By the nineteenth century the phrase had been detached from the furnace. In the Hermetic Order of the Golden Dawn and the orders descended from it, the Great Work names the whole course of initiation, and its central attainment is described as the knowledge and conversation of the Holy Guardian Angel, a term the order took from an earlier grimoire tradition. The alchemical vocabulary is retained as a map of stages rather than as a set of instructions about matter.
Aleister Crowley used the phrase constantly and defined it in several ways: as the union of opposites, as the accomplishment of the True Will, and as the process by which an individual identity is brought into right relation with what lies beyond it. In Thelemic writing the operation of the alchemists and the operation of the magician are treated as one thing described in two vocabularies.
Carl Jung read the literature a third way, as a record of processes in the psyche projected onto matter by people who had no other language for them, and he mapped the colour stages onto stages of individuation. His reading has been enormously influential and is also the one that historians of alchemy most often push back against, on the ground that it treats the chemistry as merely a screen.
The two senses of the phrase are best understood as one term used by two trades that overlapped for centuries, and a reader meeting it has to ask which trade is speaking.