Post 47 Slumping test

Some time ago I posted about Pyroplastic Deformation with a picture showing how the bisqued clay could deform under the glaze firing conditions. That test sparked a little research and an idea to test a number of ceramic materials as additives to the clay body to assess any impact on the slumping character.

I started with 8 materials, then realised I needed a control sample, then read about grog and silica sand. That resulted in 10 samples, listed in the table below.

Thes pictures show the sample bars arranged in 'bend order', best at the front to worst to the back, in the above picture and clearly left to right below.

It is interesting to observe a change in colouration too. The bars which have deformed the most have darkened in colour. The additives in these bars have obviously 'fluxed' the clay; in effect enabling the clay to mature at a lower tempreature than without the additive. The one doubt about this conclusion is that the worst two have relatively high water absorption or porosity (for stoneware).

At the other end of the scale, the two sand additive bars, have shown the lowest deformation and the highest absorption, which will eliminate these as prospects. The control absorpton result was 0.8% which is very good.

For the best 8 samples the deformation difference between them isn't really signifiacant. So if that's the case then the next level of selection is water absorption. On that basis Kaolin is the one that stands out. Perhaps this should be no surprise as Kaolin is basically quite refractory and is a key ingredient in high firing ceramics.You might expect that it would elevate the maturity temperature, but the absorption is unchanged for the control sample.

Further reading suggests that you may need to get to 10% additive before really seeing the impact of many materials. I can feel another test series coming up - next time to evaluate a number of different levels of Kaolin addition and bringing in another test - shrinkage as an additional indicator of maturity.