You can delete materials from the explorer as would any other scene element. When deleting materials, keep the following in mind:
Deleting a material removes it from the scene entirely. If you want to remove a material from an object without deleting it, you can unassign it, as described in Removing Materials From Objects.
You cannot delete a material when it's the only one in the scene.
If you delete the material that's currently assigned to the object, the default scene material is then assigned to the object.
If you delete the default scene material, the least recently created (oldest) material in the scene becomes the new default material and is assigned to all objects to which the previous default material was assigned.
You can only delete materials from a material library's hierarchy. Deleting a material from the hierarchy of an object to which it is assigned simply unassigns the material, regardless of whether it is the only object using the material.
Rather than individually locating and deleting unused materials, you can remove all of a library or scene's unused materials at once.
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