The first American demand notes of 1861 were called “greenbacks” because they featured a green ink on the reverse side to curb counterfeiting activities. The same technique was also used to produce the first legal tender notes from 1862 to 1863. A private patent date for this technique was also mentioned on these notes as PATENTED 30 JUNE 1857.

One of the writers of the Bank of Canada Museum, Graham Iddon believes that the technique was first applied on Canadian notes as the ink was invented there. He also notes that counterfeiting was easy in the 19th century due to the advent of photography. Blank pages were used to print fake money as no printing plates were required.

Notes were then printed using coloured inks. But counterfeiters removed the coloured ink, photographed the remaining black portions and reprinted the coloured portions. An American professor at Laval University in Quebec named Dr. Thomas Sterry Hunt was requested by the president of Montreal City Bank, to invent a special ink that would be harder to remove. He invented “Canada Bank Note Tint” ink made of an “anhydrous sesquioxide of chromium”. Chromium was decomposed by heating it an almost oxygen-free environment. The residue was mixed with linseed oil to make a green-coloured ink. It was chemically or physically hard to remove this ink form paper. Since Hunt was not a British subject, he could not patent the ink. A chemist named George Matthews from the Montreal City Bank filed a patent for Hunt and forwarded him the royalties.

It was later found out that the ink could be removed by boiling a note in sulfuric acid. It could also be removed without touching the other parts of the note. Finally, in August 1857 it was officially decided that the Patent Green Ink cannot be used as a security measure on banknotes. However, the Treasury Department or its private bank note printers accepted this method in 1861 and the greenback notes were introduced.

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