iandol on sun 22 dec 02
Dear Jim Murphy,
I think we are getting into post graduate terretory. Definitely not Clay =
and Glaze 101!!
As they say, many a slip twixt cup and lip. What I write may be wrong =
and should be tested by trial and examination. Validate everything =
empirically!! Cross reference as well. Take a second opinion. What's the =
Ferrenghi "Rule" here?
Given that you have achieved vitrification so that your ceramic is now =
Mullite bonded with a Soda Glass you ask <digitally controlled kiln, enhance or hinder my chances of trapping the =
alkalis (KNaO) in an alumino-silica glass matrix and developing Mullite =
crystals for greater body strength, or must there be a trade-off between =
the two ?>>
Once the glass has formed the Sodium ions are trapped. Depending on the =
purity of your ingredients there may be a little iron and calcium there =
as well as the Aluminium which was in the Nepheline Syenite. This seems =
to me to be very like a "beer glass" glass, or at least a good =
approximation. Soda Glass is not known to dissolve in water or simple =
acids, weak or strong. Normal "Shut off Cooling" should preserve this =
structure.
So what about "Arrested" or "Slow" cooling? This will, in all =
probability, have no effect on the Mullite but it can have an effect on =
the glass. Depending on the composition of this, and it is currently of =
"Unknown" composition, it may start to crystallise. The slower the =
cooling rate the greater the chance of this happening and the greater =
the degree to which it will happen. Composition will rule the roost. =
With luck you will grow Albite or Nepheline (these will detract from =
translucency). But if the glass is rich in Sodium Oxide and low in =
Aluminium ions you may also get Sodium Disilicate. This is the "Bug" it =
is water soluble. To overcome the chances of this happening it may pay =
to substitute a proportion of your 50% Neph Sy with good old Talc or =
Wollastonite if you wish to keep in phase with Cone 6 firings.
Hope I didn't spill too much down my bib.
Best regards,
Ivor
Jim Murphy on mon 23 dec 02
Seasons Greetings Ivor,
First, let me apologize Ivor as I hit the send-key Saturday, without
properly "addressing" your name in my previous message, while rushing out to
the theater to see the "Christmas Spectacular" show starring the Radio City
Rockette dancers. I'm glad you responded.
on 12/22/02 12:54 AM, iandol at iandol@TELL.NET.AU wrote:
> Given that you have achieved vitrification so that your ceramic is now Mullite
> bonded with a Soda Glass you ask <> controlled kiln, enhance or hinder my chances of trapping the alkalis (KNaO)
> in an alumino-silica glass matrix and developing Mullite crystals for greater
> body strength, or must there be a trade-off between the two ?>>
Yes, had I given myself just a little more time, a better question would
have concerned clay body "glass" and mullite development using different
oxidation firing schedules whereby the same amount of Cone 6 heatwork is
performed using different firing times and at different maximum
temperatures. This could be achieved by kiln firing control "on the way up
only" or by control both on the way up and use of "firing-down".
> So what about "Arrested" or "Slow" cooling? This will, in all probability,
> have no effect on the Mullite but it can have an effect on the glass.
> Depending on the composition of this, and it is currently of "Unknown"
> composition, it may start to crystallise. The slower the cooling rate the
> greater the chance of this happening and the greater the degree to which it
> will happen. Composition will rule the roost. With luck you will grow Albite
> or Nepheline (these will detract from translucency).
Interesting ... reminds me now of why glass artists (fusing/casting) rapidly
cool from max. temp down to the glass annealing temp - to avoid
detrification (growth of crystals) - and only then do they slow down and
control the rate of cooling to the glass "strain" point to alleviate
stresses in the glass.
Hmmm, although the particle physics of a clay body composition are not the
same as soda-lime glass, it appears I'd probably be wasting my time
experimenting with clay body firing schedules involving "firing-down" slowly
from max. temperature. Something tells me, for clay bodies, I needn't be
concerned with crash-cooling either (hopefully that's the better-half of my
conscious talking).
Seems longer firing times (firing up only) is the ticket. I'll have to
determine whether longer firing times at lower maximum temperature - with
Cone 6 heatwork the same - is necessary to avoid warping/slumping for my
particular application.
> But if the glass is rich in Sodium Oxide and low in Aluminium ions you may
also get Sodium Disilicate. This is the "Bug" it is water soluble. To
overcome the chances of this happening it may pay to substitute a proportion
of your 50% Neph Sy with good old Talc or Wollastonite if you wish to keep
in phase with Cone 6 firings.
Perhaps this is why 2% Talc was used along with 48% Neph Sy in the Cone 6
body recipe I noted in my earlier message. I was thinking it was there only
to provide auxiliary flux material (MgO). Thanks for the heads-up on Sodium
Disilicate.
I've got to get myself a new "bib".
Thanks again Ivor,
Jim Murphy
(Incidently, the "Christmas Spectacular" show was well - SPECTACULAR ! - as
were those 18 pair of Rockette's legs.)
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