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When you look at a dollar bill, you are looking at a very particular color green. That color green was invented in 1857 by a Canadian chemist named Thomas Sterry Hunt. Like other chemists of his day, he used natural pigments and laboratory chemicals to invent new colors with special properties. In this case he used chromium trioxide to make the green ink. So why is this such a big deal?

It was the habit of counterfeiters to erase the denomination on the paper money (the part that tells you how much it’s worth) and then fill in a larger value. But unlike the other inks that came before, Hunt’s new ink was extremely difficult to erase. To further stymie would-be counterfeiters, this ink color could not be successfully photographed or photocopied! And it’s practically indestructible! Acid won’t affect it and neither will most other chemical agents.

Hunt sold his invention to the U.S. government and the green ink has been used since 1862 to print U.S. banknotes (paper money). And that is how our American paper money became known all over the world as the “greenback.”