White clay is bought so that glazes read true and decoration shows, and the trap is that the colour of wet clay predicts almost nothing about the colour of fired clay. Bodies that look grey in the bag fire white; bodies that look pale can fire buff. The supplier's fired sample is the only reliable guide.
Why wet colour misleads
Wet clay looks darker and greyer than the same body will be after firing, because water and organic matter both darken it and both leave in the kiln. Draycott White Stoneware looks unremarkable in the bag and fires a clean white. A body that looks white when wet may carry enough iron to come out cream. The only test that means anything is a fired test tile from the range you actually fire to.
What makes a body white
Low iron. Iron is what gives most natural clays their buff, grey or red colour, and a white body is built from china clay, ball clays and fluxes chosen for the absence of it. Those materials cost more than the general-purpose clays in a standard body, which is why white bodies sit a little above buff ones on most price lists.
Where white earns its price
Under colour. A glaze over a buff body reads muted and a transparent glaze over a white body reads clean, so any work relying on bright glaze colour or on underglaze decoration wants white underneath. Work that relies on the body's own character, on a speckled or toasted surface, or on a heavy reduction glaze, does not, and paying for whiteness there is money spent on something nobody will see.
Keeping white clay white
White bodies pick up contamination from everything: a board that has had red clay on it, a wheel head that was not washed, a tool used on a stained body. Iron marks show as brown specks after firing and cannot be removed. If you work in both, keep separate tools and boards or wash thoroughly between them, and reclaim white and coloured clays in separate buckets.
Questions people ask about white clay
Which clay fires the whitest?
Porcelain, followed by white stoneware bodies. Both are built from low-iron materials, which is what the whiteness is.
Why does my white clay fire cream?
Either the body carries more iron than you expected, or the firing atmosphere is affecting it. A fired test tile from your own kiln settles it.
Is white clay more expensive?
Slightly, because the low-iron materials in it cost more. The difference is small next to the difference between suppliers for the same body.