It would make an interesting Core option to have a 'spellcraftsman' class that can create spells with his craft skill (such as potions, in your specific case) and carry them around to use later.
Depending on the craft, one could cut gemstones and crystals to harness their innate magical properties, distill elixers and ungeunts, carve tiny wooden totems and figures, harvest and preserve various animal parts to tap the properties of the animal / monster in question, inscribe mathematical formulae or astrological charts onto parchment to tap into principles of sacred geometry or cosmic power later, fold tiny origami figures and 'breathe' them into life as summoned constructs, create tiny mechanical clockworks to wind up and activate later, etc, etc.
Each spellcrafter would have one craft skill, and have only the smallest list of actual castable spells. Most of his spells would have to be prepared over hours and days, and be carried around as prepared foci that he constructed / shaped in his laboratory, on his loom, in his study, at his forge, etc. and he could activate them later like scrolls (but cheaper to create, as they are his principle spellcasting method, and limited to a certain number, to prevent him from carrying around 10,000 of them).
As for the Alchemist as a Core class, the best I have ever seen was in some old 1st Edition product called the Compleat Alchemists Handbook, by Bard Games. Nothing I've read since was half so interesting.