Alchemy is the science of the transformation of matter. In the crucible, the alchemist places the prima materia, the base matter, which, initially, dies and rots down; this operation corresponds to the colour black.
Next, the matter is dissolved and purified and becomes white. Then come distillation and conjunction where the matter turns red, and finally, sublimation where the colour turns to gold.
These operations are only meaningful to us if we see them as different stages of our inner life.
The work alchemists perform on the matter in a crucible is, in fact, the work of regeneration disciples perform in the crucible of their own body, a work to which they must devote their entire life. The regenerated matter comes out of the crucible transformed into gold: a regenerated human has died to their lower nature to been born to their higher nature.
This is also the meaning of the words of Jesus, ‘Unless a grain of wheat falls into the earth and dies, it remains just a single grain; but if it dies, it bears much fruit.’ This sentence can be taken as a summary of the alchemical process: dying to our lower nature so as to bear the fruit of our higher nature.