Jorge Nabel on wed 28 aug 02
Clayarters, I=B4m in a dilemma. Im firing an L&l electric kiln with this
computerized control. Reading some posts I decided to control the cooling
process so I made the kiln turn on again in 600C and off in 500C in a 60C=
/h
speed. But as the computer tries to make all parts same temperature I fee=
l
that the pots are going a dozen times through the quartz inversion up an
down,up and down.So, Im thinking this is worse than leaving it naturally.
Any idea??
Jorge en Buenos Aires
Carol Tripp on sun 1 sep 02
Hello Jorge,
Looks like no one responded on Clayart, so I'll put my opinion. I don't
understand why you are letting the tempurature fall all the way to 600C and
then doing a controlled cool down to 500C. From what I have learned, the
important zone for slow cooling is from your top tempurature down to 700C.
Others stop the controlled cool at 800C but I fire a lot of iron red glazes
and like to keep things slow to 700C. I have tried lots of cooling rates
but my latest is this;
top temp 1205C with a soak until ^6 falls, then cool naturally (that's fast
in my L&L) down to 1037C, then set the controller to drop 50C/hour down to
987C and then 25C/hour down to 700C. I haven't actually tried this yet as I
am not firing due to outside temps of 45C but this is the plan. What I have
done up until now is more simple:
top temp of 1205C, soak, then 50C/hour down to 700C and that's it.
I hope this is clear and that it is of some help to you.
Best regards,
Carol
Dubai, UAE
Jorge wrote:
>Clayarters, I´m in a dilemma. Im firing an L&l electric kiln with this
>computerized control. Reading some posts I decided to control the cooling
>process so I made the kiln turn on again in 600C and off in 500C in a 60C/h
>speed. But as the computer tries to make all parts same temperature I feel
>that the pots are going a dozen times through the quartz inversion up an
>down,up and down.So, Im thinking this is worse than leaving it naturally.
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Alistair Gillies on mon 2 sep 02
Hi,
There does seem to be a measure of disagreement and confusion about the
best cooling procedures so here is my twopenny worth.
Unless your glaze needs special slow cooling after whatever soak you use
almost all clays and certainly all commercial clays will be fine if you
just turn off your kiln and leave it to cool. The rate of descent at top
temperatures is really concerned with the maturing of glazes.
A rapid cooling down to 750-800 is sometimes used to speed up production
where time is money [I used work in pottery where I had to empty the
same kiln of 420 biscuit fired mugs every morning - the faster the temp
came down the cooler it was for me]. If one is going to the drastic
measure of opening the bung at 1060 or above then you must ensure that
there are no inward draughts and that this descent is even.
The problems start to arise because of the uneven expansion and
contraction of certain types of silica [cristobalite and I have
forgotten the other]. These materials are awkward because rather than
expanding/contracting in a linear manner they have a sudden jump at
approx 226 and 550 [it is some time since I was tested on this!].
This means that all of the pot must travel through this temperature zone
at the same time - if one side cools more rapidly then it will contract
before the other and potentially crack. Refractory clays such as Raku
have the structural strength to withstand this shock.
These are the temps to be vary of and Jorge is right on being concerned
at making the pots jump repeatedly through this zone as it is a point
where there is stress on the clay. Just keeping the bung and spyhole
closed through this temp range and the clay should be fine.
All the best,
Alistair Gillies
Shropshire, England
In message , Carol Tripp
writes
>Hello Jorge,
>Looks like no one responded on Clayart, so I'll put my opinion. I don't
>understand why you are letting the tempurature fall all the way to 600C and
>then doing a controlled cool down to 500C. From what I have learned, the
>important zone for slow cooling is from your top tempurature down to 700C.
>Others stop the controlled cool at 800C but I fire a lot of iron red glazes
>and like to keep things slow to 700C. I have tried lots of cooling rates
>but my latest is this;
>top temp 1205C with a soak until ^6 falls, then cool naturally (that's fast
>in my L&L) down to 1037C, then set the controller to drop 50C/hour down to
>987C and then 25C/hour down to 700C. I haven't actually tried this yet as I
>am not firing due to outside temps of 45C but this is the plan. What I have
>done up until now is more simple:
>top temp of 1205C, soak, then 50C/hour down to 700C and that's it.
>
>I hope this is clear and that it is of some help to you.
>Best regards,
>Carol
>Dubai, UAE
>
>Jorge wrote:
>
>>Clayarters, I´m in a dilemma. Im firing an L&l electric kiln with this
>>computerized control. Reading some posts I decided to control the cooling
>>process so I made the kiln turn on again in 600C and off in 500C in a 60C/h
>>speed. But as the computer tries to make all parts same temperature I feel
>>that the pots are going a dozen times through the quartz inversion up an
>>down,up and down.So, Im thinking this is worse than leaving it naturally.
>
>
>_________________________________________________________________
>Send and receive Hotmail on your mobile device: http://mobile.msn.com
>
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>
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>
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Alistair Gillies
Deborah Thuman on wed 2 jun 10
I know it's the quartz going from alpha to beta and from beta to alpha.
Unfortunately, I've no idea what that means. What's the alpha form and
what's the beta form?
TIA.
Deb Thuman
http://debthumansblog.blogspot.com/
http://www.etsy.com/shop.php?user_id=3D5888059
http://www.facebook.com/pages/Deb-Thumans-Art-Page/167529715986
Louis Katz on thu 3 jun 10
Here is the Wikipedia entry on this, my comments below it.
When polymorphism exists as a result of difference in crystal packing, it=
=3D
is
called packing polymorphism. Polymorphism can also result from the existe=
=3D
nce
of different conformers of the same molecule in conformational polymorphi=
=3D
sm.
In pseudopolymorphism the different crystal types are the result of
hydration or solvation. An example of an organic polymorph is glycine, wh=
=3D
ich
is able to form monoclinic and hexagonal crystals. Silica is known to for=
=3D
m
many polymorphs, the most important of which are; α-quartz, &=3D
;#946;-quartz,
tridymite, cristobalite, coesite, and stishovite.
If you were packing grapefruit into boxes, you would might load the botto=
=3D
m
layer in a square grid. The next layer of grapefruit, might sit directly
atop the layer below, or they might sit in the spaces between, staggering=
=3D
the orientation of the layers.
The first layer may not be in a square grid but might have staggered rows=
=3D
.
Each of these orientations has a different density, or size per grapefrui=
=3D
t.
The different polymorphs of silica similarly have different densities or
sizes, different thermal expansion properties and other qualities. Alpha
quartz is one arrangement of silica atoms into a 3 dimensional space, Bet=
=3D
a
is a similar arrangement, but with different angles between the individua=
=3D
l
atoms. Beta Quartz takes up more space.
I hope this helps, I really think my little Alpha-Beta Quartz "dance"
explains it better, but this is a text document.
Louis
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