jonathan byler on mon 23 jan 12
So I keep reading here on clayart (esp from Ron Roy) about how evil
cristobalite is. But I was just reading in a book, "Electric Kilns"
by Harry Fraser about glaze faults, and he lists lack of cristobalite
formation as a primary reason why glazes craze. His take on this is
that as the glaze cools to 437F the body contracts rapidly and puts
the glaze into compression, keeping it from crazing. He says: " If
cristobalite has not been allowed to develop properly in the body,
there may be little or no compression in the glaze and crazing will
take place very easily."
soooo. which is it? do you need cristobalite for good glaze fit, or
is it the worst thing ever to happen to your claybody, since it can
increase the risk of dunting?
-jon
Ben Shelton on tue 24 jan 12
"soooo. which is it? do you need cristobalite for good glaze fit, or=3D0Ais=
i=3D
t the worst thing ever to happen to your claybody, since it can=3D0Aincreas=
e =3D
the risk of dunting?"=3D0A=3D0A=3D0AI don't think it works that way and I h=
ope th=3D
ose more knowledgable will chime in.=3D0AI think it all depends on the expa=
ns=3D
ion curve of the clay vs the curve of the glaze.=3D0AThe curves must not be=
i=3D
ncompatible.=3D0A=3D0ATheoretically... if you could make a glaze with a hug=
e cr=3D
istobalite "hump" in its dilatometer curve that matches the curve of a clay=
=3D
with lots of cristobalite, they should fit each other.=3D0ADoes anyone kno=
w =3D
of a glaze with a cristobalite hump?=3D0A=3D0AOn the other hand, it is easy=
to =3D
make clay bodies with a huge cristobalite hump and easy to control cristoba=
=3D
lite formation.=3D0ASo, with respect to cristobalite, it is easier to contr=
ol=3D
it in the clay body.=3D0A=3D0ADo you need cristobalitein a clay body and h=
ow m=3D
uch? I think it depends on the expansion curve of the glaze you want to put=
=3D
on it.=3D0A=3D0A=3DA0=3D0ABen
jonathan byler on tue 24 jan 12
My understanding was that the glaze does not/cannot form
cristobalite. But my understanding is a little lacking.
On Jan 24, 2012, at 7:37 AM, Ben Shelton wrote:
> "soooo. which is it? do you need cristobalite for good glaze fit, or
> is it the worst thing ever to happen to your claybody, since it can
> increase the risk of dunting?"
>
>
> I don't think it works that way and I hope those more knowledgable
> will chime in.
> I think it all depends on the expansion curve of the clay vs the
> curve of the glaze.
> The curves must not be incompatible.
>
> Theoretically... if you could make a glaze with a huge cristobalite
> "hump" in its dilatometer curve that matches the curve of a clay
> with lots of cristobalite, they should fit each other.
> Does anyone know of a glaze with a cristobalite hump?
>
> On the other hand, it is easy to make clay bodies with a huge
> cristobalite hump and easy to control cristobalite formation.
> So, with respect to cristobalite, it is easier to control it in the
> clay body.
>
> Do you need cristobalitein a clay body and how much? I think it
> depends on the expansion curve of the glaze you want to put on it.
>
>
> Ben
jonathan byler on tue 24 jan 12
google translate will give a decent answer in whatever the french =3D20
equivalent of chinglesh is. but if you would translate it properly, =3D20
that would be AWESOME.
thanks.
On Jan 24, 2012, at 1:50 PM, Edouard Bastarache wrote:
> Denis Caraty, aka Smert.Conseil, take on cristobalite in clays.
> It is in French ,I may translate it later and send to the list.
> He deals with that on an everyday basis at the Gien Factory,
> in France.
> http://www.gien.com/
>
> "La cristobalite est n=3DE9cessaire dans les produits de fa=3DEFence dure=
=3D
de =3D20
> type
> feldspathique. Cela ne concerne pas les p=3DE2tes vitrifi=3DE9es ni les =
=3D20=3D
> fa=3DEFences
> calcaires."
> =3D3D "Cristobalite is necessary in hard earthenware feldspathic clays,
> but not in vitrified nor in calcium earthenware clays."
>
> Iznogood Bastarache
>
> Gis,
>
> http://www.flickr.com/photos/30058682@N00/
> http://edouardbastarache.blogspot.com/
> http://smart2000.pagesperso-orange.fr/bloggs_edouard.htm
> http://www.facebook.com/edouard.bastarache
>
>
>
>
>
>
> ----- Original Message -----
> From: "Smart"
> To: "Edouard Bastarache"
> Sent: Tuesday, January 24, 2012 2:07 PM
> Subject: Re: cristobalite/claybodies
>
>
>> Salut ED,
>>
>> Ce sujet est tr=3DE8s int=3DE9ressant, il est au cour de mon quotidien =
=3D
dans =3D20
>> la
>> fabrication des fa=3DEFences feldspathiques.
>>
>> Il manque dans le message de Jonathan l'information essentielle qui =3D2=
0=3D
>> est
>> que la cristobalite est n=3DE9cessaire "dans les produits de fa=3DEFence=
=3D20=3D
>> dure de
>> type feldspathique". Cela ne concerne pas les p=3DE2tes vitrifi=3DE9es n=
i =3D
les
>> fa=3DEFences calcaires.
>>
>> Lors du refroidissement de la gla=3DE7ure, la mise en compression par =
=3D
le
>> crochet de dilatation de la cristobalite (Diminution de 2.8% du =3D20
>> volume
>> cristallin entre 250 et 220=3DB0C) du tesson est n=3DE9cessaire pour ce =
=3D20=3D
>> type de
>> p=3DE2te. Cela agit comme une pr=3DE9contrainte de la gla=3DE7ure qui =
=3D
supporte =3D20
>> mieux
>> le gonflement du biscuit de fa=3DEFence lors du vieillissement du =3D20
>> produit, car
>> les biscuits de fa=3DEFences feldspathiques se dilatent en =3D20
>> vieillissant. Cela
>> est d=3DFB =3DE0 une lente r=3DE9-hydroxylation quasi-in=3DE9vitable pro=
pre =3DE0=3D
ce =3D20
>> type de
>> composition c=3DE9ramique poreuse... Si la gla=3DE7ure n'est pas =3D20
>> pr=3DE9contrainte par
>> un crochet cristobalite elle est soumise =3DE0 un effort de traction =3D=
20=3D
>> par le
>> biscuit qui se dilate petit =3DE0 petit et finit pas tr=3DE9sailler =3D2=
0
>> (crazing) car
>> les gla=3DE7ures sont peu r=3DE9sistantes aux efforts de traction. La =
=3D20
>> cristobalite
>> peut =3DEAtre produite dans la p=3DE2te par la cuisson =3DE0 partir de l=
a
>> transformation du quartz ou bien ajout=3DE9e sous forme de cristobalite =
=3D
=3D20
>> d=3DE9j=3DE0
>> form=3DE9e (galets de mer calcin=3DE9s broy=3DE9s).
>>
>>
>> Cordialement, Denis
>>
>> ----- Original Message -----
>> From: "Edouard Bastarache"
>> To: "Smart.Conseil"
>> Sent: Tuesday, January 24, 2012 5:54 PM
>> Subject: Fw: cristobalite/claybodies
>>
>>
>>> Courte traduction:
>>>
>>> C'est l'auteur Harry Fraser qui dit que =3DE7a prend une certaine =3D20
>>> quantit=3DE9 de
>>> cristobalite dans l'argile pour pr=3DE9venir le tr=3DE9saillement.
>>> J'encule Ron Roy,,,
>>>
>>> Gis,
>>>
>>> Edouard Bastarache
>>> Spertesperantisto
>>>
>>> Sorel-Tracy
>>> Quebec
>>>
>>> http://www.flickr.com/photos/30058682@N00/
>>> http://edouardbastarache.blogspot.com/
>>> http://smart2000.pagesperso-orange.fr/bloggs_edouard.htm
>>> http://www.facebook.com/edouard.bastarache
>>>
>>>
>>>
>>>
>>>
>>>
>>> ----- Original Message -----
>>> From: "jonathan byler"
>>> To:
>>> Sent: Monday, January 23, 2012 7:08 PM
>>> Subject: cristobalite/claybodies
>>>
>>>
>>>> So I keep reading here on clayart (esp from Ron Roy) about how evil
>>>> cristobalite is. But I was just reading in a book, "Electric =3D20
>>>> Kilns"
>>>> by Harry Fraser about glaze faults, and he lists lack of =3D20
>>>> cristobalite
>>>> formation as a primary reason why glazes craze. His take on this =3D2=
0=3D
>>>> is
>>>> that as the glaze cools to 437F the body contracts rapidly and puts
>>>> the glaze into compression, keeping it from crazing. He says: " If
>>>> cristobalite has not been allowed to develop properly in the body,
>>>> there may be little or no compression in the glaze and crazing =3D20
>>>> will
>>>> take place very easily."
>>>>
>>>> soooo. which is it? do you need cristobalite for good glaze =3D20
>>>> fit, or
>>>> is it the worst thing ever to happen to your claybody, since it can
>>>> increase the risk of dunting?
>>>>
>>>>
>>>> -jon
>>>>
>>>
>>>
>>
>>
jonathan byler on tue 24 jan 12
mmm. just realized that link doesn't actually take me to an
article... could you send that please?
On Jan 24, 2012, at 1:50 PM, Edouard Bastarache wrote:
> http://www.gien.com/
Edouard Bastarache on tue 24 jan 12
Its the Gien Factory's URL where Smart.Conseil has been working for 35 =3D
years. It wont send you to an article on cristobalite.
His own site is=3D20
http://perso.wanadoo.fr/smart2000/index.htm
Maybe one day he will write an article on cristobalite to be put on his =3D
personal site. Who knows, more translating for me!!!
Plejkore,
Edouard Bastarache=3D20
Spertesperantisto=3D20
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://smart2000.pagesperso-orange.fr/bloggs_edouard.htm
http://www.facebook.com/edouard.bastarache
----- Original Message -----=3D20
From: jonathan byler=3D20
To: Edouard Bastarache ; Clayart=3D20
Sent: Tuesday, January 24, 2012 4:55 PM
Subject: Re: cristobalite/claybodies
mmm. just realized that link doesn't actually take me to an article... =
=3D
could you send that please?
On Jan 24, 2012, at 1:50 PM, Edouard Bastarache wrote:
http://www.gien.com/
ivor and olive lewis on wed 25 jan 12
Dear Jon,
Interesting question ! !.
Does anyone know how to calculate, estimate or measure the proportion of
Cristobalite to Mullite and vitreous phase in any fired body?
As I see it, if there is an over abundance of Cristobalite in a mature body
the glaze will shiver.
Am I mistaken when I say that it is uneven temperature during cooling that
leads to Dunting ?
Given the temperature of the cristobalite phase change there is a good
reason not to accelerate cooling rate of a cooling kiln until the interior
of your pots has cooled well below the phase change temperature.
Regards,
Ivor Lewis,
REDHILL,
South Australia
ronroy@CA.INTER.NET on thu 26 jan 12
First of all - anyone interested in this subject should download a =3D20
copy of the article by Peter Sonhgen from the Studio Potters web site =3D20
- especially anyone firing stoneware from cone 9 and up.
http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
It would be very unusual to find any cristobalite in a glaze - as Ivor =3D2=
0
says it would have to be very underfired.
The huge problem with cristobalite is - it goes through it's volume =3D20
change (bigger when heating - smaller when cooling) at oven =3D20
temperatures - so if you are making ovenware you will guarantee =3D20
premature failure. What you need for oven ware is lower expansion =3D20
bodies - not higher - especially at ovenware temperatures.
It is true that cristobalite helps reduce crazing - because it =3D20
increases clay body contraction during cooling. So it does help to =3D20
keep glazes under compression and prevent crazing.
It can also make glazes with lower expansions shiver and cause =3D20
dunting. Why anyone would want to risk that instead of adjusting =3D20
glazes to fit without the help of cristobalite is beyond me.
If all your glazes suffer from the same fit problem - lets say crazing =3D2=
0
- then changing the clay body makes sense - other wise you risk =3D20
upsetting some of your glazes while fixing others.
It is also true that industry does - sometimes introduce cristobalite =3D20
to help cure crazing in low fired ware - but it has to be strictly =3D20
controlled because the effect gets stronger with the amount added.
By the way cristobalite does not happen naturally at earthen ware =3D20
temperatures - it starts to be formed above 1100 C but takes time to =3D20
develop. The more time it takes to get to the top temperature =3D20
(starting at 1100C) and the time it takes to get down again to 1100C =3D20
the more cristobalite is formed.
You can see how the length of firing and cooling will have an effect - =3D2=
0
hard for us to control.
The way to stop cristobalite is to have enough sodium, potassium and =3D20
lithium oxide around to melt the very fine particles that start to =3D20
emerge around 1100C. As Peter has determined - about 10 spar in a =3D20
stoneware body. Looks like the new Custer is not going to do the job =3D20
at 10% by the way - calculation will tell you how much you need in =3D20
that case.
I have never - seen any cristobalite in any porcelain by the way - =3D20
just too much spar for it to happen.
I'll be happy to explain this further if anyone wants that - The Hamer =3D2=
0
dictionary has a good explanation as well.
To answer the question directly - do you need cristobalite to control =3D20
crazing? the answer is no and there is some considerable risk solving =3D20
the problem that way,
RR
> "soooo. which is it? do you need cristobalite for good glaze fit, or
> is it the worst thing ever to happen to your claybody, since it can
> increase the risk of dunting?"
>
>
> I don't think it works that way and I hope those more knowledgable =3D20
> will chime in.
> I think it all depends on the expansion curve of the clay vs the =3D20
> curve of the glaze.
> The curves must not be incompatible.
>
> Theoretically... if you could make a glaze with a huge cristobalite =3D20
> "hump" in its dilatometer curve that matches the curve of a clay =3D20
> with lots of cristobalite, they should fit each other.
> Does anyone know of a glaze with a cristobalite hump?
>
> On the other hand, it is easy to make clay bodies with a huge =3D20
> cristobalite hump and easy to control cristobalite formation.
> So, with respect to cristobalite, it is easier to control it in the =3D20
> clay body.
>
> Do you need cristobalitein a clay body and how much? I think it =3D20
> depends on the expansion curve of the glaze you want to put on it.
>
> =3DA0
> Ben
>
jonathan byler on thu 26 jan 12
Thanks Ron,
That sounds like a good explanation. If you want to explain more, I
would be glad to read more of your explanations. I should also see if
I can find a copy of the Hamer dictionary in our library, and if not
have our librarian order a copy.
On Jan 26, 2012, at 2:07 PM, ronroy@CA.INTER.NET wrote:
> First of all - anyone interested in this subject should download a
> copy of the article by Peter Sonhgen from the Studio Potters web
> site - especially anyone firing stoneware from cone 9 and up.
>
> http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
>
> It would be very unusual to find any cristobalite in a glaze - as
> Ivor says it would have to be very underfired.
>
> The huge problem with cristobalite is - it goes through it's volume
> change (bigger when heating - smaller when cooling) at oven
> temperatures - so if you are making ovenware you will guarantee
> premature failure. What you need for oven ware is lower expansion
> bodies - not higher - especially at ovenware temperatures.
>
> It is true that cristobalite helps reduce crazing - because it
> increases clay body contraction during cooling. So it does help to
> keep glazes under compression and prevent crazing.
>
> It can also make glazes with lower expansions shiver and cause
> dunting. Why anyone would want to risk that instead of adjusting
> glazes to fit without the help of cristobalite is beyond me.
>
> If all your glazes suffer from the same fit problem - lets say
> crazing - then changing the clay body makes sense - other wise you
> risk upsetting some of your glazes while fixing others.
>
> It is also true that industry does - sometimes introduce
> cristobalite to help cure crazing in low fired ware - but it has to
> be strictly controlled because the effect gets stronger with the
> amount added.
>
> By the way cristobalite does not happen naturally at earthen ware
> temperatures - it starts to be formed above 1100 C but takes time to
> develop. The more time it takes to get to the top temperature
> (starting at 1100C) and the time it takes to get down again to 1100C
> the more cristobalite is formed.
>
> You can see how the length of firing and cooling will have an effect
> - hard for us to control.
>
> The way to stop cristobalite is to have enough sodium, potassium and
> lithium oxide around to melt the very fine particles that start to
> emerge around 1100C. As Peter has determined - about 10 spar in a
> stoneware body. Looks like the new Custer is not going to do the job
> at 10% by the way - calculation will tell you how much you need in
> that case.
>
> I have never - seen any cristobalite in any porcelain by the way -
> just too much spar for it to happen.
>
> I'll be happy to explain this further if anyone wants that - The
> Hamer dictionary has a good explanation as well.
>
> To answer the question directly - do you need cristobalite to
> control crazing? the answer is no and there is some considerable
> risk solving the problem that way,
>
> RR
>
>> "soooo. which is it? do you need cristobalite for good glaze fit, or
>> is it the worst thing ever to happen to your claybody, since it can
>> increase the risk of dunting?"
>>
>>
>> I don't think it works that way and I hope those more knowledgable
>> will chime in.
>> I think it all depends on the expansion curve of the clay vs the
>> curve of the glaze.
>> The curves must not be incompatible.
>>
>> Theoretically... if you could make a glaze with a huge cristobalite
>> "hump" in its dilatometer curve that matches the curve of a clay
>> with lots of cristobalite, they should fit each other.
>> Does anyone know of a glaze with a cristobalite hump?
>>
>> On the other hand, it is easy to make clay bodies with a huge
>> cristobalite hump and easy to control cristobalite formation.
>> So, with respect to cristobalite, it is easier to control it in the
>> clay body.
>>
>> Do you need cristobalitein a clay body and how much? I think it
>> depends on the expansion curve of the glaze you want to put on it.
>>
>>
>> Ben
>>
ronroy@CA.INTER.NET on fri 27 jan 12
Hi Jonathan - that is correct - but slips could.
RR
Quoting jonathan byler :
> My understanding was that the glaze does not/cannot form
> cristobalite. But my understanding is a little lacking.
>
> On Jan 24, 2012, at 7:37 AM, Ben Shelton wrote:
>
>> "soooo. which is it? do you need cristobalite for good glaze fit, or
>> is it the worst thing ever to happen to your claybody, since it can
>> increase the risk of dunting?"
>>
>>
>> I don't think it works that way and I hope those more knowledgable
>> will chime in.
>> I think it all depends on the expansion curve of the clay vs the
>> curve of the glaze.
>> The curves must not be incompatible.
>>
>> Theoretically... if you could make a glaze with a huge cristobalite
>> "hump" in its dilatometer curve that matches the curve of a clay
>> with lots of cristobalite, they should fit each other.
>> Does anyone know of a glaze with a cristobalite hump?
>>
>> On the other hand, it is easy to make clay bodies with a huge
>> cristobalite hump and easy to control cristobalite formation.
>> So, with respect to cristobalite, it is easier to control it in the
>> clay body.
>>
>> Do you need cristobalitein a clay body and how much? I think it
>> depends on the expansion curve of the glaze you want to put on it.
>>
>>
>> Ben
>
ronroy@CA.INTER.NET on fri 27 jan 12
Hi Ivor,
I do not know how to calculate how much cristobalite will be formed in
any given clay body but - the formula would have to take into account
time spent above 1100C and mesh size of any free quartz. Given the
natural variations in our materials I would say - not in any reliable
way.
Yes - shivering can be a problem and - with glazes ending up too big
for the clay - dunting as well.
The same mechanism that produces dunting because of the quartz
inversion at 573C can also cause dunting around 200C during the
cristobalite conversion although the volume change during the
cristobalite is more gradual.
RR
Quoting ivor and olive lewis :
> Dear Jon,
>
> Interesting question ! !.
>
> Does anyone know how to calculate, estimate or measure the proportion of
> Cristobalite to Mullite and vitreous phase in any fired body?
>
> As I see it, if there is an over abundance of Cristobalite in a mature bo=
dy
> the glaze will shiver.
>
> Am I mistaken when I say that it is uneven temperature during cooling tha=
t
> leads to Dunting ?
>
> Given the temperature of the cristobalite phase change there is a good
> reason not to accelerate cooling rate of a cooling kiln until the interi=
or
> of your pots has cooled well below the phase change temperature.
>
>
>
> Regards,
>
>
>
> Ivor Lewis,
> REDHILL,
> South Australia
>
ronroy@CA.INTER.NET on sat 28 jan 12
There is just to much KNaO in glazes for any crystalline silica to exist.
I have dilatometered many glazes at al temperatures and only found a
hint in one sample which was clearly underfired.
When cristobalite first appears during a firing (around 1100C) it is a
very fine crystal and - if there is enough sodium, potassium or
lithium oxide around it is easily melted - not in a crystalline state
any more - amorphous silica which does not go through any inversions
such as cristobalite and quartz crystals do.
In Perter Sohngen's article he proves that if you have enough KNaO,
cristobalite is melted as it forms and so cannot not propagate as it
normally does.
RR
Quoting Des & Jan Howard :
> Ron
> Would you care to expand on that comment?
> Des
>
> On 1/28/2012 6:20 AM, ronroy@CA.INTER.NET wrote:
>> Hi Jonathan - that is correct - but slips could.
>>
>> Quoting jonathan byler :
>>> My understanding was that the glaze does not/cannot form
>>> cristobalite. But my understanding is a little lacking.
>
>
> --
> Des & Jan Howard
> Lue Pottery
> Lue NSW
> Australia
> 2850
>
> 02 6373 6419
> www.luepottery.hwy.com.au
> -32.656072 149.840624
>
Des & Jan Howard on sat 28 jan 12
Ron
Would you care to expand on that comment?
Des
On 1/28/2012 6:20 AM, ronroy@CA.INTER.NET wrote:
> Hi Jonathan - that is correct - but slips could.
>
> Quoting jonathan byler :
>> My understanding was that the glaze does not/cannot form
>> cristobalite. But my understanding is a little lacking.
--
Des & Jan Howard
Lue Pottery
Lue NSW
Australia
2850
02 6373 6419
www.luepottery.hwy.com.au
-32.656072 149.840624
ivor and olive lewis on sat 28 jan 12
Take notice of Ron Roy's conclusion
"To answer the question directly - do you need cristobalite to control
crazing? The answer is no and there is some considerable risk solving the
problem that way ".
Without laboratory help from knowledgeable and well trained Scientists we
will never know when, or how much, cristobalite there may be in a ceramic
body. But, like the phenomenon of Creep in metals, the low temperature phas=
e
transformation of Beta Cristobalite to Alpha Cristobalite expected at 226
deg Celsius may be delayed for a considerable time after the work has
reached ambient (room) temperature. The sound, when it happens, is, in my
experience similar to that of a 3030 rifle.
Regards,
Ivor Lewis,
REDHILL,
South Australia
ivor and olive lewis on mon 30 jan 12
Ron,
I suppose if people are willing to read or teach what Hamer, Parmelee and
the Singers have to say and apply their information in a critical way to
what may be known or assumed about the materials being used it becomes
possible to set up precautionary working processes that help to avoid some
of these mishaps.
I emphasise the distinction between fired clay and a glaze at the selected
maturing point. In general terms the former is always a mixture of solids
which may be permeated with a lower melting point fluxing agent while the
latter is a fluid, a mixture which is a solution comprising a solvent
through which is uniformly dispersed a number of solutes. ( Some glaze
ingredients are insoluble. Some melt to become immiscible liquids These
exceptions do not invalidate the general idea )
Sincere regards,
Ivor.
> Hi Ivor,
>
> I do not know how to calculate how much cristobalite will be formed in
> any given clay body but - the formula would have to take into account
> time spent above 1100C and mesh size of any free quartz. Given the
> natural variations in our materials I would say - not in any reliable
> way.
>
> Yes - shivering can be a problem and - with glazes ending up too big for
> the clay - dunting as well.
>
> The same mechanism that produces dunting because of the quartz inversion
> at 573C can also cause dunting around 200C during the cristobalite
> conversion although the volume change during the cristobalite is more
> gradual.
>
> RR
>
> Quoting ivor and olive lewis :
>
>> Dear Jon,
>>
>> Interesting question ! !.
>>
>> Does anyone know how to calculate, estimate or measure the proportion o=
f
>> Cristobalite to Mullite and vitreous phase in any fired body?
>>
>> As I see it, if there is an over abundance of Cristobalite in a mature
>> body
>> the glaze will shiver.
>>
>> Am I mistaken when I say that it is uneven temperature during cooling
>> that
>> leads to Dunting ?
>>
>> Given the temperature of the cristobalite phase change there is a good
>> reason not to accelerate cooling rate of a cooling kiln until the
>> interior
>> of your pots has cooled well below the phase change temperature.
>>
>>
>>
>> Regards,
>>
>>
>>
>> Ivor Lewis,
>> REDHILL,
>> South Australia
>>
>
>
>
>
ronroy@CA.INTER.NET on mon 30 jan 12
Hi Ivor,
The experiments and conclusions by Peter Sohngen give the best answer
on how to avoid (or get) cristobalite in a clay body. I wish you would
download the article and tell us what you think about his method and
conclusions.
http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
Best regards - RR
Quoting ivor and olive lewis :
> Ron,
> I suppose if people are willing to read or teach what Hamer,
> Parmelee and the Singers have to say and apply their information in
> a critical way to what may be known or assumed about the materials
> being used it becomes possible to set up precautionary working
> processes that help to avoid some of these mishaps.
> I emphasise the distinction between fired clay and a glaze at the
> selected maturing point. In general terms the former is always a
> mixture of solids which may be permeated with a lower melting point
> fluxing agent while the latter is a fluid, a mixture which is a
> solution comprising a solvent through which is uniformly dispersed a
> number of solutes. ( Some glaze ingredients are insoluble. Some
> melt to become immiscible liquids These exceptions do not invalidate
> the general idea )
>
> Sincere regards,
>
> Ivor.
>
>
>
>
>> Hi Ivor,
>>
>> I do not know how to calculate how much cristobalite will be formed
>> in any given clay body but - the formula would have to take into
>> account time spent above 1100C and mesh size of any free quartz.
>> Given the natural variations in our materials I would say - not in
>> any reliable way.
>>
>> Yes - shivering can be a problem and - with glazes ending up too
>> big for the clay - dunting as well.
>>
>> The same mechanism that produces dunting because of the quartz
>> inversion at 573C can also cause dunting around 200C during the
>> cristobalite conversion although the volume change during the
>> cristobalite is more gradual.
>>
>> RR
>>
>> Quoting ivor and olive lewis :
>>
>>> Dear Jon,
>>>
>>> Interesting question ! !.
>>>
>>> Does anyone know how to calculate, estimate or measure the proportion =
of
>>> Cristobalite to Mullite and vitreous phase in any fired body?
>>>
>>> As I see it, if there is an over abundance of Cristobalite in a mature =
body
>>> the glaze will shiver.
>>>
>>> Am I mistaken when I say that it is uneven temperature during cooling t=
hat
>>> leads to Dunting ?
>>>
>>> Given the temperature of the cristobalite phase change there is a good
>>> reason not to accelerate cooling rate of a cooling kiln until the inte=
rior
>>> of your pots has cooled well below the phase change temperature.
>>>
>>>
>>>
>>> Regards,
>>>
>>>
>>>
>>> Ivor Lewis,
>>> REDHILL,
>>> South Australia
>>>
>>
>>
>>
>>
>
>
Taylor Hendrix on mon 30 jan 12
So to recap the general characeristics of silica and cristobalite in
clay bodies:
Silica inverts at 573 degrees F, inverts rapidly, and the inversion
measures around 2% in volume change
Cristobalite inverts at 200 something degrees F, inverts somewhat more
gradually, and its inversion measures about 7% in volume change.
Someone want to clean up the particulars? I just want to have these
little tidbits correct before I store them away in my dusty cabeza.
Taylor, in Rockport TX
wirerabbit1 on Skype (-0600 UTC)
http://wirerabbit.blogspot.com
http://wirerabbitpots.blogspot.com
http://www.flickr.com/photos/wirerabbit/
https://youtube.com/thewirerabbit
On Fri, Jan 27, 2012 at 1:35 PM, wrote:
> Hi Ivor,
>
> I do not know how to calculate how much cristobalite will be formed in
> any given =3DC2=3DA0clay body but - the formula would have to take into a=
ccou=3D
nt
> time spent above 1100C and mesh size of any free quartz. Given the
> natural variations in our materials I would say - not in any reliable
> way.
...
Paul Herman on mon 30 jan 12
Ron and All,
The Studio Potter article "Cristobalite: the Hump" by Sohngen was
preceded (by ten years) by an earlier article on the same subject,
"Body Building for Potters" by Jim Robinson. I think people should
read that one first because it lays the groundwork for the later
article. In the first article, Jim presents a series of glazes with
differing percentages of thermal expansion, from very high to very
low, to be used as diagnostic tools for evaluation of clay bodies. One
of the nice things about his method is that it is possible for a
studio potter to do it at home, without any fancy equipment.
Mr. Sohngen states in his footnotes:
"We all owe Jim Robinson of Phoenix, Oregon a tremendous debt for his
clear thinking and inspiration. My effort here is a mere footnote to
Jim's groundbreaking article in Studio Potter ten years ago."
I think it's good to give credit where credit is due.
Mr. Sohngen also sounds very much like he thinks that some
cristobalite isn't always such a bad thing after all, and it can help
glaze fit in some instances. As always, there is a place of balance to
be sought. There really are no "evil" materials, just materials.
People have a monopoly on evil.
Good mixing,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Jan 30, 2012, at 10:32 AM, ronroy@CA.INTER.NET wrote:
> Hi Ivor,
>
> The experiments and conclusions by Peter Sohngen give the best answer
> on how to avoid (or get) cristobalite in a clay body. I wish you would
> download the article and tell us what you think about his method and
> conclusions.
>
> http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
>
> Best regards - RR
>
> Quoting ivor and olive lewis :
>
>> Ron,
>> I suppose if people are willing to read or teach what Hamer,
>> Parmelee and the Singers have to say and apply their information in
>> a critical way to what may be known or assumed about the materials
>> being used it becomes possible to set up precautionary working
>> processes that help to avoid some of these mishaps.
>> I emphasise the distinction between fired clay and a glaze at the
>> selected maturing point. In general terms the former is always a
>> mixture of solids which may be permeated with a lower melting point
>> fluxing agent while the latter is a fluid, a mixture which is a
>> solution comprising a solvent through which is uniformly dispersed a
>> number of solutes. ( Some glaze ingredients are insoluble. Some
>> melt to become immiscible liquids These exceptions do not invalidate
>> the general idea )
>>
>> Sincere regards,
>>
>> Ivor.
>
jonathan byler on mon 30 jan 12
you have your fahrenheit and celsius mixed up.
On Jan 30, 2012, at 12:38 PM, Taylor Hendrix wrote:
> So to recap the general characeristics of silica and cristobalite in
> clay bodies:
>
> Silica inverts at 573 degrees F, inverts rapidly, and the inversion
> measures around 2% in volume change
> Cristobalite inverts at 200 something degrees F, inverts somewhat more
> gradually, and its inversion measures about 7% in volume change.
>
> Someone want to clean up the particulars? I just want to have these
> little tidbits correct before I store them away in my dusty cabeza.
>
> Taylor, in Rockport TX
> wirerabbit1 on Skype (-0600 UTC)
> http://wirerabbit.blogspot.com
> http://wirerabbitpots.blogspot.com
> http://www.flickr.com/photos/wirerabbit/
> https://youtube.com/thewirerabbit
>
>
>
> On Fri, Jan 27, 2012 at 1:35 PM, wrote:
>> Hi Ivor,
>>
>> I do not know how to calculate how much cristobalite will be formed
>> in
>> any given clay body but - the formula would have to take into
>> account
>> time spent above 1100C and mesh size of any free quartz. Given the
>> natural variations in our materials I would say - not in any reliable
>> way.
> ...
Edouard Bastarache on mon 30 jan 12
Saluton Ed
The problem of feldspathic earthenware clays is
very interesting, it is part of my daily duties at the
manufacture.
Cristobalite is necessary in =AB hard earthenware
products of the feldspathique type =BB. It does not
concern vitrified bodies nor calcium earthenware
clays.
During cooling of the glaze, the setting in compression
by the dilatation =AB hook =BB of cristobalite (Reduction
of 2.8% of crystalline volume between 250 and 220=B0C)
of the shard is necessary for this type of body.
That acts as a pre-stressing of the glaze which supports
better the swelling of the earthenware biscuit during
the ageing of the product, because the feldspathic
earthenware biscuit dilates upon growing old.
This is due to the nearly inevitable D-hydroxylation
specific to this type of porous ceramic composition.
If the glaze is not pre-stressed by a hook of cristobalite,
it is subjected to a traction effort by the biscuit which
dilates gradually and it does not stop crazing because
the glazes are not very resistant to traction efforts.
Cristobalite can be produced in the clay by the firing
from the transformation of quartz, or added in the
form of cristobalite already formed.
Cristobalite is formed above 1100=B0C and its formation
depends directly on the time spent above this temperature.
That is what occurs in feldspathic earthenware bodies.
These industrial feldspathic earthenware bodies are
baked between 1180 and 1230=B0C in tunnel kilns with
long time cycles. Thus cristobalite develops by firing
in hard earthenware bodies of high temperature.
Plejkore,
Denis Caraty
aka Smart.Conseil
http://perso.wanadoo.fr/smart2000/index.htm
http://www.gien.com/
John Britt on mon 30 jan 12
Off my hip,
Quartz inversion
1063 F a 1 - 2% exp./cont.=3D20
and=3D20
cristobalite=3D20
between 220=3DB0C and 280=3DB0C (428=3DB0F and 536=3DB0F).
got Google? Potter's Dictionary?
john britt pottery
ronroy@CA.INTER.NET on tue 31 jan 12
Hi Paul,
Jim is a friend and he deserves our thanks for many reasons - his
article on shino's was ground breaking.
Peter does believe that some cristobalite will help keep glazes from
crazing - that is true - but - because we all fire differently we are
going to get different amounts of cristobalite in the same clay body.
I think bodies that don't form any are the safer way to go - adjusting
glazes so they don't craze.
When we used to buy silica there was some correlation between what
mesh was posted on the bag and what was inside - when you bought 200
mesh the majority of the silica in that bag was around 200M.
Now with more efficient milling machinery only about 5% is 200 mesh
with much of the remaining is micro fine - which easily converts to
cristobalite.
Anyone who wants to so some experiments - my dilatometer still works -
glad to work with those who want to publish articles that will be
useful to potters.
Quoting Paul Herman :
> Ron and All,
>
> The Studio Potter article "Cristobalite: the Hump" by Sohngen was
> preceded (by ten years) by an earlier article on the same subject,
> "Body Building for Potters" by Jim Robinson. I think people should
> read that one first because it lays the groundwork for the later
> article. In the first article, Jim presents a series of glazes with
> differing percentages of thermal expansion, from very high to very
> low, to be used as diagnostic tools for evaluation of clay bodies. One
> of the nice things about his method is that it is possible for a
> studio potter to do it at home, without any fancy equipment.
>
> Mr. Sohngen states in his footnotes:
>
> "We all owe Jim Robinson of Phoenix, Oregon a tremendous debt for his
> clear thinking and inspiration. My effort here is a mere footnote to
> Jim's groundbreaking article in Studio Potter ten years ago."
>
> I think it's good to give credit where credit is due.
>
> Mr. Sohngen also sounds very much like he thinks that some
> cristobalite isn't always such a bad thing after all, and it can help
> glaze fit in some instances. As always, there is a place of balance to
> be sought. There really are no "evil" materials, just materials.
> People have a monopoly on evil.
>
> Good mixing,
>
> Paul Herman
>
> Great Basin Pottery
> Doyle, California US
> www.greatbasinpottery.com/
>
>
>
>
> On Jan 30, 2012, at 10:32 AM, ronroy@CA.INTER.NET wrote:
>
>> Hi Ivor,
>>
>> The experiments and conclusions by Peter Sohngen give the best answer
>> on how to avoid (or get) cristobalite in a clay body. I wish you would
>> download the article and tell us what you think about his method and
>> conclusions.
>>
>> http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
>>
>> Best regards - RR
>>
>> Quoting ivor and olive lewis :
>>
>>> Ron,
>>> I suppose if people are willing to read or teach what Hamer,
>>> Parmelee and the Singers have to say and apply their information in
>>> a critical way to what may be known or assumed about the materials
>>> being used it becomes possible to set up precautionary working
>>> processes that help to avoid some of these mishaps.
>>> I emphasise the distinction between fired clay and a glaze at the
>>> selected maturing point. In general terms the former is always a
>>> mixture of solids which may be permeated with a lower melting point
>>> fluxing agent while the latter is a fluid, a mixture which is a
>>> solution comprising a solvent through which is uniformly dispersed a
>>> number of solutes. ( Some glaze ingredients are insoluble. Some
>>> melt to become immiscible liquids These exceptions do not invalidate
>>> the general idea )
>>>
>>> Sincere regards,
>>>
>>> Ivor.
>>
>
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