Bill Arnold on fri 8 mar 02
One of the most frequent complaints expressed on the list is the =
observation that glazes do not turn out as expected. Since the release =
of John and Ron's book these comments have increased. A common solution =
to glaze problems is to soak the kiln at the end of the firing or to =
slow the firing as the kiln approaches shut-off. What is neglected is =
what the effect of following such advice will have on glazes and =
clay.When a glaze is above its melting point, the length of time spent =
is almost as important as the final temperature.Once a glaze begins to =
melt , it will continue its various chemical and physical changes until =
it hardens.One may cure pimholes in one glaze by soaking for an hour, =
only to ruin other glazes in the same firing, Some Evenheat Kilns have a =
"Soft Fire" feature which slows the rate of temperature increase towards =
the end of the firing. In their instuction book they point out that if =
you choose not to use this feature, then you should fire almost 40oF =
higher for the same cone. An hour soak could be the equivalent of two =
cones higher. The effect on clay is similar. Once vitrification begins, =
the process will proceed as long as the clay is at or above that =
temperature. This means that a clay body that has a good fit with your =
glazes and suitable thermal shock resistance could lose these =
characteristics due to excessive soaking. The only way to know for sure =
is to test. Small test kilns will tell you very little. You must test =
under your normal firing conditions. The resulting problems from =
excessive soaking may be more than aesthetic. The lawsuit from a cubic =
foot of hot lasagno landing on someone's foot might distract you from =
your creative activities. Some clays and some glazes have wide firing =
ranges. Some do not. To quote a wise man: test test test
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