Mikvaot, Chapter Ten, Mishnah One
1) Any handles of vessels which have been fixed not in their usual manner, or, if fixed in their usual manner, have not been fixed firmly, or, if fixed firmly, have been broken, they block.
2) If a vessel was immersed with its mouth downwards, it is as though it had not been immersed.
3) If immersed in the regular manner but without the attachment, [it becomes clean] only if turned on its side.
4) If a vessel is narrow at each end and broad in the center, it becomes clean only if turned on its side.
5) A flask which has its mouth turned inwards becomes clean only if a hole is made at the side.
6) An inkpot of laymen becomes clean only if a hole is made at the side.
a) The inkpot of Joseph the priest had a hole at its side.
Section one: Handles of vessels that have not been attached properly, or were attached properly but broke, block successful immersion. However, if the handle is properly attached to the vessel, it does not block immersion.
Section two: If the vessel is immersed with its mouth downwards, water will not fully enter it. Therefore, the immersion does not count.
Section three: The attachment referred to here seems to be some sort of drainage pipe placed on the side of the vessel. If he immerses it in the normal manner, the water will not enter this attachment. Therefore, it is not pure until he immerses it on its side so that water will enter the drainage pipe.
Section four: The water will enter this vessel only if it is immersed on its side.
Section five: Water will not enter this vessel through its mouth, therefore to successfully immerse it he must make a hole in its side.
Section six: The inkpot used by laypeople will not allow water into its mouth, if it is immersed. Therefore, one must make a hole in it to allow the water in. It would seem that the inkpots of professional scribes were made differently.
Joseph the priest seems to have been well-known for making a permanent hole in his inkpot so that it would be easy to immerse. Assumedly, when using it, he would plug it up temporarily.