Note 236 — Chapter XIV · (XXVIII.)

The word ‘bolus’ is derived from the Greek βῶλος, a lump or clod, and means, according to Murray’s Dictionary, a pill, or a small rounded mass of any substance, and also a kind of reddish clay or earth, used medically for its astringent properties, that was brought from Armenia, and called by the pharmacologist ‘bole armeniac.’ Its use in the arts is due to its unctuous character, which made gold adhere to it. See below. In mediaeval illuminations a ‘bolus’ or small lump of a properly prepared gesso is generally laid on the parchment where gold is to come, so that the raised surface may give the polished metal more effect. The gold over the bolus was always burnished. It may be noticed that our word ‘size’ is really ‘assise,’ the bed or layer under gilding, for which a gluey substance was suitable.