Alchemy is the preparation of substances that carry mystical potency — elixirs, powders, tinctures, and worked materials — by taught recipe and procedure. The alchemist does not work on the world directly; they make a thing, and the thing does the work afterwards, in another hand and possibly in another year. That difference is the whole of what separates alchemy from incantation, and it is why alchemists are valuable to people who have no arcane gift at all.
The work is slow, sited, and expensive. A preparation is a routine of days or weeks in a laboratory, and the star quality of that laboratory bears on everything: what can be attempted, how far a completed preparation can be augmented, and whether an itinerant alchemist can work at all. Elixirs decay of their own accord, so a stock of them is a standing obligation rather than a finished achievement.
Invoking it. An alchemical working is a Success Test like any other Mystical Ability — at this ability’s own mastery level, or at that of the skill governing it. Most preparations are read as a Success Value rather than a bare success level, because what is wanted is not “did it work” but how strong the result is and whether an augmentation may be attempted.
What bears on the test. The star quality of the workshop and its equipment; the time allowed, since an abbreviated routine buys speed at the cost of strength; assistance, where the routine admits it; and the quality of the materials to hand. Beyond that, the two standing limits on all mystical practice: nothing may be prepared while the alchemist carries Aural Shock, and no Mystical Ability test may ever be fated.
The rules for the discipline, including the optional provisions for itinerant practice, are set out under Alchemy.