Which type of clay is waterproof even without glaze?

Stoneware is a type of clay that is known for its durability and resistance to water even without a glaze. This characteristic comes from its firing temperature and composition. Stoneware is typically fired at higher temperatures compared to earthenware, which makes it more vitrified, or glass-like, thus providing a non-porous surface that can hold liquids without leaking.

In contrast, earthenware is not waterproof without glazing; instead, it remains porous and requires a glaze to create a barrier against moisture. Kaolin, also known as china clay, is primarily used in porcelain and has its own specific qualities, but it is not typically recognized for its waterproof properties without firing at high temperatures. Colored slips are mixtures of clay and water meant to provide color during decoration and don’t contribute to the waterproof nature of the finished piece. Thus, stoneware stands out as the correct choice for being inherently waterproof.