The longer and hotter they are fired, the better the minerals within the clay will harden or vitrify. When selecting pots, you can easily detect low quality pots by running your fingernail across the pot. Low quality pots scratch easily. Walls of inferior pots begin to flake, pit and crumble as mineral salts leach through. Durability is affected by the purity of clay used.
All clay contains lime impurities.
In quality pot construction, the lime is filtered out of the clay before manufacturing. Inferior pot manufacturers usually don't bother with this process and allow lime deposits to remain. Lime particles absorb more moisture than clay particles which causes blistering and flaking of the pot surface. If the pot you are selecting has white, gritty spots on the surface, pass it by, it's an inferior pot. Clay that contains sand is made into quality pots as long as the firing process is done correctly.