The potion flask acts as a jumbo vial that can hold up to six doses of potion rather than the four doses of a traditional vial. It may also be used to hold untradeable potions such as overloads. Upon finishing the potion, the flask is automatically destroyed. Completing As a First Resort is an indirect requirement for making potion flasks, as robust glass machines (used to make robust glass) can only be used after completion of the quest.
Potion flasks are made by using a glassblowing pipe with robust glass, yielding 100 Crafting experience. Creating flasks requires 89 Crafting; players may boost their level if below 89 but can not be assisted. By crafting flasks there is a chance of receiving a strange rock for Crafting. After a sip of potion has been taken from the flask, it cannot be sold over the Grand Exchange; however, they can still be traded with other players. Assuming a player does their daily mining of red sandstone, they will gain a minimum of 5,000 crafting experience each day making flasks (7,500 if they completed the elite Desert Tasks), as well as 218,700 coins if sold on the Grand Exchange.
Empty flasks are filled by decanting from multiple vials. However, vials may not be decanted into partially-filled flasks. Potions in flasks may also be decanted back into empty vials. Teplin Macagan can assist with decanting large numbers of flasks, even in noted form. However, for either of these tasks, only flasks containing 4 doses or less may be decanted; players will not be allowed to move potion from a 5 or 6 dose flask into a vial. Also, the flask is not kept upon decanting. Whenever decanting a potion out of a flask it is destroyed as if it were drunk.
Despite being created entirely of non-tradeable materials, the flask itself is tradeable, as are any flasks containing tradeable potions. The resources required to craft robust glass are non-tradeable and have a maximum of 50 (75 if the player has completed the elite Desert Tasks) available to players daily (or more with the extra ores bonus from Shooting Star and the saved ores from the resourceful aura).
4-dose perfect juju potions can be decanted into potion flasks to make 6-dose perfect juju potion flasks.