Potion flasks are a vessel of storage for potions. Storing 6 decanted doses of potion, they are a tradeable item - when empty - and are made through the Crafting skill at level 89. The Crafting level can be boosted. Players also need a Glassblowing pipe.
Making a potion flask out of robust glass is not assistable. When a flask is completely full it is tradeable, unless the potion inside is not.
Another factor to consider regarding potion flasks is that they can only be filled once, and only with 6 doses of potion, though one can use one four-dose and one two-dose potion to fill it, rather than only two three-dose potions.
Once filled, flasks of 6 doses (or 5 doses) can only be consumed - one dose at a time. They can not be decanted into another container, and more potion cannot be added to "top them up." This makes it a less flexible storage method than normal potion vials, so careful consideration should be used before filling large numbers of flasks for long-term storage and use.
Flasks of 1, 2, 3, or 4 doses CAN be decanted into new flasks provided the total number of doses in various containers in inventory add up to at least 6. Or they can be decanted into regular empty vials, but in either case the flask will still "shatter" and disappear.