Tim Willey

Adaptive Construction & Open Firing

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Clays for Open Firing
Clays, traditionally used for rapid open firings, were empirically developed, that is to say, they were chosen purely on a trial and error basis and the acquired knowledge and understanding passed down through the generations, possibly over hundreds and even thousands of years.

It is still possible to find natural clays which have many, if not all of the attributes needed for rapid open firings. Indeed it may be possible to buy ‘off the shelf’ products that might work, up to a point. However, an understanding of the mechanics, chemistry and mineralogy involved are really useful, at least in understanding why some clays blow apart and others don’t.

For simplicity, I’ve listed what I think are the main criteria for an open-firing body. The list is not exhaustive and by no means scientific (in its strictest sense) but from a makers point of view this is what I think we need to be aware of:

  • A clay body can be viewed as plastic clay-minerals binding together other materials (natural non-plastics and additional tempering)
  • It is important to have very fine non-plastics in the body, together with much courser fractions
  • Fine non-plastic materials become coated with the plastic clay minerals and aid workability
  • Course non plastics (opening materials) provide an open structure to the body, allow gases (steam) to escape and interrupt any developing cracks
  • Many natural clays contain the right blend of clay mineral and non-plastic natural inclusions as a basis for an open-fired body

As a starting point I have found that natural clays are the way to go. Even really course clays can be graded down (levigation will remove gravels and stones).

Usually it’s a case of finding the right clay and adding tempering to get the balance right and looking at the evidence, this is very much the way traditional open-firing potters went about it, often travelling considerable distances to source just the right materials.
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This is a magnified image of a section of ceramic fabric after firing. It clearly shows the wide size-differential of the opening material.

The finest non-plastics are not so easily identified as they are largely coated in clay (in this case, the clay minerals, montmorillonite and illite) and so form the plastic matrix of the body, lending workability.

The ceramic fabric is critically, very open, and inter-connecting fissures can be seen throughout the section. These fissures will allow rapid expanding steam to escape without breaking open (spalling) the fabric. Also, any cracks that do develop are effectively interrupted by the tempering.