Alchemy, being a "sacred science" (a marriage of science and spirituality, flourishing at times when there was little seperation made between the two), is often misrepresented as a strictly medieval European phenomenon. However, alchemy is known in many different countries, rooting itself in the histories of various cultures; Egypt, India, China, Greece, Rome, and the Islamic Empire all at one time or another were home to the furnaces and laboratories of devoted alchemists.
Alchemy in Europe
The oldest extant alchemical text was written by the Gnostic Christian, Zosimus of Panopolis, sometime around 300 A.D. Despite his title, Zosimus lived in Alexandria, home to the famous Library of Alexandria. This text, titled Isis the Prophetess to Her Son Horus, claims that the origins of alchemy come from the Nephilim, the Fallen Angels, who taught it to Isis in exchange for sexual intercourse (not to be confused with any kind of demonic entities, the Nephilim were, in certain traditions, said to be great teachers and guides for the human race). Zosimus also claimed as his sources the writings of the Persians and the Jews.
Around the same time, in the same area of Hellenized Egypt, thrived the Hermetic tradition which still remains vital and alive even today, having provided fertile ground for the Rosicrucian Order, which in turn gave rise to the Hermetic Order of the Golden Dawn. Although its roots reach back into the ancient, inner esoteric pantheism of the Egyptian religious polytheism, it was around the first couple centuries AD that the actual body of Hermetic writing arose, known as the Corpus Hermeticum, authored by the mythical Hermes Trismegistus. The Corpus Hermeticum, an extremely important body of writing for both the ancient and modern alchemist, was probably influenced by the Egyptians and the Greek Philosophy of Democritus.
Basic Alchemical Principles
The Four Elements Alchemy is based on the idea that the world is composed of the four elements fire, air, water and earth. These elements are considered to be contained in all things, the most common example being wood which "obviously" contained earth (which was why it was hard) and water (sap when squeezed out) and air (smoke) and fire (it could be burnt).
The basic idea of the atom as the "smallest unit of divisibility" was also formed during this time - though this truly bears little relation to the present-day concept of the atom (which is merely named after the Democritian concept). Atoms were considered to be tiny, perfect spheres and made up of one of the four above-mentioned elements.
The Philosopher's Stone One of the concepts was that, because lead and gold (the perfect metal) were from the earth element, earth could therefore be turned into lead and gold using an "appropriate catalyst" known as the Philosopher's Stone. This was a magical substance which could turn water into wine, heal the sick, and as mentioned, turn really boring lumps of metal into gold and silver. Most of it though was just processes like mixing copper with arsenic to give it a silver finish, or a philosophical trick by which the alchemist's apprentice, through years of hard work and training, was turned from lead (i.e. an ignorant klutz) into gold (i.e. an alchemically educated scholar).
The quest to turn lead into gold was also a spiritual one, as it represented the change of an impure substance into a pure substance - the equivalent of returning Man from his current state of sin to a state of Grace last known before the Fall.
Despite never discovering the Philosopher's Stone, alchemists did come up with a number of interesting reactions and the knowledge gathered eventually led to the foundations of modern chemistry.
Panacea Alchemy also gave rise to concept of the panacea, the "medicine" to cure all illnesses - also never found, but a search for it led to an interesting number of ways to poison oneself (mercury, silver and lead poisoning being numbered under the chief ways that alchemists died).
Alchemy in the SCA
Although not common, it is not unheard of for SCAdians to practice alchemy, often as a demonstration activity at events. Drawing from original alchemical texts, "alchemists" in the SCA attempt to recreate experiments and chemical processes. However, since many medieval alchemical substances were extremely toxic, a great deal of care should be taken in handling them.
Alchemy can be a specific category in the Arts and Sciences, and it includes all chemical processes related to the time period. Some examples of chemical processes are: cooked and non-cooked non-culinary recipes (e.g., soap, candles, potpourri, perfume, etc.).
This is not to be confused with actual practicioners of modern alchemy, who are often considered Neo-Pagans.