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.
The cellar comes with 33 Casks. It's possible to fill the cellar with 125 casks and still leave paths to reach all of them without displacing any of them.
It's also possible to completely fill the cellar with Casks. This requires 189 casks, made of 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. In order to harvest the finished product the player will need to remove the empty casks as they go, carving a path to the furthest casks. This can be time-consuming so it may be a desirable option for only the most valuable products (e.g., Starfruit or Ancient Fruit wine).
All layouts described here are shown in the Gallery below.
Note that players using a controller instead of mouse/keyboard have reported difficulty accessing casks in the corners of the Cellar.
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.