Sample Ring From the Ring Maker
Crispin Pemberton-Pigott, New dawn Engineering, July 30, 2006
I am sending you a relatively clear picture of the ring made last week here in Swaziland. It was formed in the ring maker from New Dawn Engineering. The water content when formed was surprisingly high I think because of the amount absorbed by the charcoal.
It is 2/3 clay (being itself a blend of two clays) and 1/3 pounded charcoal. It was fired at 1000. The clays come from the same pit, from two different strata. It is near Maputo. The mix is for terracotta tiles of good quality.
It has a nice 'ring' to it when struck sounding 'ceramic' and a density of 1.0
From everything I can gather, it is important when firing clay with a large amount to charcoal (or organics) in it to stay at 600 C for quite some time to get rid of it. Additionally, the emissions when doing so are largely CO so it is important to ventilate the place.
There should be air flow through the kiln (typically two 1 inch holes, side and top) to let in more oxygen.
When the carbon content is as high as this it should be held at 600 for about 24 hours in that oxydizing atmosphere.
It seems to me one could tell if it was ready to fire to a higher temperature by weighing it. The chracoal should be almost entirely missing. If it is, it can be raised to 1000 or so. As it is insulative and the clay is not a low thermal expansion material, the temperature rise has to be slow in order for the middle to catch up in size with the outer skin. It is likely that not observing this caution will result in a greatly weakened ring as a result of internal micro-fracturing.
Best regards from