A swelling, also known as an oak apple, that forms on the bark of an oak tree after it has been stung by an insect laying its eggs. Tannic and gallic acids contained in gallnuts can be soaked out in water, the gall solution forming the basis of ink. Gall can also be used in tanning processes.

Michelle Brown
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Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).