Frederich, Tim on thu 11 jul 02
Hello,
Vince said that quartz inversion is of little concern in heating a bisque
firing. It may not be a problem with some clay bodies but it can be with
others. I feel that quartz inversion is always important. This happens at
573 degrees C(1063 degrees F). The quartz phase of silica goes from alpha
quartz to beta quartz. Cracking can occur if the ware is fired too fast
through this change. It is even more important on the way back down when it
changes back. It shrinks in size as it changes back and can cause cracking
called dunting. This change happens every time it passes through this
temperature unless the quartz has become glass (as in a glaze surface).
Slow firing is always the best. This has been said over and over. You do
not save enough money by firing fast to take the risk in my opinion. Slow
firing also allows time for all of the organic material and gases to burn
out and escape the clay body.
Best regards,
Tim Frederich
Ron Roy on fri 12 jul 02
I don't think the quartz inversion is a problem on the way up - and here is
my thinking.
Looking at the inversion consequences on the way down - it makes sense to
me that dunting can occur because the rim - cooling faster than the base -
tries to get smaller and the base - says - no way - I'm not ready to get
smaller yet (it's not cooled enough to go through the inversion yet) - if
you want to get smaller you are going to have to suffer the consequences -
like crack.
On the way up the rim - reaches the inversion temperature (573C) usually
before the base and gets bigger. In fact it comes under compression (as
opposed to tension on cooling) which makes it stronger.
I have not done any experiments to prove this but - I have fired tons of
Porcelain (25% silica) fast - in bisque firings and glaze firings and have
not had any cause for concern.
Now it may seem from what I just said that I am advocating fast firing of
bisque - I am not - it is crucial - especially with iron bearing clays -
that they be bisqued slow enough to eliminate the carbonous materials that
lead to reduced iron and over vitrified clay during the later glaze
firings.
I do think that if we understand why things happen we have an enormous
advantage when solving problems. So I offer this not as a critical comment
but as encouragement to understand more about the processes we all need to
know more about.
RR
>Vince said that quartz inversion is of little concern in heating a bisque
>firing. It may not be a problem with some clay bodies but it can be with
>others. I feel that quartz inversion is always important. This happens at
>573 degrees C(1063 degrees F). The quartz phase of silica goes from alpha
>quartz to beta quartz. Cracking can occur if the ware is fired too fast
>through this change. It is even more important on the way back down when it
>changes back. It shrinks in size as it changes back and can cause cracking
>called dunting. This change happens every time it passes through this
>temperature unless the quartz has become glass (as in a glaze surface).
>
>Slow firing is always the best. This has been said over and over. You do
>not save enough money by firing fast to take the risk in my opinion. Slow
>firing also allows time for all of the organic material and gases to burn
>out and escape the clay body.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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