Re: Hardening of Outdoor Oven clay?

Posted by Rado (144.134.37.198)

In Reply to: Hardening of Outdoor Oven clay? posted by Robert Richard

Absolutely it makes sense, because after putting clay structure together clay still contains a large amount of water that has to dry out before it is fired. Otherwise the heated water changes to steam inside the clay body expanding it and bang! Remember those banging wet rocks around campfires? Steam has great power, try to put small piece of nearly dry clay into microwave oven, but stem further from it when you set it on for 1 minute ;o)

Why clay drying should go for so long? It is not only water as we see it daily, there is also water called 'chemical water' that's being there by crystallization in kaolin - the water chemically combined with crystals in the clay is critical at 300°C - 572°F, this water should be driven out by slow temperature increase: 100°C - 212°F in no shorter time then one hour untill you reach highest temperature, after this 'first time slow heat up' you can fire quicker. When you fire out this chemical water you will sense burning particles in the air that are burning off, they're leaving the clay and won't come back again.

However you can speed it up. But first make sure clay is totally dry before applying assisting drying fires. When clay is wet it's dark in color, drying makes it lot lightish. First it dries on outside but naturally it is or can be still wet inside (in the body.) Drying should be slow to limit creations of cracks by shrinkage, any form of support to copy a shape could be lined by thicker wet-newspaper layer or cardboard, this will shrink together with clay - if the oven was formed right on e.g. wooded form the form will not change size together with shrinking clay. Best if the built structure is covered by plastic sheet for the first week, because the oven is large this makes the drying much more even. r

Re: Hardening of Outdoor Oven clay?