LAC'MUS, noun A blue pigment, formed from archil, a species of lichen. [See Archil.] It is prepared by bruising the archil, and adding quick lime and putrefied urine, or spirit of urine distilled from lime. The mixture, after cooling and the evaporation of the fluid, becomes a mass of the consistence of paste, which is laid on a board to dry in square lumps.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI