Comment by throwup238

It’s one and the same. The sodium and other atoms from the molten cryolite intercalate into the carbon cathode structure and swell it by a few percent. Once in use, a cathode is held together by the steel shell and thermal equilibrium of the running pot. Once it cools the cracking is inevitable.

You also can’t fully drain a pot. You can siphon most of the aluminum and cryolite off but at those temperatures they behave like a proper liquid with surface tension and the metal wicks into the pot like solder instead of flowing with gravity.