Ball clay is a fine-particle, highly plastic secondary clay, valued in clay body and glaze recipes for the workability it contributes rather than for its own fired color or strength. Unlike kaolin, a primary clay mined close to where it formed, ball clay is a secondary clay: weathering and water transport ground its particles down to a much finer size over geologic time, and that fine particle size is exactly what gives it such high plasticity.
Most white and light-colored clay bodies include a meaningful percentage of ball clay specifically to make the body workable enough to throw or hand-build, since kaolin alone is often too short (insufficiently plastic) to hold a shape under pressure. A porcelain or white stoneware body might include somewhere around 10 to 25% ball clay for exactly this reason, balancing the plasticity it contributes against the whiteness and refractoriness a high-kaolin body is chosen for in the first place. The tradeoff is that ball clay typically carries more organic matter and iron than kaolin, which can slightly darken a fired body and requires a longer, more careful bisque firing to burn out cleanly. A bisque fired too fast, or in a poorly ventilated kiln, risks black coring: the organic carbon inside a thicker wall doesn't fully oxidize out before the surface seals over, leaving a dark gray to black band trapped in the cross-section even though the outside looks normally fired. Because organic carbon needs both time and available oxygen to burn out completely, a slower bisque with adequate venting matters more for a body with a meaningful ball clay percentage than for a leaner, low-organic body.
Ball clay also appears in slip recipes and some glazes, again mainly for the plasticity and suspension properties it contributes to a wet mixture. Ball clay deposits are named for how the clay was historically cut and transported from the pit as roughly ball-shaped lumps, not for any property of the fired material itself.
The name traces to England's Devon and Dorset ball clay industry, where clay was originally dug by spade in roughly cube-shaped blocks that rounded into ball-like lumps during subsequent transport. The region's clay has been worked since Roman times, and was mined commercially from the 16th century onward, first for tobacco pipes and later, from the 18th century, as a key ingredient for Josiah Wedgwood and the wider Staffordshire pottery trade, shipped from Dorset's Poole Harbour to reach manufacturers elsewhere in England.