Alchemy

Alchemy (錬金術) is a subbranch 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, the only known one being Aureolus Izzard.

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, which 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 or Ars Magna. Or simply put, humans are incomplete gods and through training and making one complete, humans can become gods.

Indeed, according to Stiyl Magnus alchemy is an unrefined knowledge, 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. Furthermore Stiyl also pointed out several times that as the goal of Alchemy is to simulate the entire world, any result that they happen to get before reaching the ultimate objective would, or should be seen as useless by the alchemist.

Ars Magna
Ars Magna (黄金練成 (アルス＝マグナ) ) 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 mold the real world would 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. Kamijou Touma deducted that this why the prospect of why actual existing vampires are sought after by the magic side, by both alchemists and normal magicians, since one can achieve to finish the chants of the level of 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.

Limen Magna
Limen Magna (瞬間錬金 (リメン＝マグナ) ) is a type of Alchemy that was used by Aureolus Dummy and possibly Aureolus himself could use it if he so desired; in normal conditions the basic procedures for the alchemy of turning lead into gold are very bothersome and quite expensive, this makes it so that the actual cost and effort employed in the task results in such that the produced Gold is of little reward when compared to the annoyance of the process. Limen Magna is an ability that Aureolus Dummy used that collected and condensed "natural mana" from the building and created the "turn into gold" process in an instant, even though it's not nearly as impressive as Ars Magna; the pinnacle of alchemy, we have that Stiyl commended the versatility of Limen Magna was still something beyond the current limits of modern alchemy, even thought it's just doing something that's being done in a new manner.

Deep Blood Arc
The concept of alchemy was prominently featured only in the Deep Blood Arc but was introduced earlier. It is also referenced much later, usually in the company with eerily 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.

La Regina del Mare Adriatico Arc
While not present in the actual story in any form, Biagio Busoni mentioned that the spell that empowered the self destruction of of the Queen's fleet was reinforced with Alchemy.

Trivia

 * Stiyl commented that alchemists are like scientists of the magic side, who are constantly researching on subjects, looking for a basis or rule for it.
 * Stiyl also mentioned that besides Paracelsus no alchemist has really done significant progress with their studies and thus "wasn't strong" . This, however, was a statement made before he could experience Ars Magna first hand.