this year, there was a campaign to have Element 117 be named Octarine, although Tennesine ended up being chosen. (I should have told my students this when one group used in their “invention of new compounds” assignment this past semester, but I did not know about the campaign until recently.)

Who would have thought colors were so interesting? I’m already leaning to colors and light being the theme of my General Chemistry class next Fall. Wow, I’m really thinking ahead now! First, I have to design my Potions themed class. But maybe I can sneak in some ideas about colored potions!

[For the previous blog post on Periodic Tales, click here. I’ve posted more on this book than any other thus far, excluding the Harry Potter series.]