Arsenic reached medieval alchemists not as a metal but as two brightly colored minerals, orpiment, a golden-yellow arsenic trisulfide whose Latin name auripigmentum means gold pigment, and realgar, a scarlet arsenic sulfide. Both had been mined and ground for pigment across the ancient and medieval world for centuries before anyone isolated the element itself, and both are seriously poisonous, which did not stop their extensive practical use.
The isolation of elemental arsenic from these ores is credited to Albertus Magnus, working around 1250, by heating orpiment together with soap, a preparation earlier Arabic alchemical writers had likely anticipated in some form though Albertus's account is the one that entered the Latin European tradition and carried his name forward. The achievement mattered beyond arsenic itself: orpiment's gold-like color had long made it a natural candidate for containing some fraction of gold's own essence in the alchemical scheme, and arsenic compounds genuinely do whiten copper into an alloy resembling silver, a visible transformation that helped keep the broader project of transmutation credible to practitioners watching it happen.
Arsenic never fit neatly into the alchemical system built around the seven classical planets and their metals; like antimony, bismuth and zinc, it was worked with constantly but never assigned a planetary correspondence of its own. Albertus's isolation of it stands, in the history of chemistry, as one of the few genuinely traceable "firsts" to survive a period whose chemical claims are mostly anonymous or legendary, credited to a specific friar at a specific and unusually well-documented moment in the thirteenth century.