The Bark Knife is an item added by Roots. It is used to harvest Bark, like Oak Bark. Right-clicking a Wood Log (like Oak Wood) with it will consume one durability from the Bark Knife and drop the Wood's corresponding Bark (like Oak Bark). Additionally, the Wood Log will have a 30% chance of breaking when this is done. It will not drop anything besides Bark if this happens.
Trees are very important to natural magic. Using some wood and saplings, you can create a knife capable of softly harvesting the bark from trees, which can be used in recipes and rituals. Right-click a log block to harvest bark from it, with a chance of breaking the log block.
— Runic Tablet
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