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 (錬金術師), the only known one being Aureolus Izzard.

Principles
With it simply being a sub-branch of magic, naturally it involves several applications and theories that are present in magic such as the teachings of, which the Golden Dawn advocates. One of these teachings is that alchemy is not only that of turning lead into gold, but also a process in which one can make a natural body into perfection, the process being called or Ars Magna - put another way, that humans are incomplete gods and through training and making one complete, humans can become gods.

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

One of the alchemists known to have made significant progress is, who developed a number of concepts which were incorporated into the field and other forms of magic, including the association of spirits known as (such as Gnomes and Salamanders)  with each of the four elements. He also developed the concept of the (tria prima) - salt (body or base matter), sulfur (soul) and mercury (mind) (not to be confused with the more mundane substances of the same name), and the relationship between them.

Another core concept which various alchemical concepts were developed around is the, which were identified by humanity in prehistoric times and the only ones known prior to the discovery of in the 13th century. Associated with the seven known celestial bodies among other things, the seven metals were (Sun, ☉),  (Moon, ☽),  (Mercury, ☿),  (Venus, ♀︎),  (Mars, ♂︎),  (Jupiter, ♃) and  (Saturn, ♄).

There are a number of legendary substances which are spoken of and pursued by alchemists; these include the elixir of life, a drug which is said to convey immortality upon its drinkers, the ', said to aid in the production of the elixir and enable the transmutation of metal into gold, and ', a cure-all medicine.

Ars Magna
Ars Magna (黄金練成 (アルス＝マグナ) ) is Latin for "Great Art". 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 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 actual existing vampires are sought after by the magic side, by both alchemists and normal magicians, since one could finish the chants of the level of Ars Magna if one was 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 increasing its effects 120 times. In his use, the Ars Magna distorts reality through his thoughts, and requires 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 who 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 is such that the produced Gold is of little reward when compared to the annoyance of the process. When used by the Aureolus Dummy, Limen Magna collects and condenses "natural mana" from the building, and creates the "turn into gold" process in an instant. Even though it's not nearly as impressive as Ars Magna, the pinnacle of alchemy, Stiyl commended the versatility of Limen Magna as still something beyond the current limits of modern alchemy, even thought it's just doing something that's being done in a new manner.

Background
According to Samuel Liddell MacGregor Mathers, William Wynn Westcott 'brought alchemy back at the end of the 19th century'.

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.

Coronzon Arc
Aleister Crowley referred to the relationship between in alchemy, during his explanation of the Lady of the Masquerade Ball's Taphthartharath. Mina Mathers later referred to the, specifically silver, when detecting Coronzon's imminent attack.

Christmas Eve Arc
Neoka Norito made use of the seven metals, utilizing their association with the planets and from that the Sephirah, for the magic he had acquired from R&C Occultics.

Trivia

 * Stiyl commented that alchemists are like scientists of the magic side, who are constantly researching subjects, looking for a basis or rule for them.
 * Stiyl also mentioned that, besides, 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.