By Gall-y!

Until the mid 1900's, most of the world's supply of tannic acid was obtained from the Aleppo gall (also called the Smyrna gall), found on oak trees (Quercus infectoria) in Asia Minor (Figure 1).   The trees produce gall tissue in response to a chemical substance secreted by the larvae of tiny wasps (Cynips gallae-tinctoriae -- Hymenoptera: Cynipidae) that infest the trees (Figure 2).   Larvae of these gall wasps live and grow inside the galls which unwittingly provide both food and shelter for the insect invaders.   50-75% of the gall's dry weight is composed of tannic acid.

Commercial interest in oak galls reached its peak in the United States around 1945.   In that year, more than 550,000 pounds of Aleppo galls were imported from Turkey.   Much of this tannic acid was mixed with copperas (ferrous sulfate) to make a high-quality, indelible ink that was used for keeping court records and for minting paper money by the US Treasury, the Bank of England, the German Chancellery, and the Danish Government.   The ink can easily be made using the following formula:

It is also possible to make "invisible ink" by omitting the copperas in the above formula.   A page with this "invisible" writing can be immersed in a dilute solution of ferrous sulfate to reveal the words.

Today, there is no longer a commercial market for oak galls because tannic acid can be extracted more economically from the quebracho tree (Schinopsis quebracho-colorado), a South American member of the sumac and poison ivy family (Anacardiaceae).