Porcelain is a type of ceramic made from kaolinite. Porcelain items are white in color and have a material value of 10, equal to that of iron, silver, and crystal glass. Large pots made out of porcelain are water-tight, and do not need to be glazed to hold liquids like booze.
Unlike earthenware and stoneware, porcelain can only be made in limited quantities, determined by the amount of kaolinite available in your embark region (and whatever small quantities you can request from your outpost liaison), making it particularly similar to crystal glass.
Porcelain is made in the same way as other ceramics: kaolinite, though technically a type of rock, is also a kind of clay, so "Make clay large pot" (for example) can make a porcelain large pot.