Such thinking is commonly communicated through allegory and symbolism, both because it is understood that direct explanation is not adequate and because such spiritual knowledge is considered dangerous to those not able to understand the concepts contained.
The most common alchemical terminology revolves around chemistry procedures of the time. For this reason, the common impression of alchemists is that they seek material wealth through the transmutation of lead into gold. In truth, this description symbolically represents the transformation the material, common and mundane into the rarefied and divine i.e. the elevation of the soul and the perfecting of one's nature.
At its most basic, alchemy can be broken down into three processes: negredo, albedo, and rubedo. Be aware the different sources will vary in terminology and details in their explanation of them because each alchemist had their own personal understanding of alchemy, as do modern readers of their works.
Negredo (Blackening)Negredo is the stage of decomposition. Before something new can be created, the old must be broken down. Here the breakdown needs to be complete as if through decomposition or burning by fire. In both cases, the object is reduced to its most basic components, allowing impurities to be separated and discarded.
Matter is understood to come from a single primordial state of matter.
Physically, you cannot fully break down an object into this substance: setting something on fire does not leave prime matter behind. Soot is still more complicated than prime matter. But allegorically, this is about breaking down the self into its most basic state, stripping away everything superfluous in order to reach the core of being. Ultimately, you break down primal matter itself (allegorically speaking) until all you have is divine essence, understanding, a complete lack of attachment to physicality.
To spiritually strip oneself down leaves one fertile for new growth, just as rotting matter and ash fertilize soil so that life cycles can begin again.
The process is not an easy one. Rejection is always painful. Primal matter is chaotic. The breaking down of anything involves trauma. But through this suffering one will eventually emerge purified.
Albedo (Whitening)Albedo is the purification and separation of the results of the blackening. It is revelation of the self, the divine light within, the spark of life, and one's ultimate nature. A large part of this nature is the understanding of duality: that humans are both material and spiritual beings, symbolically both hot and cold, dry and wet, and male and female, which have been common philosophical divisions for centuries.
These parts are broken down during the blackening. Now these opposites are reconciled so that they can be reunited in harmony rather than in conflict as they are in most human beings.
Modern understanding of optics states that black is the absence of color while white is the presence of all color. Thus, blackening reduces us to nothing, while whitening brings together diverse parts.
Rubedo (Reddening)Rubedo is the final transformation where the results of previous processes are fully realized and permanently fixed. It is the fixing of malleable, perfectly shaped clay into ceramic or the fixing of molten metal into a perfect object.
Spiritual understanding becomes imprinted upon physical reality; the volatile has become durable. After all, we are still physical creatures. Now, however, our spiritual and physical natures are in harmony, purified and united.
Interactions Between the StagesAlchemy is not a simple progress through the three stages. They occur in cycles: components are broken down, purified and fixed. Revelation in one stage lead to more transformation in other stages.
More Allegorical AssociationsNegredo is the lead of the process. Lead is heavy, coarse and material. Albedo is silver, a much more rarefied but still imperfect metal, while the final form is gold, considered the most perfect of the metals.
Mythologically, negredo is the descent into the underworld, albedo is the elevation of the spirit into heaven, and rubedo is the rebirth into the world as a being purified and transformed. Likewise, it is the setting of the sun every night, often mythologically seen as a descent into the underworld. It is the traversing of that underworld, and the realization of the return of the sun at daybreak, associated in many cultured with rebirth and resurrection.