Creating Stuff with Alchemy
Alchemy is usually used to create temporary items such as magical potions, oils, and poultices, but it can be used to create all manner of magical things. Temporary, single-use items are the easiest sort of magic item to make, while permanent magic items take a great deal of effort and time. In Erandor, it's not uncommon for an adventurer to seek out such items at an alchemist's shop or friendly temple, but permanent magical items are never for sale except by private arrangement. Permanent items take too much time and cost too much in terms of time and material to be viable trade goods. In addition, most require some considerable investment on the part of the owner to gain full use of it.
Here we'll focus mainly on the creation of potions and other temporary magic.
Alchemical creations require three things: knowledge, special ingredients, and time. The acquisition of the ingredients and their special preparation takes time and Resources, while knowing how to put it all together to create something useful requires time and Lore. Then the concoction might be successfully made. This all boils down to a few steps.
1. Determine the Desired Effect
To create a magical potion, or any sort of magic item, you must have aptitude with magic. This means an aspect describing that aptitude and a stunt that gives you access to at least one school or domain of magic. You can only create potions or other magic items with magical effects to which you have access. So for example, you can't make a potion of Flight if you have no access Air or Aether magic.
You can use the chart for spell preparation in the Magic section as a guide. This will tell you what difficulty you will be facing for the rest of the project. Once you have it and the GM agrees, you can proceed with the next step.
2. Acquire Materials
This might be materials found in game and should manifest as an aspect. If not, then you need to make a Resources roll against the difficulty you found in step 1 to acquire the basic ingredients. If the roll is failed, then you might not have been able to find ingredients of suitable quality or you ran out of funds. Successful or not, the time involved is an one hour per level of difficulty. So if you were trying to acquire items for a Good (+3) potion, then it took you three hours of searching and haggling whether you were successful or not.
If you can provide a good reason to use a skill other than Resources to acquire your materials and the GM agrees, you can use that skill instead.
Success means you have an aspect such as Special Ingredients you can use in the final phase with a free invocation (two free invocations if you succeed with style). A tie means you can place the aspect but you have no free invocations. Failure means you have ruined your potion and must start over completely.
Next you must prepare the ingredients. This might require brewing for a certain amount of time, special incantations, or any number of other tasks. At the end of another span of hours equal to the difficulty of the potion you determined in step one, you make Craft skill roll against the same difficulty. Success means you have an aspect such as Enchantment Materials you can use in the final phase with a free invocation (two free invocations if you succeed with style). A tie means you can place the aspect but you have no free invocations. Failure means you have ruined your potion and must start over completely.
This is the hard part. You have an effective skill of Mediocre (+0) for this, but you can invoke the aspects you placed during material acquisition (step 2) and preparation (step 3). You can use the free invokes and of course you can always spend a Fate point on either or both of those aspects. If you are successful, then viola! You have a spell effect in a bottle. Or cream. Or poultice. You get the idea. Using the potion lets you cast the spell using whatever skill is most appropriate. It just takes an action to drink, apply, or throw it and then you can take advantage of whatever effect the potion was designed to have.
But of course if you tie or fail the difficulty, then you just had a bad day and the temporary magic item just didn't turn out.
Such items can be used by anyone who gets their hands on the item. It's like getting to cast a spell someone else prepared for you. These basic items usually require an action to activate (quaffing a potion, smearing on some unguent, reading from a scroll, etc.). After that, the spell goes into effect. Some temporary magic might be an attack spell, in which case after activating the magic, you must make some kind of attack roll. If you miss, it does not count as failure -- the magic worked whether your target got out of the way or not. Because spell failure is not a factor in using items like this, it's considered a safe way to access magic.
Purchasing Temporary Magic
Items like this are typically valued at the same level as the difficulty to create them, or one step higher. So a Potion of Restoration, normally a Good (+3) spell, will usually require a Resources roll against a Good (+3) or Great (+4) difficulty.
GMs who would like to limit the acquisition of these temporary magic items should consider using one of the options for Resources provided on page 123 of Fate Core (or just click this link to see the options).
Permanent magic items might also be produced by Alchemy. You might consider increasing the times required by 2 to 10 times normal and perhaps increasing the difficulty to create by +2. But the true cost is that to acquire permanent magic items the character needs to invest stunts or skills or an aspect in it.
There are lots of good examples of permanent magic items in Section 2 of the Bonus Extras page for Faeln.
Also, take a look at Special Metals of Erandor. Many permanent magic items available in Erandor are made of adamant, mythral, or orichalc. The mentioned section discusses the basic attributes of these magical substances.