Post 61 More clay formulation tests

This is going to be a little technical for most folks but for the ceramics affectionados it might carry some interest.

Some time ago I ran a series of tests adding various materials to the clay body I have been using to evaluate the impact on deformation, shrinkage and porosity. Nothing stood out as a winner which was good enough to go to the trouble of adding and combining a dry material to the clay.
The best result came from the addition of Kaolin, but it wasn't good enough to take up the option.

A little further research suggested two further alternative prospects. One is like a calcined version of Kaolin called Molochite and the other is a calcined Alumina. Calcination drives out all forms of water both free and molecular. The composition of Molochite is principally silicon and aluminium oxides and the Alumina clearly just aluminium oxide.


So I designed as set of test samples using these materials at concentrations up to 20% using two Clayworks stoneware bodies - RGH and CTS.

 The photograph above shows the stack of 12 deformation test samples, stacked in order of performance. The RGH samples - darker colour - performed less favourably to the CTS.
CTS also showed less shrinkage which was consistent with the deformation characteristic.
Unfortunately this superior performance was achieved it seems, by the action of the additives in elevating the vitrification temperature of the clay body, demonstrated by relatively poor levels of permeability at increasing levels of the additives. The CTS also had an unsatisfactory permeability when fired to the same schedule as the RGH.

Taking the clay to vitrification temperature is all about achieving impermeability and so to include a refractory additive which might improve the physical dimensional characteristics (stability) at the cost of vitrification and permeadility would be going in the wrong direction. That's likely to be what the Chinese mass bonsai pot market producers seem to be doing. This might be ok if you are making a sculptural object where the dimensional and shrinkage properties have a higher priority but for a bonsai container exposed to the elements it's the wrong end of the compromise stick.

Interesting as the idea of an additive was it's looking like a dead end. Besides incorporating a dry powder into a clay body in sufficient quantity, up to 20%, would be a huge pain in the asp so I'm pleased not to have found a compelling reason to do it. The answer might well be the simple matter of testing just how much heat work is needed to achieve vitrification; where is the tipping point, the elbow in the curve. I'll just have to fire up the test kiln and run some different firing schedules to learn a little more.