White air dry clay is bought for one reason: colour applied over it reads true, and an unpainted piece looks finished rather than raw. The complication is that white in the packet, white when wet and white when dry are three different things, and most greying happens after the clay leaves the bag.
Where the white comes from
A white air drying body is made from pale clays with little iron in them, usually with a white filler and a binder. Iron is what makes most natural clay buff, grey or red, so the whiteness is a matter of what was left out rather than anything added. That also means white bodies tend to be finer and slightly more expensive per kilogram than the general-purpose ones.
What turns a white piece grey
Hands, tools and work surfaces. White clay picks up everything: iron from an unwashed board, colour from a previous project, dust from a workshop floor. The greying people report is nearly always contamination rather than the clay changing. Work on a clean surface, wash hands and tools between colours, and keep white clay away from red bodies entirely if you use both.
Whether to paint it at all
A well-finished white piece needs no paint, and a thin wash of colour over white reads far cleaner than the same wash over a buff body. If you are painting fully, the white body still helps, because it removes the need for a primer coat to hide the ground. That is a real saving on anything with detail, where every extra coat softens the edges.
The finish is where white shows every flaw
White shows tool marks, fingerprints and uneven surfaces more clearly than any other colour, so it rewards smoothing the piece at the leather-hard stage with a damp sponge or a rubber kidney. Do that wet rather than dry sanding it later: the finish is better, and dry sanding a clay body is the one operation on this shelf that puts respirable crystalline silica into the air.
Questions people ask about air dry clay
Does white air dry clay stay white?
Yes, if it stays clean. Most reported greying is contamination from hands, tools or a work surface rather than a change in the clay.
Is white air dry clay better for painting?
Yes. Colour reads true over white and you can often skip a primer coat, which keeps fine detail crisp.
Why is white clay more expensive?
It is made from low-iron clays and finer fillers, which cost more than the general-purpose materials in a buff body.