How a kaolin and ball clay compare in a dry performance test

Monday 24th October 2011

These are DFAC drying performance disks of a large-particle insight-live.com/material/925">kaolin (OptiKast) and a ball clay (Plainsman A2). This test reveals a clay's response to uneven drying (these disks are dried with the center portion covered). The kaolin feels smoother yet its ultimate particles are ten to one hundred times bigger than a typical ball clay. Thus it shrinks much less. The ball clay has dramatically lower water permeability, water from the center protected portion resists migration to the outer edge during drying. When the inner section finally dried the outer was already rigid so it split the disk in two and pulled all the edge cracks. Most ball clays shrink more and crack worse than this (cracks concentric to the center also appear). So why use ball clay? This kaolin is so lacking in plasticity it was barely possible to even make this disk. And it is so weak that it can easily break just by handling it. Still, it is useful to make casting bodies. But the ball clay, when used as a percentage of a body mix, can produce highly plastic bodies than can be dried without trouble if done evenly.

Pages that reference this post in the Digitalfire Reference Library:

Ball Clay, Kaolin, OptiKast Kaolin, DFAC Drying Factor, It dry shrinks much more yet cracks less. How is that possible?, Ultimate Particles, Primary Clay, Ball Clay

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