LITMUS, in the arts, is a blue pigment, formed from archil. It may be prepared by adding quicklime and putrified urine, or spirit of urine distilled from lime, to archil previously bruised by grinding. The mixture being cooled, and the fluid suffered to evaporate, becomes a mass of the consistence of a paste, and is laid on boards to dry in square lumps. It is only used in miniature paintings, but cannot be much depended on, because the least approach of acid changes it instantly from blue to red.