The incredible plasticity of bentonite. And a lesson it teaches.

Wednesday 17th April 2019

The 20cm vase on the left is thrown from what I thought was a very plastic body, I achieved close to the same thickness top-to-bottom (5mm). The one on the right was the same original height, 20cm. But it has dried down to only 18cm high, it shrinks 14% (vs. 6% for the other). The thinnest part of the wall is near the bottom, only 2mm thick! How is it possible to throw that thin? The body is 50% ball clay and 50% bentonite. Bentonite, by itself, cannot be mixed with water, but dry-blended with fine-particled ball clay it can. It took about 4 days to dewater the slurry on my plaster table. This is the poorest drying body one could possibly use. But there is the lesson here: Even this can be dried crack-free. How? One month under cloth and plastic to assure water content throughout! This means that pretty well any other body can be dried without cracks if done sufficiently evenly.

Pages that reference this post in the Digitalfire Reference Database:

Bentonite, The Black Art of Drying Ceramics Without Cracks, Plasticity

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