Carl Jung thought that alchemists projected their unconscious on the chemical experiments they performed but there is a more simple and logical explanation to explain what alchemists depicted in their grimoires.
In Psychology and Alchemy, Carl Jung asserts his hypothesis that alchemists were projecting unconscious contents on their chemical experiments. He wrote
The real nature of matter was unknown to the alchemist: he knew it only in hints. In seeking to explore it he projected the unconscious into the darkness of matter in order to illuminate it. In order to explain the mystery of matter he projected yet another mystery—his own unknown psychic background—into what was to be explained: Obscurum per obscurius, ignotum per ignotius! This procedure was not, of course, intentional; it was an involuntary occurrence.(CW 12, par. 345)
Jung obviously take for granted that alchemists were really doing chemical experiments which is not at all certain. When you understand that alchemists were always talking about the development of consciousness and the mystical experiences or conjunction of opposites which stem from that process, you discover that real alchemists never did the chemical experiments displayed in their books.
By stating the obscure by the means of the most obscure (obscurum per obscurius), in the quote above, the alchemists expressed that their recipes were symbolic of very precise processes. It was not an indication that there was something obscure or something that they could not explain in their experiences as Jung suggested. They knew exactly what they were talking about but they had to bury their teachings under a layer of symbols to protect themselves from the Church and the Inquisition. The key to decipher these processes relied on the elucidation of the personal symbols they used in their grimoires.
Jung quote Hyle und Coahyl from Jurain:
Take of common rainwater a good quantity, at least ten quarts, preserve it well sealed in glass vessels for at least ten days, then it will deposit matter and faeces on the bottom. Pour off the clear liquid and place in a wooden vessel that is fashioned round like a ball, cut it through the middle and fill the vessel a third full, and set it in the sun about midday in a secret or secluded spot.
When this has been done, take a drop of the consecrated red wine and let it fall into the water, and you will instantly perceive a fog and thick darkness on top of the water, such as also was at the first creation.
Then put in two drops, and you will see the light coming forth from the darkness; whereupon little by little put in every half of each quarter hour first three, then four, then five, then six drops, and then no more, and you will see with your own eyes one thing after another appearing by and by on top of the water, how God created all things in six days, and how it all came to pass, and such secrets as are not to be spoken aloud and I also have not the power to reveal. Fall on your knees before you undertake this operation. Let your eyes judge of it; for thus was the world created. Let all stand as it is, and in half an hour after it began it will disappear.(CW 12, par. 347)
To Jung, that fragment was one proof of the projections the alchemist might expect. His proof is very thin because it does not explain why suddenly, the alchemist would stop being symbolic about the process and begin detailing the visions that might occur.
To support his hypothesis, he added another quote from a treatise by Theobald de Hoghelande (sixteenth century):
They say also that different names are given to the stone on account of the wonderful variety of figures that appear in the course of the work, inasmuch as colours often come forth at the same time, just as we sometimes imagine in the clouds or in the fire strange shapes of animals, reptiles, or trees. I found similar things in a fragment of a book ascribed to Moses: when the body is dissolved, it is there written, then will appear sometimes two branches, sometimes three or more, sometimes also the shapes of reptiles; on occasion it also seems as if a man with a head and all his limbs were seated upon a cathedra.(CW 12, par. 349)
Jung added in the next paragraph:
Hoghelande’s remarks prove that during the practical work certain events of an hallucinatory or visionary nature were perceived, which cannot be anything but projections of unconscious contents. KCW 12, par. 350l
Here again, the proof is extremely thin. Why would Hoghelande stop using symbols and begin describing visions? It would be against their desire to protect themselves. What is most probable is that the entire paragraph is a symbolic description of a psychological process. The appearance of the branches, the reptiles and the man with his head and limbs are not descriptions of hallucinatory visions that the alchemist would experienced, but symbols of the stages in the development of consciousness. Tree branches are well-known symbols of ideas and spiritual competences. Reptiles have long been associated with initiation and rebirth because they shed their skins. The man with head and four limbs is a symbolic depiction of the four elements and the quintessence, the fifth element.
In their time, alchemists knew that consciousness was composed of four sides. They sometimes used the four temperaments to depict them (as in the pucture above). More often, they used the four elements Air, Fire, Water and Earth. To identify the position of each one of those four elements in the individual consciousness they refered to the labels body, spirit and soul or Sulfer, Mercury and Salt. Those names correspond today with Jung's auxiliary, tertiary and inferior cogntive functions.
Sulfur (auxiliary) ------ Mercury (tertiary)
Salt (inferior)
What the short quote from Hoghelande says, is that when the auxiliary cognitive function is sacrificed (the body is dissolved), a new way of seeing the world springs. A new cognitive function, the tertiary is developed to compensate the missing one. This is the process that is linked to rebirth.
One should always be cautious when reading Jung's interpretation of alchemy. He had the tendency to bend alchemy to fit the premisses of his Analytical psychology.