Jars are used to preserve fruit for longer periods of time. Jars start as Empty Jars, obtained through Glassworking. Then, a Jar Lid must be smithed from Tin. Crafting these together creates an Empty Jar With Lid.
In a pot, boil Sugar with 2-4 pieces of Fruit. When the recipe is done, Right Click with the empty jar with lid to create a Sealed Jar of Jam. The sealed jar will last quite a long time on its own.
Unsealing a jar is done by crafting. The lid is not able to be retrieved. An unsealed jar only lasts for a few days!
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Jam is used for making sandwiches. Jam sandwiches can contain Dairy, Cooked Meats, and Jam. An Empty Jar is left over.
Jars can be placed on solid surfaces with Right Click. A block can contain four jars of any kind.
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Jar Shelves can be attached to solid walls. They can be placed directly or by clicking on an existing block of placed jars. The top of the jar shelf counts as a solid surface for placing items, more jars, or other blocks.