Nikom Chimnok on sun 16 apr 00
> ----------------------------Original message----------------------------
>Jeff Lawrence wrote:
> "Free silica" is often used to describe that silica which will form into
> crystobalite during firing and break pots on the cooldown.
**************************************************************************
Jeff,
The presence of some cristobalite does not necessarily mean that a pot will
break during cooldown. I know this because the nice lady at the Thai Center
for Ceramics Development is currently testing everybody's local clays for
cristobalite, using her X-ray Diffraction machine, and our clay develops
quite a bit of cristobalite when fired to Cone 7 reduction. It figures,
because the clay has lots of silica under 4 microns, and not much flux.
During the same week in which she sent her printouts, we got a new fiber
module kiln, and the builder did a test firing, then left the damper wide
open and cooled it from 1250 C to 165 C in 8 hours, at which point we opened
the door.
No cooling cracks, surprisingly.
It may be that the 20% grog, ranging from 125 to nearly 2000 microns, and
very refractory, prevented any cracking. I don't claim to understand it, but
those are the facts.
Nikom
Ron Roy on mon 24 apr 00
This is not a simple question to answer - some cristobalite can be
necessary to counter act crazing of some glazes - it can also lead to
dunting when combined with certain lower expansion glazes.
It is generally recommended that cristobalite be controlled and minimized
in ovenware - the reason is - if a casserole is closer to one element in an
electric oven - one side will get heated faster - go through the
cristobalite conversion - (get bigger) faster than the other side. The
cracking can happen immediately or over time.
It is certainly not wanted in any body that is going to be used with an
open flame or on the stove top.
Much depends on the compatibility of clays and glazes from the fit
perspective. Picture a tea pot with a glaze inside that has not contracted
as much as the clay on cooling - my dilatometer experiments and Peter
Songhens observations show that aside from the spectacular increase in
expansion of cristobalite around 200C is one thing - but the overall
expansion of the body is increased as well. So you have this glaze - too
big for the clay - pushing out from the inside of the tea pot - you pour in
boiling water - for a split second - it makes the liner glaze get bigger -
expansion on heating - the tea pot cracks - sometimes sooner sometimes
later. Some times not - the clay is thick enough to resist the force of the
glaze - and/or the glaze is thinner this time.
If the inside glaze is crazed - smaller than the clay - this expansion with
boiling water has no effect - look at the number of crazed tea pots around
- and compare with the number that are not crazed. Gotta be more crazed
ones.
RR
>----------------------------Original message----------------------------
>> ----------------------------Original message----------------------------
>>Jeff Lawrence wrote:
>> "Free silica" is often used to describe that silica which will form into
>> crystobalite during firing and break pots on the cooldown.
>**************************************************************************
>Jeff,
>The presence of some cristobalite does not necessarily mean that a pot will
>break during cooldown. I know this because the nice lady at the Thai Center
>for Ceramics Development is currently testing everybody's local clays for
>cristobalite, using her X-ray Diffraction machine, and our clay develops
>quite a bit of cristobalite when fired to Cone 7 reduction. It figures,
>because the clay has lots of silica under 4 microns, and not much flux.
>During the same week in which she sent her printouts, we got a new fiber
>module kiln, and the builder did a test firing, then left the damper wide
>open and cooled it from 1250 C to 165 C in 8 hours, at which point we opened
>the door.
>No cooling cracks, surprisingly.
>It may be that the 20% grog, ranging from 125 to nearly 2000 microns, and
>very refractory, prevented any cracking. I don't claim to understand it, but
>those are the facts.
>
>Nikom
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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