Coade Stone Totum – Kiln Fired

The Clay components had to be air dried in a warm room for between 7-10 days.  If any moisture remained within the body of the clay, the components would be susceptible to cracking or explosion.  In addition to this, if through the process of wedging air pockets remain, or the clay too dry and therefore the ingredients not mixed enough this would also result in cracking.

P1260862These are the test shots of the four different batches of clay.  I will explore the mechanical and  physical properties of each batch and compare.  I have used Batch One for my main Totum components.  Batch One consists of : 70% Ball Clay; 5% Crushed Flint; 10% Quartz and 15% Grog (Calcined China Clay).

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The kiln was in cold state when the components were put into it.  Because of the silica content within the components it was necessary to  put a protective layer of ceramic paper down.  (fibre glass layers)  This ensured that any excess silica would not result in the components  being stuck within the kiln.  The first layer consisted of the test shots and test cubes.  The cubes were put toward the rear as they still felt slightly heavy (retain moisture) and there was a concern that they may explode.

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The second layer was set up with the main body of the totum components.  Toward the front were four rods, these were samples of each batch.  They were propped on some ceramic paper, this was to see if any of them would bend and thus an indication of warping.  Also at the front were the broken pieces of pineapple leaves.  These had broken in transit and mainly due to the fact that the leaves had been modelled at an earlier  stage than the head of the pineapple.  You cannot fix leather hard clay with green clay.

All components in place, the kiln was programmed to run for a period of 38 hours and a cooling down period of a further four hours at which                                                                                                     point the kiln was still hand hot.  The programme was finely detailed to ensure the chemical process occurs at the right time and mitigates any potential stresses or failure, it took half and hour to set.  The firing regime was to have a slow rise in temperature to allow any remaining moisture to escape through  the fissures within the clay created by the grog.    Following the low temperature state, the increase in temperature to above 100 degrees C  would allow the ingredients to go beyond any stress points and then the rise of temperature gradually to a maximum of 1050 degrees C before reducing back down to 30 degrees at the same rate.  If the temperature and time is not set correctly for the materials propoerties the stone will fail, too hot too soon, explode, not hot enough not long enough, crack and crumble, too hot or too long, explode or crack and dis-colour or burn.

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The firing complete and the components all intact, no explosions, no cracks, and no splits!  Even the rods to test for bending  have remained fairly straight, with batch two having a slight hint of a bend.  It seems that the grog originating from china clay  has given my artificial stone a ‘porcelain’ tinge.  The overall result is worth all the effort.

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P1260946This is how it will look when the fixings have been placed.

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