Craig Martell on wed 10 aug 05
Lili was asking:
>So. If a glaze has a low c.o.e, and if it does not shiver peel etc on a
>given body, what is the need to "fix" it. I have some pots with a glaze on
>it that has a c.o.e of 61 and has been on those pots for 20 some years, no
>problem at all.
Hello Lili:
Calculated expansions are just guidelines and tell you which way you are
moving with regard to more or less expansion in a fired glaze. My feeling
is that they almost never tell what the "actual" coefficient of expansion
is. If you want the straight poop on that, you'll need to have a test run
with a dilatometer. Spendy, so use the money for coffee and pie.
Calculated expansions relate to bright, fully fused glazes with no crystal
development. When there is crystal development in a glaze, the expansion
goes up. Also, the analyses that your computer uses to calculate the COE
may not be the same as the raw materials you are actually using in the
glazes. Crazing and shivering also relate to the clay that the glazes are
applied to. Different clays have different COEs too. Your glazes are
perfectly ok on the clay you've used but they could shiver on other claybodies.
If your low COE glazes are ok for 20 years they will probably be fine until
hell freezes over and you don't need to fix them. But the low COE values
tell you to be vigilant about using them on other clays.
regards, Craig Martell Hopewell, Oregon
John Hesselberth on wed 10 aug 05
On Aug 10, 2005, at 8:53 PM, Lili Krakowski wrote:
> So. If a glaze has a low c.o.e, and if it does not shiver peel etc on
> a
> given body, what is the need to "fix" it. I have some pots with a
> glaze on
> it that has a c.o.e of 61 and has been on those pots for 20 some
> years, no
> problem at all.. Silica is 2.4 . It is a stable agreeable glaze.
Hi Lili,
No need at all to 'fix' it if it fits that body. I find that COEs more
around 68-70 fit 'typical' cone 6 bodies. 75 will often craze,
particularly on porcelains. But the point of COE calculations is to
find what fits your body. Then as you develop new glazes you can be
more aware of what might work and what might cause problems.
It is also important to note that we don't include colorants in COE
calculations--we usually just don't have enough knowledge to do so,
although we do for iron oxide. For example if you had a fair amount of
iron in your 61 glaze that would effectively raise the COE to a more
normal number. For example, that is why Waxwing in MC6G works. Its base
glaze has a calculated COE of 60 or 61 but it also has a lot of iron.
If you include the iron in the calculation it comes up to 69. I
wouldn't want to use that glaze as a base glaze without the
iron--shivering/dunting would very likely result on my clay body and on
most cone 6 bodies.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Lili Krakowski on wed 10 aug 05
I just added some glazes to my GlazeMaster list and find that some have a
low co-efficient of expansion. As I recall RonJohn speak of around 75.
Meanwhile I was reading Michael Bailey and he points out that while crazing
can show up pretty slowly, shivering and peeling show up within a few days
of firing.
So. If a glaze has a low c.o.e, and if it does not shiver peel etc on a
given body, what is the need to "fix" it. I have some pots with a glaze on
it that has a c.o.e of 61 and has been on those pots for 20 some years, no
problem at all.. Silica is 2.4 . It is a stable agreeable glaze.
Lili Krakowski
Be of good courage
Ron Roy on thu 11 aug 05
Hi Lili,
It's good to know the relative expansion of glazes - so you know which ones
to keep an eye on.
We tend to think the clays we use today are the same as those we will use
next year - well some times they are - but sometimes they have changed or
we have changed our firings.
Chapter 5 in our book goes into great detail about this - that - combined
with the set of fit testing glazes will provide all the info necessary for
cone 6 - and I use the same criteria for cone 10 porcelain for instance.
The ideal number for the clays I use is 71 for instance but that is
misleading - I know the expansion rates of the clays I use. A glaze with a
calculated expansion of 71 can easily crack a clay that has enough
cristobalite in it.
Put the glaze on a cylinder - thrown or slab - big mug size - keep the clay
about 1/4 inches thick - glaze the inside and fire it - if it's not cracked
then - freeze it for 24 hours - if it's still not cracked - put it in a
sink, still frozen and pour in boiling water.
If it survives that you can sleep well - knowing your customers will
probably not stress it more - unless they apply direct intense, uneven
heat.
RR
>I just added some glazes to my GlazeMaster list and find that some have a
>low co-efficient of expansion. As I recall RonJohn speak of around 75.
>
>Meanwhile I was reading Michael Bailey and he points out that while crazing
>can show up pretty slowly, shivering and peeling show up within a few days
>of firing.
>
>So. If a glaze has a low c.o.e, and if it does not shiver peel etc on a
>given body, what is the need to "fix" it. I have some pots with a glaze on
>it that has a c.o.e of 61 and has been on those pots for 20 some years, no
>problem at all.. Silica is 2.4 . It is a stable agreeable glaze.
>
>
>Lili Krakowski
>
>Be of good courage
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
David Hewitt on thu 11 aug 05
Lili,
Whether or not a glaze crazes or shivers depends on the expansion of the
clay body as well as the glaze. This can well explain the different
figures you quote. You can see more of what I mean by looking at my
website and going on to Pottery Techniques / Calculating Crazing. This
is also one of the articles on this subject that Mike Bailey and I have
written on this subject and which have been published in Ceramic Review.
A more recent article of ours, 'Glaze Fit on Porcelain', appeared in
Ceramic Review Issue 214 in July/August this year, and this may also be
of interest to you if you have not already seen it.
http://www.dhpot.demon.co.uk
David
In message , Lili Krakowski writes
>I just added some glazes to my GlazeMaster list and find that some have a
>low co-efficient of expansion. As I recall RonJohn speak of around 75.
>
>Meanwhile I was reading Michael Bailey and he points out that while crazing
>can show up pretty slowly, shivering and peeling show up within a few days
>of firing.
>
>So. If a glaze has a low c.o.e, and if it does not shiver peel etc on a
>given body, what is the need to "fix" it. I have some pots with a glaze on
>it that has a c.o.e of 61 and has been on those pots for 20 some years, no
>problem at all.. Silica is 2.4 . It is a stable agreeable glaze.
>
>
>Lili Krakowski
>
>Be of good courage
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
Kathi LeSueur on thu 11 aug 05
Lili Krakowski wrote:
>I just added some glazes to my GlazeMaster list and find that some have a
>low co-efficient of expansion. As I recall RonJohn speak of around 75.
>
>Meanwhile I was reading Michael Bailey and he points out that while crazing
>can show up pretty slowly, shivering and peeling show up within a few days
>of firing.
>
>So. If a glaze has a low c.o.e, and if it does not shiver peel etc on a
>given body, what is the need to "fix" it. I have some pots with a glaze on
>it that has a c.o.e of 61 and has been on those pots for 20 some years, no
>problem at all.. Silica is 2.4 . It is a stable agreeable glaze.>>>>>
>
I had a problem with a family of glazes some years ago. All of them
shared the same base. Suddenly, they all shivered in a firing. The glaze
would come off of the rims as I was taking them from the kiln. It turned
out to be a clay problem. The supplier replaced it without argument and
I still use his clay today. Moral #1? Glazes either fit or they don't.
If they suddenly don't fit look for a change in something. Moral #2?
Treat your customers right and they will stick with you even when there
is a problem.
kathi
>
>
>
Ron Roy on sat 13 aug 05
Hi Craig,
I hope you don't mind me asking this on the list - I would like others to
jump in case they have something to add to this subject line.
I'm not sure calculated expansion is affected with some small
recrystallization in glazes - but I also realize I need to do some
experimentation to actually find out.
I'm sure there is a point when it really does begin to have a large affect
as well.
I am also curious about "recrystallization always increases expansion."
While I understand it is true about crystalline silica in clays - my
understanding for glazes is - calculation of matte glazes is not reliable.
I took that to mean the expansion could increase or decrease as well -
depending on the type of crystals involved.
Do you have different information?
In a similar vein - have you ever been successful in curing crazing in a
matte glaze and retained the same look and mattness?
RR
Hope you are well and looking forward to finally meeting you next March - RR
>Calculated expansions relate to bright, fully fused glazes with no crystal
>development. When there is crystal development in a glaze, the expansion
>goes up. Also, the analyses that your computer uses to calculate the COE
>may not be the same as the raw materials you are actually using in the
>glazes. Crazing and shivering also relate to the clay that the glazes are
>applied to. Different clays have different COEs too. Your glazes are
>perfectly ok on the clay you've used but they could shiver on other claybodies.
>
>If your low COE glazes are ok for 20 years they will probably be fine until
>hell freezes over and you don't need to fix them. But the low COE values
>tell you to be vigilant about using them on other clays.
>
>regards, Craig Martell Hopewell, Oregon
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Craig Martell on sat 13 aug 05
Ron was asking:
>I hope you don't mind me asking this on the list - I would like others to
>jump in case they have something to add to this subject line.
Hello Ron:
Of course I don't mind. These sorts of things should be discussed and
viewed be all the list members. Especially if there are questions and
concerns about validity and application of what any of us say about
technical issues as well as all the other stuff.
>I'm not sure calculated expansion is affected with some small
>recrystallization in glazes
I would have to agree with this. I'm not sure either but I tend toward
thinking that crystals will have some sort of effect on expansion but it's
hard to know at what point crystals will shift the expansion enough to
cause fit problems.
>I am also curious about "recrystallization always increases expansion."
>While I understand it is true about crystalline silica in clays - my
>understanding for glazes is - calculation of matte glazes is not reliable.
My understanding is that crystalline silica will increase expansion in
glazes by robbing the glass matrix of silica. When enough silica is
captured by crystal development the expansion of the glaze may go up enough
to cause the glaze to craze. Could we say that this is sort of like having
two glazes on one pot without actually using two different
glazes? Essentially, it's just a crystal phase existing in a glassy
matrix. I'm mainly thinking and talking about satin matts and glazes that
are not reliant on a silica shortage to produce the matt surface.
I have a magnesia matt, actually two variations of this glaze, that
crazes. The magnesia is at about 0.3 molecular equivalents and the silica
is in the durable range. I can send the formula and recipe if there's any
interest. Anyway, bright glazes with an expansion value of 7.4 and lower
will fit my claybody. I've lowered the expansion of this glaze to 6.7 and
it will still craze if there are enough crystals. When the glaze cools
quickly and is glassier, it fits ok. I've seen this happen with calcium
matts too.
My information on this phenomenon comes from Hamer and Hamer and
Kingery. I will see if I can find some time to dig this stuff out so I can
at least be more specific about information sources. My feeling about
calculated expansions only relating to bright, fused glazes is that there's
never any discussion about phase differences effecting expansion and I'm
assuming that the COEs are just based on the calculated expansions of the
various oxide components of a glaze and not the actual state of the glaze
in the finished piece.
regards, Craig Martell Hopewell, Oregon
Craig Martell on mon 15 aug 05
a few days ago Ron Roy was saying
>I hope you don't mind me asking this on the list - I would like others to
>jump in case they have something to add to this subject line.
Yeah Ron! It's really a good thing that you posted this query to the
list. The amount of discussion has been unbelieveable. 8>) Perhaps this
microscopic stuff has little or no interest to anyone else. I like to
consider all this stuff and think about it but I'm sort of an odd duck
anyway. I do get a lot of pots made though.
Then again, people may just be busy washing their Bentleys because Jeeves
is cleaning the pool and can't wash the car for them. Maybe they're
checking out some new venues for more 401Ks. I don't know. I'm going back
into lurk mode. I'll get even MORE pots made.
later on, Craig Martell Hopewell, Oregon
Michael Wendt on tue 16 aug 05
Ron, Craig and others,
When I was a student at the University of Idaho in the early 70s, we
sometimes had shivering occur. It turned out the clay mixing technician had
not added any Feldspar. Does that account for it?
My take was the feldspar is needed when a clay body like Helmer (which we
used then) is mixed because it forms glass, takes up the ejected
cristobalite and in this way, both strengthens the body and removes the
cristobalite contraction at the end that might be enough to shiver the glaze
off the clay.
The glaze came off in large sheets, it was so severe.Gloss glazes.
Ideas?
Back to the wheel now.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Craig wrote:
a few days ago Ron Roy was saying
>I hope you don't mind me asking this on the list - I would like others to
>jump in case they have something to add to this subject line.
Yeah Ron! It's really a good thing that you posted this query to the
list. The amount of discussion has been unbelievable. 8>) Perhaps this
microscopic stuff has little or no interest to anyone else. I like to
consider all this stuff and think about it but I'm sort of an odd duck
anyway. I do get a lot of pots made though.
Then again, people may just be busy washing their Bentleys because Jeeves
is cleaning the pool and can't wash the car for them. Maybe they're
checking out some new venues for more 401Ks. I don't know. I'm going back
into lurk mode. I'll get even MORE pots made.
later on, Craig Martell Hopewell, Oregon
Craig Martell on tue 16 aug 05
Michael responded:
>When I was a student at the University of Idaho in the early 70s, we
>sometimes had shivering occur. It turned out the clay mixing technician had
>not added any Feldspar. Does that account for it?
Hello Michael:
Well, yes, but not in this case actually. Absense of spar in a high fire
body would definitely promote cristobalite development and cause glazes to
shiver. The question to Clayart was actually a "non issue". Lily was
calculating some of her glazes and asked why some of the low expanders
weren't shivering on pots she'd had around for 20 years or so. It was a
hypothetical, sort of, glaze question.
But at least someone responded and I thank you for that. I'm still waiting
on the edge of my seat for some Moose Creek Spar.
regards, Craig Martell Hopewell, Oregon
bonnie staffel on wed 17 aug 05
I had a shivering problem when I was firing my Alpine gas kiln. I had found
some black iron sand on the beach of Lake Michigan. Wanting iron spots as
was the mood of the 70's, I would add some to the clay body that I mixed
myself. In my research, someone suggested that I fire a cone cooler, so
dropped from Cone 10 to Cone 9. Worked great and still had the iron
spotting. My glazes also looked fine, think I was using Rhodes 32 from the
book.
Regards,
Bonnie Staffel
http://webpages.charter.net/bstaffel/
http://vasefinder.com/bstaffelgallery1.html
Charter Member Potters Council
Craig Martell on thu 18 aug 05
Bonnie sez:
>I had a shivering problem when I was firing my Alpine gas kiln. In my
>research, someone suggested that I fire a cone cooler, so
>dropped from Cone 10 to Cone 9. Worked great
Hello Bonnie:
If the shivering was a result of cristobalite, lowering the firing
temperature and the duration of the fire at high temp would lessen the
amount of cristobalite developed thereby curing the shivering
problem. Cristobalite develops at high temperatures and is an ongoing
thing. If there's enough cristobalite formed the claybody goes thru two
inversions on cooling. The first being the quartz inversion at about 1032
F and the second is the cristobalite inversion at about 435 F. This second
inversion will cause shivering and sometimes shattering of the pots if
enough cristobalite has been formed. This kind of problem is best
understood on a "case by case" basis. It depends on the composition of the
clay, length and temp of the fire, glazes used etc. This may not have been
what happened in your case but I think what I've stated may very well have
been the actual scenario. ad infinitum................
regards, Craig Martell Hopewell, Oregon
Ron Roy on thu 18 aug 05
Hi Michael,
Yes - that would do it. I have done many measurements of raw clays - fired
at cone 10 in reduction - there are very few that don't show a lot of
cristobalite - simply because there is not the KNaO present to melt the
fine silica - either ejected or added to clay bodies.
It's the microfine silica that converts easily.
Peter Sohngen did the definitive article on this - see Studio Potter vol 28 #1.
Have at least 10% spar to melt the cristobalite as it forms - using graded
quartz (not microfine) makes a big difference when you need free quartz to
get a bigger beta to alpha quartz inversion - to help prevent crazing.
I did the dilatometery for the article - once you see those charts you will
understand what you need to know to eliminate cristobalite from high fired
stoneware clay.
Never any cristobalite in any porcelains I have measured by the way -
always more than 10% spar is the reason - no matter how much microfine
silica is present.
The burning question is - how much crystalline formation in glazes before
the crystals begin to affect the expansion of a glaze - and does it alway
increase the expansion?
I know Craig is right - that as crystals form they rob the surrounding
glaze of silica - which will make that phase of the glaze tend to craze -
and it will also reduce the durability - because silica is so tied in with
durability.
I am currently asking related questions of ceramic engineers - they are
taking a long time to answer - so I suspect we are into an area that is not
common experience.
RR
>Ron, Craig and others,
>When I was a student at the University of Idaho in the early 70s, we
>sometimes had shivering occur. It turned out the clay mixing technician had
>not added any Feldspar. Does that account for it?
>My take was the feldspar is needed when a clay body like Helmer (which we
>used then) is mixed because it forms glass, takes up the ejected
>cristobalite and in this way, both strengthens the body and removes the
>cristobalite contraction at the end that might be enough to shiver the glaze
>off the clay.
>The glaze came off in large sheets, it was so severe.Gloss glazes.
>Ideas?
>Back to the wheel now.
>Regards,
>Michael Wendt
>Wendt Pottery
>2729 Clearwater Ave
>Lewiston, Idaho 83501
>USA
>wendtpot@lewiston.com
>www.wendtpottery.com
>Craig wrote:
>a few days ago Ron Roy was saying
>>I hope you don't mind me asking this on the list - I would like others to
>>jump in case they have something to add to this subject line.
>
>Yeah Ron! It's really a good thing that you posted this query to the
>list. The amount of discussion has been unbelievable. 8>) Perhaps this
>microscopic stuff has little or no interest to anyone else. I like to
>consider all this stuff and think about it but I'm sort of an odd duck
>anyway. I do get a lot of pots made though.
>
>Then again, people may just be busy washing their Bentleys because Jeeves
>is cleaning the pool and can't wash the car for them. Maybe they're
>checking out some new venues for more 401Ks. I don't know. I'm going back
>into lurk mode. I'll get even MORE pots made.
>
>later on, Craig Martell Hopewell, Oregon
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sat 20 aug 05
Hi Craig,
Very strange - you would think that all those professionals would at least
give us some insight into their experiences with fit problems brought on by
changes in cooling rates over the years - from all those pots they have
made.
Do you think they have never had any fit problems?
I get the feeling this is a tar baby issue.
This is not such a difficult thing to test for - slow cool and fast cool a
glaze and measure the difference - anyone willing to do the firing - I'll
do the dilatometery.
RR
>a few days ago Ron Roy was saying
>>I hope you don't mind me asking this on the list - I would like others to
>>jump in case they have something to add to this subject line.
>
>Yeah Ron! It's really a good thing that you posted this query to the
>list. The amount of discussion has been unbelieveable. 8>) Perhaps this
>microscopic stuff has little or no interest to anyone else. I like to
>consider all this stuff and think about it but I'm sort of an odd duck
>anyway. I do get a lot of pots made though.
>
>Then again, people may just be busy washing their Bentleys because Jeeves
>is cleaning the pool and can't wash the car for them. Maybe they're
>checking out some new venues for more 401Ks. I don't know. I'm going back
>into lurk mode. I'll get even MORE pots made.
>
>later on, Craig Martell Hopewell, Oregon
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
bonnie staffel on sun 21 aug 05
Thank you Craig, for your analysis. My clay body was either of two recipes.
One of my favorites was straight APGreen Fireclay used back then. The other
was a recipe of Jordan, APGreen Fireclay, and a small amount of Iron Oxide.
I thought maybe the problem could be easily solved in this way if it sounds
plausible.
Warm Regards,
Bonnie Staffel
http://webpages.charter.net/bstaffel/
http://vasefinder.com/bstaffelgallery1.html
Charter Member Potters Council
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