There are two specializations on crafting: strength and frequency. If you have a frequency specialization, it increase the number of free uses you get out of an item. It's also possible to have crafting strength and crafting frequency focus items.
Example:
Brutha has a base lore of 4. He puts his free specialization point from thaumaturgy into crafting strength. In addition, he uses one of his focus item slots to make a crafting frequency +1 focus.
So, assuming he has his focus handy when he makes something, he has an effective crafting strength of 5 and a frequency of two.
So, for example, if he wanted to make a single slot defensive item, it would have a block strength of 5 and 2 uses.
This also carries over to potions, for example, your potion of invisibility can make two people invisible, or be use to make you invisible on two different occasions.
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Now, how do you determine what the potions you have in your pockets can do?
There are three ways:
1 at the start of the story, say you have potions prepared to do some specific effect
2 during the story, spend a scene making a potion to some specific effect (also possibly replacing a potion of a specific effect you declared before hand). This also gives you the chance to invoke aspects/spend fate points to make a more powerful potion that you would otherwise be able to.
3 during the action, make a lore declaration (generally difficulty 3) or spend a fate point to declare that one of your 'free' potions slots 'just happens' to be the type of potion you want right now.
The 'problem' with just leaving all your potions undeclared and relying on a lore declaration to have the one you want, is that you can fail this roll. If you happen to be falling from a great height, for example, you really want to be able to pull a potion of feather falling out of your pocket. So you might want to spend a fate point for a sure thing.
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If you are using a potion to make an attack or offensive maneuver, the targeting roll is generally your discipline.
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So, if your lore isn't that good, you generally want to have a few potions that are already 'declared' ahead of time that you might find useful in an emergency (escape, veils, attacks, defenses, etc)
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Another thing to remember is that a 'potion' is probably better described as a single use magical item. So, for example, you might have an origami crane that lets you jump really far, or a set of (temporarily magical) handcuffs, or a single magical bullet, or magical land mine, or ...