The American “greenback,” probably the most famous paper currency in the world, owes its vibrant hue to a Canadian discovery. The “green” is chromium oxide, a refined version of chromite, a naturally occurring ore in the Earth’s crust. This ore is also the source of chromium metal which is produced by heating the ore in the presence of silicon and aluminum. The metal derives its name from the Greek “chroma” meaning colour, because all of the compounds of chromium are coloured. Lead chromate, for example, was the original pigment used in paints for school buses and “yellow” cabs. Because of the toxicity of chromium VI and lead ions that make up this pigment, its use has declined.
The brilliant red colour of rubies is due to traces of chromium III ions embedded in the aluminum oxide that constitutes the base of this gem. Emeralds are composed of beryllium aluminum silicates but the stunning green colour is due to traces of chromium. When a small amount of chromium is present in the mineral beryllium aluminate, or chrysoberyl, we have one of the rarest of gemstones, called alexandrite. It was supposedly named in honour of Tsar Alexander II because the mineral was first found in the Ural Mountains of Russia. The amazing feature of alexandrite is that its colour changes depending on the source of the light in which it is viewed. It is deep red by fire light and green or pink in daylight. It takes a lot of greenbacks to buy any of these gems.
The first U.S. bills to be coloured with chromium oxide were introduced in 1862 during the American Civil War. President Lincoln needed money for the war but the nation’s 1600 or so private banks wanted interest of more than 20% as insurance should the South prevail. Lincoln wasn’t going to end slavery and then become a slave to the banks. The problem was that there wasn’t enough gold or silver to back the minting of any more currency, but after much discussion, Congress passed a bill authorizing the printing of full legal tender treasury notes that were backed by nothing more than the government’s saying that they were to be accepted by everyone. The green colour was a proprietary version of chromium oxide that had been developed by Thomas Sterry Hunt, a chemist who worked for the Canadian Geological Survey and also lectured at McGill University.
Hunt was American born and attended Yale before taking up a post in Canada where he became one of the original members of the Royal Society of Canada and eventually its president. He didn’t profit much from his invention of using chromium oxide to dye paper, selling the idea to the Americans at too low a price. Chromium oxide turned out to be ideal for currency because it stood up well to exposure to acids and bases and could not be easily copied by photographic methods. Hunt can lay claim to another contribution to science. In 1878 he was the first to propose the theory that climate change could be linked to concentrations of carbon dioxide in the atmosphere.