What It Actually Is

Butter of Antimony is identified as antimony trichloride, SbCl3, an identification with no real historical dispute, the compound's fuming, corrosive, waxy character matching the historical descriptions closely and its preparation from stibnite or antimony metal with mercuric chloride tracking a genuine, chemically sound route to the modern compound.

Antimony trichloride is properly classed today as a Lewis acid and a powerful hydrolyzing agent, meaning it reacts vigorously with water to produce the very antimony oxychloride precipitate, powder of algaroth, that alchemical chymists observed and exploited without the underlying acid-base chemistry to explain why pouring the butter into water produced a solid white powder rather than simply a diluted liquid. This entry's identification is one of the more secure in the entire table, resting on a clean match between recipe, observed behavior, and modern chemical formula. The compound remains in modern industrial use, chiefly as a catalyst and as an intermediate in producing other antimony compounds, a mundane afterlife quite removed from its dramatic seventeenth century reputation as one of the antimony literature's most discussed preparations.