One of several alchemical symbols for realgar as well as one for litharge.
Realgar is a a ruby-red paste compounded of sulphur and arsenic. It is used in the production of glass, small shot, signal lights, grounding paint for ships' hulls, etc. It was a substance that alchemists worked with continuously during the Middle Ages.
This particular version of the symbol is also used to indicate litharge, a protoxide of lead that is used used in the manufacture of glass, enamel, varnishes, paints and insecticides.