Survey of Clays

A non-exhaustive list of tests on various North American raw clays.

Pseudo Soda Firing

I simulated soda firing for slips created from one type of clay.

1. A slip was made from 60% clay and 40% Nepheline Syenite
   1. The slip was not sieved
2. The slip was applied to leather hard test tiles
   1. The test tiles were made from a white burning stoneware
3. The test tiles were bisque fired to cone 05.
4. The test tiles were then soaked in a 10wt% soda ash solution
5. The test tiles were fired to cone 6 in reduction

The following clays were tested:

• Kaolin
  • Grolleg Kaolin
  • Tile 6 Kaolin
  • Helmer Kaolin
• Ball Clay
  • Kentucky OM4 Ball Clay
  • Bell Dark Ball Clay
• Stoneware Clay
  • Goldart
  • Hawthorne Bond

• Iron percentage and silica-alumina ratio were not correlated to fired color and degree of Alkali Flashing
• This test confirms the popular uses of certain clays
  • Grolleg fired gray. Grolleg is known to be a very white-burning clay with low iron content.
  • Helmer flashed deep red-brown, much more than I expected from its iron content
    • Helmer is a common ingredient in flashing slips for wood and soda firing
• The ball clays had similar fired appearance
  • Some degree of sheen on the surface, likely due to soluble salts in the clay
  • Light brown flashing
• Goldart flashed a light orange-red with iron speckling
• Hawthorne flashed a deep orange-red
  • Iron speckles are more numerous than Goldart
  • Surface is very grainy due to the large particles
• These tests were made in 2023 and 2024
  • Most clays should be recently mined, but some may be a few years old

Created 2024-04-15.

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