Because their methods and materials are innumerable, the alchemists most certainly never did the experiments that were described in their treatise. Those symbolic processes were put forward to protect their knowledge from the powerful Church and the Inquisition.
In Psychology and Alchemy, Carl Jung wrote
The alchemical opus deals in the main not just with chemical experiments as such, but with something resembling psychic processes expressed in pseudochemical language. The ancients knew more or less what chemical processes were; therefore they must have known that the thing they practised was, to say the least of it, no ordinary chemistry. (Par. 342)
Jung is correct. The alchemists knew that their books were not about chemical experiments. If they would have been so, why would they have hidden them under multiple symbols? To reveal the process to change lead into gold? The alchemists knew that it was impossible and that does not explain why numerous books sought to get the philosopher's stone which has nothing to do with gold.
If you can find the courage to read an alchemical grimoire, you will understand immediately that those recipes were not real experiments that would be done in a laboratory. What they revealed were recipes to access specific psychological results.
In the same paragraph, Jung continues and ask two important questions:
If the alchemist is admittedly using the chemical process only symbolically, then why does he work in a laboratory with crucibles and alembics?
And if, as he constantly asserts, he is describing chemical processes, why distort them past recognition with his mythological symbolisms?
The answer to first question is simple. Yes, some alchemists did have laboratories with alembics and chemical substances but they were intended to protect the owner from the Church and the Inquisition. It is important to highlight that the alchemists were the finest intellectuals of their time and they knew that alchemy was not about the process to make gold or the stone of immortality. Those were only symbols of psychological endeavours to access a particular state of consciousness.
Thus, the real alchemists never did chemical experiments. They knew the key to read the books of their colleagues because alchemists always talked about consciousness.
Jung's second question is flawed because alchemical grimoires were not, as we have said, chemical experiments. The goal of the mythological symbolism was to hide a specific knowledge that was too dangerous to bring to light.
People normally see alchemists as strange and lonely madmen who worked days and nights to make gold but this a romantic and defective view. These men were highly educated intellectuals who invested both time and large sum of money in the publishing of their books that were intended to other intellectuals. What was under the symbolic description of their recipes was the hidden knowledge to develop consciousness and access mystical experiences or conjunction of opposites.
In The Relations Between the Ego and the Unconscious (CW 7), Jung gives the key to understand the transcendent function:
"This remarkable capacity of the human psyche for change, expressed in the transcendent function, is the principal object of late medieval alchemical philosophy, where it was expressed in terms of alchemical symbolism… The secret of alchemy was in fact the transcendent function, the transformation of personality through the blending and fusion of the noble with the base components, of the differentiated with the inferior functions, of the conscious with the unconscious." (CW 7 p. 219-220)
Alchemy was about the development of cognitive functions known to them as the four elements (Fire, Water, Earth and Air) and the four temperaments (sanguine, choleric, melancholic, and phlegmatic).
For more, see
Carl Jung's Alchemical Secret: the Process to Reach Mystical Experiences