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


Monday 24th October 2011

These are DFAC insight-live.com/glossary/170">drying performance disks of a large-particle kaolin (Opticast) and a ball clay (Plainsman A2). The DFAC test displays a plastic clay's response to very uneven drying (these disks are dried with the center portion covered to set up a water content differential). These materials both feel super-smooth, in fact, the kaolin feels smoother. But the ultimate particles are ten to one hundred times smaller than kaolin, thus it shrinks much more. The ball clay also has much lower water permeability, being unable to channel water from the center protected portion fast enough. 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, DFAC Drying Factor, It dry shrinks much more yet cracks less. How is that possible?, Ultimate Particles, Primary Clay, Ball Clay


This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.