Alchemy

Alchemy (錬金術) is a sub-branch of magic that is traditionally associated with turning base metals such as lead into gold and creating an elixir of immortality. However, the true goal of alchemic branch of magic is to model everything in the world.

A practitioner of alchemy is called an alchemist; currently, Aureolus Izzard is the only character introduced that has that distinction.

Principles
With it simply being a sub-branch of magic, naturally, several applications and theories that are present in magic such as the teachings of Hermes Trismegistus(which the Hermetic Order of the Golden Dawn advocates). With the teaching that alchemy is not only that of turning lead into gold, but is a process in which to make a natural body into perfection, the process is called Magnum Opus or Ars Magna, or simply put, human's are incomplete gods and through training and making one complete, humans can become gods.

Indeed, according to Stiyl Magnus alchemy is an unrefined knoweldge, and is a magician is referred to an alchemist if they deal with subject matter closely related to it if they are untalented enough for other roles.

Ars Magna
Ars Magna (黄金練成) is Latin for Great Work. It is the spell in which alchemist would achieve their goal of simulation of the world inside the mind. With Ars Magna, they can bring out thoughts into the real world, essentially, making those who can accurately simulate or mode the real world be able to control it.

However, according to Stiyl, since alchemy is an unrefined knowledge, modeling the entirety of the world and its laws into a chant takes centuries even with the assistance of descendants and removing of unnecessary lines. That is why the prospect of actual existing vampires are sought after by the magic side, by both alchemists and normal magicians, since one can achieve to finish the chant for Ars Magna if one is able to figure out vampire's immortality. Hence forth, no one has yet to achieve Ars Magna under the above means.

Aureolus Izzard is the only person who has managed to achieve Ars Magna. By utilizing the Gregorian Chant Replica to chant the spell simultaneously, he was able to bypass the time required to chant the spell in only a mere seven days as well as increase its effects 120 times. In his use, the Ars Magna distort reality through his thoughts, and require a vast amount of concentration to accurately model a thought into reality with it.

Chronology
The concept of alchemy has been prominently featured in only the Deep Blood Arc, but has been introduced earlier. It is also referenced much later after it's introduction, usually in the company with eeriely similar concepts such as the teachings of Kabbalah and Thelema, as well as Esper powers, Touma even thought that the basic principles of alchemy were similar to those of espers.