Almost every glaze recipe you will read contains a clay, and it is usually china clay, ball clay, or both. They are there for alumina, which stops a melted glaze running off the pot, and for the working properties that keep a glaze suspended in the bucket. Knowing which does what makes a recipe adjustable rather than a spell.
China clay: pure, white, and not very plastic
China clay, also called kaolin, is the purest and whitest of the clays and the closest to a single mineral. In a glaze it supplies alumina without adding iron or colour, which is why white and pale glazes lean on it. In a body it gives whiteness and firing strength. What it does not give is plasticity: a body made only of china clay would barely hold a shape.
Ball clay: plastic, darker, and the working partner
Ball clay is fine-particled and highly plastic, which is what makes a clay body throwable and what keeps a glaze in suspension rather than settled as a brick at the bottom of the bucket. It carries more iron and organic matter than china clay, so it fires darker. Most recipes use a proportion of both: china clay for purity, ball clay for behaviour.
Why alumina matters in a glaze
A glaze is glass plus flux plus stiffener. Without alumina the melt is fluid and runs off the pot onto the kiln shelf, taking both with it. The clay in a recipe is where most of that alumina comes from, and adjusting it is the standard first move on a glaze that runs. Adding a couple of percent of china clay is the usual fix, and it also stiffens the raw glaze in the bucket.
Buying and storing them
Both are sold as powders by the bag and both are cheap by the kilogram compared with colourants. They are also both silica-bearing powders, which means they are weighed with a fitted respirator in a ventilated space, and the bench is wet-wiped rather than swept afterwards. Kept dry and sealed they last indefinitely.
Questions people ask about china clay
What is the difference between china clay and ball clay?
China clay is purer, whiter and barely plastic; ball clay is finer, darker and highly plastic. Recipes use both because each supplies something the other does not.
Is china clay the same as kaolin?
Yes. Kaolin is the mineral name and china clay the traditional British trade name for the same material.
Why does my glaze run off the pot?
Usually too little alumina. Adding a small percentage of china clay stiffens the melt and is the standard first adjustment.