Almost everything said about the beginnings of alchemy runs through Zosimos, because he is the first practitioner whose own writing survives in bulk rather than in a later summary. He worked in Egypt, at Panopolis in the Thebaid, around the turn of the fourth century CE, and he wrote what appears to have been an enormous set of treatises, of which we have Greek fragments preserved in Byzantine manuscript collections and a larger body in Syriac and Arabic translation.
Several are addressed to a woman named Theosebeia, who is presented as a fellow practitioner receiving instruction. What survives is genuinely two things at once. There is careful workshop material on apparatus, on the construction of stills and furnaces, on the treatment of copper and the production of coloured surfaces, written by someone who had clearly done the work. There is also a religious framework drawn from Hermetic and Gnostic currents in which matter has a spiritual dimension and the operator's own condition is part of the operation.
Later writers who wanted alchemy to be only one of these had to read him selectively, and most did. Nothing survives of the alchemists he cites as his own predecessors except through quotation, which places him at the narrow point of the funnel: everything earlier reaches us, if at all, because Zosimos or a later compiler thought it worth copying.