This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Compound is the largest of the alchemical substance groups in this taxonomy, gathering ten preparations that share one trait: each is made by combining or transforming other substances rather than being taken from the earth or the workshop in an already elemental state. Some are straightforward by modern standards, Litharge and Luna Cornea are lead oxide and silver chloride respectively, Verdigris is a basic copper acetate grown deliberately on copper plates for pigment, and Flowers of Sulfur is simply elemental sulfur refined to a finer powder by sublimation, included here for its manufactured character rather than any change in composition. Others carry more history than chemistry, Butter of Antimony and Powder of Algaroth are linked antimony preparations, one made from the other, while Aethiops Mineral is chemically identical to bright red cinnabar despite being a dull black powder, a relationship early chymists plainly suspected without being able to confirm it.
Two members mark the group's fuzzier edges. Calx names an entire class of substances, the powdery residue left when any of the seven classical metals is roasted, so its entry describes an operation rather than one fixed compound. The Philosophers' Stone is fuzzier still, the legendary goal of transmutational alchemy itself, described across sources as everything from a red powder to a crystalline stone, with no single physical account ever agreed on and no modern chemical equivalent at all, included here as the compound the whole Great Work aimed to produce rather than one it ever reliably yielded.