Clay is an item found in vanilla Minecraft, composed of greyish blocks that yield four clay lumps each. The lumps can be smelted into bricks, which can be crafted into brick blocks or clay pots.
Clay is a naturally occuring rescource found around swamp biomes and under large or small bodies of water. It is usually completely submerged, making it difficult to find. Alternatively, you can use UU-Matter or Equivalent Exchange to craft clay, produce it in a Sludge Boiler, or produce it by smelting Factorization's Sludge, which is produced by a Mixer.
Clay can also be obtained by smelting Sludge.