"A container used to preserve materials. Place an item inside to prevent it from perishing."
Airtight Flasks are used to preserve perishable (and perished) foodstuffs and flowers, preventing them from decaying. The preserved (or "kept") versions have the same curative properties as the originals, and in many cases have the same combining properties.
Kept items are also just as good as affinity gifts as the originals.
The preserving process is permanent and cannot be reversed, nor can the flask be recovered once used.
In many cases a decayed kept item can be refreshed to an undecayed item by combining it with Spring Water - for example Giant Rotten Fish can be converted to Kept Giant Fish via preservation and reaction with Spring Water.
An Airtight Flask can be found near the tidal line on the beach in Cassardis.
Another Airtight Flask can also be found on Unusual Beach, near the cliffs. Of the three ways down to the beach, take the centre one - the flask is beyond the bottom of the stone steps, on the north side.
An Airtight Flask can be found in the battlements of the Duke's Demesne, inside the ground floor of the northern 'Observation Room'. It sits on top of a barrel.
Another Airtight Flask also can be found in the Duke's Demesne: Take first door to the left when entering the 'Audience Chamber' which leads to the servant's quarters, then enter the room to the right. It's sitting on top of a crate.
Another Airtight Flask can be found in the Dungeon, lying on the floor of the first cell on the left. A Skeleton Key is needed to open this door.
An Airtight Flask is located in the Ward of Regret, upon entering from the Duskmoon Tower, head west then north and round past the two locked cells, then when the path angles northwest, head left (west) into a hall in which the flask can by found on the floor on the right side near a pillar.
Also in the Ward of Regret, after descending through a hole in a wall and down a rough hewn cave, the passage curves to the west, proceed north to a T intersection and head west again where the path turns south. An Airtight Flask can be found on the ground here.
Within Midnight Helix, by the wall at the base of the first suspension bridge, an Airtight Flask can be found.