Normal, silver, and gold quality items can be prematurely removed from a cask at any time by striking the cask with an Axe, Hoe, or Pickaxe. However, striking a cask with an Axe, Pickaxe or charged Hoe when the cask is ready to harvest (with an iridium quality item) may crash the game.
Casks are able to produce items with "iridium star" quality. Iridium star is the highest level of quality, which doubles the value of an item. Different types of items take longer or slower to age. Wine takes the longest -- 2 seasons of aging to go from basic wine to iridium quality wine.
Casks can be placed anywhere, however they will only accept goods to refine if they are actually in the cellar.
It's worth noting that even with high-value goods such as Starfruit wine, the value gained from casks is comparatively low as an iridium-quality product is only double the original value, so maturing a bottle of starfruit wine will yield 2250g in 56 days, yielding only 40g per day per cask. The lack of maintenance required for casks means you can fill the cellar and forget about it for two seasons to get that little extra value, however.
You start with 33 Casks. It's possible to place 189 Casks in the Cellar, requiring 156 Hardwood and 3120 Wood. In order to use all 189 casks, the player must fill the casks all at once, placing additional casks as they move towards the exit. The 125 cask layout pictured below is a good place to start. In order to harvest the finished product the player will need to carve a path through the casks as they empty them. This can be time-consuming so it is recommended to do this only with a valuable product that takes a long time to reach iridium quality, such as starfruit wine.
A fully filled cellar with processing goods of different stages.
Optimal layout: 125 accessible casks.
Cellar filled with the maximum 189 casks, all in use.
Bugs
Striking a Cask that is ready to harvest (i.e., contains an iridium quality item) with an Axe or Pickaxe will crash the game. Striking a cask with a Hoe may harvest the item, or it may crash the game.