The word derives from the Latin encaustum ('burnt in'), since the gallic and tannic acids in ink and the oxidation of its ingredients cause it to eat into the writing surface. The basis of medieval ink was a solution of gall (from gallnuts) and gum, coloured by the addition of carbon (lampblack) and/or iron salts. The ferrous ink produced by iron salts sometimes faded to a red-brown or yellow. Copper salts were occasionally used too, sometimes fading to grey-green. Ink was used for drawing and ruling as well as for writing and, when diluted, could be applied with a brush as a wash.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).