LITMUS, or LACMUS, in the arts, is a blue pig-
ment, formed from archil. It is brought from Hol-
land at a cheap rate: but may be prepared by adding
quicklime and putrid urine, or spirit of urine distil-
led from lime, to the archil previously bruised by grind-
ing. The mixture having cooled, and the fluid suffered
to evaporate, becomes a mass of the consistence of a
paste, which is laid on boards to dry in square lumps.
It is only used in miniature paintings, and cannot be
well depended on, because the least approach of acid
changes it instantly from blue to red. The best litmus
is very apt to change and fly.