Jiri Minarik on sun 17 oct 04
My last two firings have produced a large number of pots with the following
flaw: large areas of the glaze surface lift off in a very thin layer and
expose the glaze underneath which looks and feels quite rough. The flakes
can be crumbled easily into a powder.The flakes come off from both vertical
and horizontal surfaces and it seems that it happens where the glaze is
thick. First I thought this is shivering but now I am not sure which of the
known glaze faults this may be. I used three different types of clay, two
stonewares and one porcelain and all three of them experienced the same
problem. The glaze I use is the Penn State shino . I mixed three batches of
the glaze so it probably is not a mistake in mixing.
Can anyone help?
Jiri in Berkeley
Ron Roy on mon 18 oct 04
Hi Jiri,
Any lithium Carb in that glaze?
RR
>My last two firings have produced a large number of pots with the following
>flaw: large areas of the glaze surface lift off in a very thin layer and
>expose the glaze underneath which looks and feels quite rough. The flakes
>can be crumbled easily into a powder.The flakes come off from both vertical
>and horizontal surfaces and it seems that it happens where the glaze is
>thick. First I thought this is shivering but now I am not sure which of the
>known glaze faults this may be. I used three different types of clay, two
>stonewares and one porcelain and all three of them experienced the same
>problem. The glaze I use is the Penn State shino . I mixed three batches of
>the glaze so it probably is not a mistake in mixing.
>Can anyone help?
>Jiri in Berkeley
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
John Britt on mon 18 oct 04
Yes, spodumene 29%.
John Britt
jrmnr on mon 18 oct 04
Penn State shino:
neph sye 14.6
F-4 34
spodumene 29
EPK 9.7
OM 4 4.9
soda ash 7.8
Liz Willoughby on mon 18 oct 04
Hello Jiri and Ron,
Just wanted to chime in here that the glaze has spodumene in it. I
have used this glaze a lot in the past, and never found that the
glaze surface lifted off in the way that you describe. I found it to
be very reliable, but stopped using it as it did not produce the kind
of carbon trapping that I desire.
Meticky Liz in Grafton, Ontario, Canada.
>Hi Jiri,
>
>Any lithium Carb in that glaze?
>
>RR
>
>>My last two firings have produced a large number of pots with the following
>>flaw: large areas of the glaze surface lift off in a very thin layer and
>>expose the glaze underneath which looks and feels quite rough. The flakes
>>can be crumbled easily into a powder.The flakes come off from both vertical
>>and horizontal surfaces and it seems that it happens where the glaze is
>>thick. First I thought this is shivering but now I am not sure which of the
>>known glaze faults this may be. I used three different types of clay, two
>>stonewares and one porcelain and all three of them experienced the same
>>problem. The glaze I use is the Penn State shino . I mixed three batches of
>>the glaze so it probably is not a mistake in mixing.
>>Can anyone help?
> >Jiri in Berkeley
daniel on mon 18 oct 04
Hi Jiri,
This is interesting. I've been thinking about this a bit. Sounds like the
glaze is of variable composition throughout its thickness.
> flaw: large areas of the glaze surface lift off in a very thin layer and
> expose the glaze underneath which looks and feels quite rough. The flakes
> can be crumbled easily into a powder
Any chance of a photo or two of this ?
> and horizontal surfaces and it seems that it happens where the glaze is
> thick.
How thick is thick in this case ?
Thanx
D
Belmont, California, USA
(ex terra australis)
jrmnr on mon 18 oct 04
Daniel,
you are right it looked like the two glaze layers did not belong
together. First I thought the glaze lifted off all the way to the clay
underneath, it looked just like rough clay surface. Then I broke the
piece and saw that the glaze was all there, except for its shiny
surface that flaked off. The thickness was perhaps 1/16 to 1/8 "
nowhere near the 3/16" John Britt talks about in his book. Also if my
recollection is correct it only happened in the last two firings,
never before.
I am just flabbergasted.
Jiri
Paul Herman on tue 19 oct 04
Hello Jiri,
I've seen the problem you described, but only with shinos that contain
lithium, from Spodumene or carbonate. I have a small cup, crazed and
shivered on the same piece. It was dropped on the concrete floor and it
bounced and didn't break.
Here's what I "think" happens with some of those lithium bearing shinos.
If you ever look at a magnified cross section of high fired ware, you
can see the glaze/body interface, a layer where the glaze has eaten down
into the clay, and dissolved some of it. So, what you have is NOT a clay
pot with a uniform layer of glass on the outside, but a layer of glass
that is graded in composition, from the surface down into the interface.
It seems to me that the outer surface would be mostly glaze, and the
interface mostly clay body, attacked by glaze. Therefore, you have a
glaze with varying thermal expansion coeficient, which, exacerbated by
that weird lithium stuff, can cause shivering.
At a workshop this last summer, from Malcolm Davis, one of my test tiles
shivered like that, the whole surface of the glaze came off in a sheet.
That particular glaze was the only one I tested using Lithium carbonate,
at five percent.
Good glazes,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: jrmnr
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: glaze fault
>Date: Tue, Oct 19, 2004, 3:10 PM
>
> Louis,
> it almost seems as if the top layer formed by the soda salts separated
> from the rest of the glaze.It is very thin. Is that a possibility?
> Jiri
Ron Roy on tue 19 oct 04
OK - so the glaze looks normal - for a shino type - now we have to look at
the clay body - is it a new batch, got a recipe? - is it a high iron body?
Sure sounds like some kind of shivering.
RR
>Penn State shino:
>neph sye 14.6
>F-4 34
>spodumene 29
>EPK 9.7
>OM 4 4.9
>soda ash 7.8
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Louis Katz on tue 19 oct 04
Hi Ron,
Just doing calculations on a shino probably gives goofy expansion
results. Depends on where all the soda ends up (I think).
My understanding is that there is a continuim of expansion from low to
high as you go from the clay body to the surface in a shino ( for this
discussion I am refering to the decendants of Wertz Shino and the other
American Soda Ash shinos). You can have a glaze that crazes on the
surface but shivers up against the clay. This seems to happen
occasionally to pots glazed inside and out with shino dried in the
wind. All to soda goes to the outside of the pot and the inside is
deficient in soda hence it has a lower expansion. I have had on and off
troubles with some high soda carbon trap glazes and this seems to be
the only theory that makes sense to me, but I must say I have gotten
stumped a couple of times and thought things must be even more complex
than this.
Louis
On Oct 19, 2004, at 12:24 PM, Ron Roy wrote:
> OK - so the glaze looks normal - for a shino type - now we have to
> look at
> the clay body - is it a new batch, got a recipe? - is it a high iron
> body?
>
> Sure sounds like some kind of shivering.
>
> RR
>
>> Penn State shino:
>> neph sye 14.6
>> F-4 34
>> spodumene 29
>> EPK 9.7
>> OM 4 4.9
>> soda ash 7.8
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
> _______________________________________________________________________
> _______
> 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.
>
jrmnr on tue 19 oct 04
Ron,
this is what I used: Stout Stoneware (not much iron), Soldate and a
porcelain. MarjB mentioned that she had a similar experience with
Malcolm's shino where it was thick.
Jiri
jrmnr on tue 19 oct 04
Louis,
it almost seems as if the top layer formed by the soda salts separated
from the rest of the glaze.It is very thin. Is that a possibility?
Jiri
Laurie Kneppel on tue 19 oct 04
Hi, I have been following this thread with interest because it sounds a
lot like a problem i have sometimes with Coleman's (purchased premixed
version) Orange Shino on certain clay bodies. At first I thought it was
the shino on a red cone 10 stoneware I like to use, but I have had it
happen on other stonewares as well. But not all of the ones I have
tried it on. The glaze will just flake off in places and leave a rough
grey surface underneath. Sometimes it happens right after the piece is
fired, sometimes it takes it a couple weeks or more. I hadn't noticed
that the glaze thickness made a difference, but I like to brush a
little iron oxide on the surface of the shino in streaks and circles
and such and it seems to flake off mainly where the iron oxide is. i
don't have the recipe handy for the Orange Shino, but it is in one of
Tom Coleman's books (which I own and just LOVE!) And i am assuming the
commercially prepared version probably uses the same or very similar
recipe. (with variations of available materials taken into account) How
odd, though. I thought maybe the red clay I had was the problem,
because they did have a bad batch of it around the time I bought the
stuff I have due to a change in I think it was the fireclay. Maybe mine
is okay after all and it is just something to do with the way it acts
with the shino.
I'll keep watching to see what you all think about it.
Laurie
Sacramento, CA
http://rockyraku.com
Potters Council, charter member
Sacramento Potters Group, member
On Oct 19, 2004, at 3:05 PM, jrmnr wrote:
> Ron,
> this is what I used: Stout Stoneware (not much iron), Soldate and a
> porcelain. MarjB mentioned that she had a similar experience with
> Malcolm's shino where it was thick.
> Jiri
>
> _______________________________________________________________________
> _______
> 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.
>
>
Louis Katz on tue 19 oct 04
Seems possible to me, almost anything does. I really don't have a clue
of all the stuff that happens in a shino glaze. It is as accurate to
say there are five layers as one or two, or three. Just a matter of how
you slice the pie. I made the mistake of adding some soda ash to some
glazes I wanted to jazz up once. Worked fine until the fifth load. Had
shivering on rims like nobodies business. I still don't have an
explanation that fits what happened.
Going home.
Louis
On Oct 19, 2004, at 5:10 PM, jrmnr wrote:
> Louis,
> it almost seems as if the top layer formed by the soda salts separated
> from the rest of the glaze.It is very thin. Is that a possibility?
> Jiri
>
> _______________________________________________________________________
> _______
> 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.
>
jrmnr on wed 20 oct 04
One place I do not find this thing happening is on rims and edges. (
I hate to call it shivering, I have been blessed with shivering
before and this is nothing like that; i.e. sharp pieces shooting of
the ware)
Also there is this flaking off and crazing going on on the same pieces.
Ron Roy on wed 20 oct 04
Hi Louis,
I was not commenting on expansion - just saying that the glaze was not
anything out of the ordinary - and I was asking about lithium Carb. because
it can do that to many glaze types.
I do think you are right about migrating salts - I have often wondered why
this has not resulted in problems with soluble materials in them - but this
is the first I have heard about this.
I know several potters that use a Shino type glaze - with lots of soda ash
in them - and dried different ways - but have never heard of this problem.
7.8 is not a lot of soda ash by the way - Malcolm's has 17% and many use
it.
I think there is something else at work here as well.
If it is related to salts migrating to the surface - would not the problem
be in certain areas - and be related to how the ware is dried and where the
salts are deposited?
Would it happen to that glaze - on that body with no or less soda ash for
instance?
Curious minds want to know - RR
>Hi Ron,
>Just doing calculations on a shino probably gives goofy expansion
>results. Depends on where all the soda ends up (I think).
>My understanding is that there is a continuim of expansion from low to
>high as you go from the clay body to the surface in a shino ( for this
>discussion I am refering to the decendants of Wertz Shino and the other
>American Soda Ash shinos). You can have a glaze that crazes on the
>surface but shivers up against the clay. This seems to happen
>occasionally to pots glazed inside and out with shino dried in the
>wind. All to soda goes to the outside of the pot and the inside is
>deficient in soda hence it has a lower expansion. I have had on and off
>troubles with some high soda carbon trap glazes and this seems to be
>the only theory that makes sense to me, but I must say I have gotten
>stumped a couple of times and thought things must be even more complex
>than this.
>Louis
>On Oct 19, 2004, at 12:24 PM, Ron Roy wrote:
>
>> OK - so the glaze looks normal - for a shino type - now we have to
>> look at
>> the clay body - is it a new batch, got a recipe? - is it a high iron
>> body?
>>
>> Sure sounds like some kind of shivering.
>>
>> RR
>>
>>> Penn State shino:
>>> neph sye 14.6
>>> F-4 34
>>> spodumene 29
>>> EPK 9.7
>>> OM 4 4.9
>>> soda ash 7.8
>>
>> Ron Roy
>> RR#4
>> 15084 Little Lake Road
>> Brighton, Ontario
>> Canada
>> K0K 1H0
>> Phone: 613-475-9544
>> Fax: 613-475-3513
>>
>> _______________________________________________________________________
>> _______
>> 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.
>>
>
>______________________________________________________________________________
>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 wed 20 oct 04
Hi Jiri,
Does it happen on all three bodies?
Does it happen in any particular area consistently - like on one side or
near the top or bottom.
Do you dry em fast or slow after glazing.
Does it happen in any particular part of the kiln?
Big pots only you say - how long between your top temp and 1100C for firing
and cooling?
Wish we could figure this out - RR
>Ron,
>this is what I used: Stout Stoneware (not much iron), Soldate and a
>porcelain. MarjB mentioned that she had a similar experience with
>Malcolm's shino where it was thick.
>Jiri
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
John Britt on wed 20 oct 04
Ron,
I have had this problem particularly in heavily reduced firings with
shinos on stoneware bodies. I think that the strength of the clay body is
being compromised by the black coring which then allows the glaze to flake
off. (mimicking shivering or a form of shivering) It may be variations in
exp/cont caused by migrating soda ash in a lithium glaze (from spodumene)
but exaserbated by the weakened body. It does not happen on porcelain.
The UMF will not anticipate this problem as physical properties of glazes
are not accounted for. UMF assumes a homogenous melt. But migrating soda
ash actually creates two glaze layers, high soda on the surface and high
lithium/alumina on the surface.
Just my opinion,
JOhn Britt
www.johnbrittpottery.com
Ron Roy on thu 21 oct 04
Hi Laurie,
I just checked some test tiles from my last firing - I have been trying my
shino glaze with different amounts of soda ash and don't see any shivering
- only small tiles however.
I tested on a dark (high iron) body and porcelain by the way.
Is it happening to you on larger pieces?
Just to be clear - you said only where there is iron brushed on or does it
happen elsewhere as well?
RR
>Hi, I have been following this thread with interest because it sounds a
>lot like a problem i have sometimes with Coleman's (purchased premixed
>version) Orange Shino on certain clay bodies. At first I thought it was
>the shino on a red cone 10 stoneware I like to use, but I have had it
>happen on other stonewares as well. But not all of the ones I have
>tried it on. The glaze will just flake off in places and leave a rough
>grey surface underneath. Sometimes it happens right after the piece is
>fired, sometimes it takes it a couple weeks or more. I hadn't noticed
>that the glaze thickness made a difference, but I like to brush a
>little iron oxide on the surface of the shino in streaks and circles
>and such and it seems to flake off mainly where the iron oxide is. i
>don't have the recipe handy for the Orange Shino, but it is in one of
>Tom Coleman's books (which I own and just LOVE!) And i am assuming the
>commercially prepared version probably uses the same or very similar
>recipe. (with variations of available materials taken into account) How
>odd, though. I thought maybe the red clay I had was the problem,
>because they did have a bad batch of it around the time I bought the
>stuff I have due to a change in I think it was the fireclay. Maybe mine
>is okay after all and it is just something to do with the way it acts
>with the shino.
>
>I'll keep watching to see what you all think about it.
>
>Laurie
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on thu 21 oct 04
Hi Paul,
Others have reported this happening with lithium carbonate before on
ClayArt. It does happen on some clay bodies.
There is always that intermediate section with high fired ware - more so in
porcelain of course - so in effect you are always dealing with at least 3
different layers - all having different expansion rates. Add slip and you
get 4 layers - or maybe five depending on how much the slip melts.
My general rule is - no more than 2% lithium carb in a glaze - and never
any in a clay body.
If you would care to send me any recipies of any glazes that craze and
shiver I will see what i can deduce from them.
I would like to know - did you test the glaze that shivered on different
clays - and was it OK on some?
RR
>I've seen the problem you described, but only with shinos that contain
>lithium, from Spodumene or carbonate. I have a small cup, crazed and
>shivered on the same piece. It was dropped on the concrete floor and it
>bounced and didn't break.
>
>Here's what I "think" happens with some of those lithium bearing shinos.
>
>If you ever look at a magnified cross section of high fired ware, you
>can see the glaze/body interface, a layer where the glaze has eaten down
>into the clay, and dissolved some of it. So, what you have is NOT a clay
>pot with a uniform layer of glass on the outside, but a layer of glass
>that is graded in composition, from the surface down into the interface.
>
>It seems to me that the outer surface would be mostly glaze, and the
>interface mostly clay body, attacked by glaze. Therefore, you have a
>glaze with varying thermal expansion coeficient, which, exacerbated by
>that weird lithium stuff, can cause shivering.
>
>At a workshop this last summer, from Malcolm Davis, one of my test tiles
>shivered like that, the whole surface of the glaze came off in a sheet.
>That particular glaze was the only one I tested using Lithium carbonate,
>at five percent.
>
>Good glazes,
>
>Paul Herman
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
jrmnr on thu 21 oct 04
Hi Ron ,
here are some details. It happened on all three bodies, no
discrimination here.
No special area; inside and outside of bowls, vertical surfaces of
cups. Wherever the glaze was thick either double dipped or pooled.
No place in the 60 cf kiln was immune. It was all over.
The pieces ranged from 20" platters to 3" cups, no discrimination here
either.
In general the glazed ware sits for several days in airy room and it
was quite hot here during that time.
It takes about 4 hours from 1100 C to cone 10 and I do not know about
the cooling speed but it should be quite slow, it is a big kiln.
And yes I too wish we could figure this out, the sound of pots hitting
the dumpster floor is not what I look forward after my next firing.
Jiri
Ron Roy on fri 22 oct 04
Hi John,
I am not sure I understand your reference to black coring - once a clay is
reduced - you cannot reduce it more - it should be grey, dark grey or black
all the way through depending on the amount of iron in the body.
If it's glassy then it may be a bisque firing problem or an overfired body
due to improper formulation or a change in materials.
I have been doing shinos for years now - on porcelain and a dark body -
well reduced - different amounts of soda ash - no problems so far.
What I said is - there is nothing unusual about that shino when I compare
it to others - using the UMF - so I conclude it is not the formulation that
is the problem - and Jiri says it happens on all his clays - including a
porcelain - so it is not the clay.
We are left with process and materials - it can't go from not happening to
happening without some reason - so we work our way through it - using the
tools we have - you can't just blame it on migrating soda ash - that has
been around for years and some potters have never had the problem.
RR
>I have had this problem particularly in heavily reduced firings with
>shinos on stoneware bodies. I think that the strength of the clay body is
>being compromised by the black coring which then allows the glaze to flake
>off. (mimicking shivering or a form of shivering) It may be variations in
>exp/cont caused by migrating soda ash in a lithium glaze (from spodumene)
>but exaserbated by the weakened body. It does not happen on porcelain.
>
>The UMF will not anticipate this problem as physical properties of glazes
>are not accounted for. UMF assumes a homogenous melt. But migrating soda
>ash actually creates two glaze layers, high soda on the surface and high
>lithium/alumina on the surface.
>
>Just my opinion,
>
>JOhn Britt
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Laurie Kneppel on fri 22 oct 04
Hi Ron,
I am going to check the ingredients in the Coleman Orange Shino when I
get home tonight. I am pretty sure it's the same one that is in his
book that I have (I also have MC6G, too!) I am curious if it contains
spodumene. It mainly flakes off on the IMCO 8-11 Red clay that I like.
Leaves a very rough grey patch where the glaze used to be. Sometimes
the patch pops off on its own, but usually it has a crack that allows
it to be rubbed off with your finger. The glaze comes off in chunks, it
doesn't powder. The fired piece lacks the normal ring and sounds like
it ought to have a crack somewhere since it has a dull sound when
tapped.
I had the cracking happen on some Rod's Bod and also Quyle's Sandstone
Buff, which is a very yellow clay with lots of yellow iron oxide in it.
The RIO I paint on is fairly dilute, but the flaking seems to be where
it has puddled a wee bit and might be a little heavier. What I have
been making are orange tabby cats and the orange shino is the perfect
color. The RIO is painted on as the tabby stripes on the cats. The
pieces are fired to cone 10 in reduction in my Geil kiln. Sometimes
it's more of a cone 9.5 in some areas in the kiln, but I believe the
glaze is marked as cone 8-10 (I need to look at the package when I get
home)
It happens on large or small pieces and not always underneath the RIO
decoration.
I will email you the glaze recipe offlist, but it does contain over 30%
spodumene.
Thanks!
Laurie
On Oct 21, 2004, at 8:57 PM, Ron Roy wrote:
> Hi Laurie,
>
> I just checked some test tiles from my last firing - I have been
> trying my
> shino glaze with different amounts of soda ash and don't see any
> shivering
> - only small tiles however.
>
> I tested on a dark (high iron) body and porcelain by the way.
>
> Is it happening to you on larger pieces?
>
> Just to be clear - you said only where there is iron brushed on or
> does it
> happen elsewhere as well?
>
> RR
Paul Herman on fri 22 oct 04
Hi Ron,
The shivering shino I tested did not shiver on porcelains that the other
students were using. The clay I used to do my tests was one of my
homemade bodies, a white stoneware, very vitreous. It shivered on some
of the dark stoneware clays other students had.
The glaze I used on the little cup that bounced is:
Shell Shino
Nepheline Syenite 60
spodumene 40
zircopax 6
Bentonite 3
I quit using it. My partner Joe and some other friends still do, and are
putting some into the firing we are doing on October 25-27. I'll keep an
eye on the results. It's a very pretty glaze, lustrous, and takes the
ash nicely.
Good glazing,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: Ron Roy
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: glaze fault
>Date: Thu, Oct 21, 2004, 8:57 PM
>
> Hi Paul,
>
> Others have reported this happening with lithium carbonate before on
> ClayArt. It does happen on some clay bodies.
>
> There is always that intermediate section with high fired ware - more so in
> porcelain of course - so in effect you are always dealing with at least 3
> different layers - all having different expansion rates. Add slip and you
> get 4 layers - or maybe five depending on how much the slip melts.
>
> My general rule is - no more than 2% lithium carb in a glaze - and never
> any in a clay body.
>
> If you would care to send me any recipies of any glazes that craze and
> shiver I will see what i can deduce from them.
>
> I would like to know - did you test the glaze that shivered on different
> clays - and was it OK on some?
>
> RR
>
>
>>I've seen the problem you described, but only with shinos that contain
>>lithium, from Spodumene or carbonate. I have a small cup, crazed and
>>shivered on the same piece. It was dropped on the concrete floor and it
>>bounced and didn't break.
>>
>>Here's what I "think" happens with some of those lithium bearing shinos.
>>
>>If you ever look at a magnified cross section of high fired ware, you
>>can see the glaze/body interface, a layer where the glaze has eaten down
>>into the clay, and dissolved some of it. So, what you have is NOT a clay
>>pot with a uniform layer of glass on the outside, but a layer of glass
>>that is graded in composition, from the surface down into the interface.
>>
>>It seems to me that the outer surface would be mostly glaze, and the
>>interface mostly clay body, attacked by glaze. Therefore, you have a
>>glaze with varying thermal expansion coeficient, which, exacerbated by
>>that weird lithium stuff, can cause shivering.
>>
>>At a workshop this last summer, from Malcolm Davis, one of my test tiles
>>shivered like that, the whole surface of the glaze came off in a sheet.
>>That particular glaze was the only one I tested using Lithium carbonate,
>>at five percent.
>>
>>Good glazes,
>>
>>Paul Herman
>
> Ron Roy
> RR#4
Ron Roy on fri 22 oct 04
Hi Jiri,
Well I am thinking about this - a lot - if it is happening to you then it
is going to happen to others sooner or later.
You may have answered some of these questions before but humor me - I can't
remember all the posts on this.
Is it true that some ware is OK? - and it is scattered because the ware
does not go into the glaze firing the way it came out of the bisque firing?
What temperature do you fire your bisque to - and is the bisque firing even
- and is it done in a gas kiln?
I have many tests out of my last two firings - shinos with Spodumene (what
kind are you using by the way) with different amounts of soda ash ranging
from 2% up to 16% - on a dark cone 10 body and porcelain - I cannot find
any shivering in any of the tests I have done.
My shino normally has 2 soda ash, 22 Canadian Spod and 36 Neph Sy for
instance - but I have tested it with up to 16 soda ash.
I has occurred to me - that the one thing the shivering shinos have in
common is Spodumene - would you like me to reformulate yours with
decreasing amounts Spodumene - replacing it with KNaO and keeping all the
other oxides the same - to see if the problem is related to the Spod?
If so send me your exact recipe again with all the materials identified as
to where mined or by who.
You should also try to remember - did the problem start with any new batch
or batches of material - especially the Spod?
Regards for now - RR
>Hi Ron ,
>here are some details. It happened on all three bodies, no
>discrimination here.
>No special area; inside and outside of bowls, vertical surfaces of
>cups. Wherever the glaze was thick either double dipped or pooled.
>No place in the 60 cf kiln was immune. It was all over.
>The pieces ranged from 20" platters to 3" cups, no discrimination here
>either.
>In general the glazed ware sits for several days in airy room and it
>was quite hot here during that time.
>It takes about 4 hours from 1100 C to cone 10 and I do not know about
>the cooling speed but it should be quite slow, it is a big kiln.
>And yes I too wish we could figure this out, the sound of pots hitting
>the dumpster floor is not what I look forward after my next firing.
>Jiri
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Earl Brunner on sat 23 oct 04
Don't just look for spodumene, see if it has Lithia.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Laurie Kneppel
Sent: Friday, October 22, 2004 9:14 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: glaze fault
Hi Ron,
I am going to check the ingredients in the Coleman Orange Shino when I
get home tonight. I am pretty sure it's the same one that is in his
book that I have (I also have MC6G, too!) I am curious if it contains
spodumene. It mainly flakes off on the IMCO 8-11 Red clay that I like.
Leaves a very rough grey patch where the glaze used to be. Sometimes
the patch pops off on its own, but usually it has a crack that allows
it to be rubbed off with your finger. The glaze comes off in chunks, it
doesn't powder. The fired piece lacks the normal ring and sounds like
it ought to have a crack somewhere since it has a dull sound when
tapped.
Laurie Kneppel on sat 23 oct 04
Yep, that's the one. I'm sort of assuming it's closely related to the
one being sold through Aardvark if not the exact same one.
Laurie
Sacramento, CA
http://rockyraku.com
Potters Council, charter member
Sacramento Potters Group, member
On Oct 23, 2004, at 5:15 AM, John Britt wrote:
> Laurie,
>
>
> Is the the recipe you have:
>
> COLEMAN ORANGE
>
>
> Neph. Sye. 7.30
> Spodumene 36.30
> EPK Kaolin 16.81
> Zircopax 2.51
> Custer Feldspar 17.94
> Kentucky Ball Clay 14.95
> Soda Ash 4.19
>
>
> John Britt
> www.johnbrittpottery.com
>
> _______________________________________________________________________
> _______
> 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.
>
>
John Britt on sat 23 oct 04
Laurie,
Is the the recipe you have:
COLEMAN ORANGE
Neph. Sye. 7.30
Spodumene 36.30
EPK Kaolin 16.81
Zircopax 2.51
Custer Feldspar 17.94
Kentucky Ball Clay 14.95
Soda Ash 4.19
John Britt
www.johnbrittpottery.com
jrmnr on sat 23 oct 04
Hi Ron,
Yes you are right some of the pots were fine.
I bisque to cone 010 in an electric kiln and the temperature is
uneven, hot in the middle and colder at the top and bottom.
Now, this thing started happening two firings ago, and the one before
the last produced only a handful of afflicted pots, so I just
pretended this was a fluke and ignored it. The last firing of course
could not be ignored, because I lost probably half of the load.
Usually I have glaze left from previous firings and mix new as needed.
This might indicate that some of the culpable material started showing
up in the first firing and hit the second firing in full force. I have
looked at my spodumene and it comes from Australia, first thing on
Monday I am going to try to find out from my supplier if they used to
carry Canadian brand before. Do you think this might be it?
original modified
neph sye 15.2 36.7
soda ash 7.8 7.8
EPK 9.8 9.8
OM4 3.9 3.9
Kona f-4 34.3 34.3
spodumene 29.4 20.0
The left column is the original formula and in the right column I
dropped some spodumene while keeping the oxides the same. Would you
think I should reduce the spod even more?
Jiri
John Britt on sat 23 oct 04
Ron,
It certainly is the spodumene (lithium) that causes shivering. If the body
is black cored then the clay/glaze interface is affected and a glaze that
previously did not shiver will then shiver. So variations in kiln
atmosphere can cause glaze problems with the same glaze batch that did not
exhibit problems before.
Shinos are the simplest glazes (feldspar and clay) and do not need to be
re-formulated. They do not fit into the limits for cone 10 glazes so why
do you want to keep all the other oxides the same? They are purposely out
of balance. Carbon trap shinos are high alumina /high sodium glazes. You
can simply exchange half of the lithium feldspar for a soda feldspar. You
can use NC-4, F-4, , Minspar, even the feldspathoid, Nepheline Syenite
etc. All the feldspars will work interchangeably. The clays are also
interchangeable, you can use OM-4 for EPK if you want more iron or add
some Redart for even more iron.
John Britt
www.johnbrittpottery.com
Ron Roy on tue 26 oct 04
Hi Jiri,
Got behind on my mail again - sorry for the delay.
We have eliminated the glaze and the clay as the problem - at least in my
mind - so we are left with materials and process - so we should pursue the
material part as much as possible - any newer materials should be checked
out. I would list spod as one of the suspects so do check it out - the
Aussi Spod is easy to spot - it sparkles. OM #4 is a variable clay by the
way so I will reformulate with EPK and some bentonite. It may be that just
adding bentonite will clog up the pores enough to slow down the migration
of salts when the glaze is drying. Perhaps you should try a test batch of
your original with some bentonite added as well to see if it helps.
By the way - I have been testing some new shino variations with the Aussi
spod - with up to 16% soda ash - no problems so far - I will have some with
17 to 23% out on Wed - I'll post the results.
We also have to consider your bisquing temperature as a possible problem -
very absorbent ware - sucking up a lot of soluble soda ash. I suggest you
do a cone 04 bisque and thicken up you glaze a bit to compensate. This will
also have the effect of keeping the glaze from settling out too fast - and
this might be part of the problem as well.
I have started with your original recipe and taken out all the spod - you
can do a line blend which will show the effect of decreasing the spod to 0
call it #5. Mix up a batch of this new recipe and dip some test ware. The
take equal volumes of this no spod version and mix it with equal volume of
your original Call it #1 - and call the combination #3. You will use #3
twice so make sure you have about twice as much as #5.
Equal volumes of #3 and #1 gives #2
Equal volumes of #3 and #5 gives #4
Do at least 5 test pieces of each mix and scatter them in the next firing -
to make sure you get a good sample.
#5 ( no spod shino)
Neph Sy - 46.5
Soda Ash - 8.0
EPK - 12.5
OM#4 - none
F4 spar - 33.0
Spod - none
Bentonite 2.0
Total - 102.0
Let me know if there are questions - I'm away Tuesday but will be back Wed.
RR
>Hi Ron,
>Yes you are right some of the pots were fine.
>I bisque to cone 010 in an electric kiln and the temperature is
>uneven, hot in the middle and colder at the top and bottom.
>Now, this thing started happening two firings ago, and the one before
>the last produced only a handful of afflicted pots, so I just
>pretended this was a fluke and ignored it. The last firing of course
>could not be ignored, because I lost probably half of the load.
>Usually I have glaze left from previous firings and mix new as needed.
>This might indicate that some of the culpable material started showing
>up in the first firing and hit the second firing in full force. I have
>looked at my spodumene and it comes from Australia, first thing on
>Monday I am going to try to find out from my supplier if they used to
>carry Canadian brand before. Do you think this might be it?
> original modified
>neph sye 15.2 36.7
>soda ash 7.8 7.8
>EPK 9.8 9.8
>OM4 3.9 3.9
>Kona f-4 34.3 34.3
>spodumene 29.4 20.0
>
>The left column is the original formula and in the right column I
>dropped some spodumene while keeping the oxides the same. Would you
>think I should reduce the spod even more?
>Jiri
>
>______________________________________________________________________________
>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 tue 26 oct 04
Hi John,
I think we are having a cross purpose dialogue due to the different meaning
we give to black coring - I see it as a fault to be corrected - are you
using the term to describe reduced clay?
I have many Shino tests on a dark clay body - fully reduces - with a
similar Shino glaze - and many others - and have never had the problem we
are discussing - so lets get our terms defined.
I just reformulated Jiri's glaze and as you can see - it looks quite
different without the spod. If you look at the analysis for Spodumene you
will see there are differences between most spars and spodumens - so in
order to keep the ratio the same the materials need to be adjusted - get
the ratio substantial different and you will not have the same glaze. In
the case of shinos - you need to adjust the alumina and silica oxides to
come up with the same look.
I don't understand why you have mentioned limits - unless you mean the
limits for Shino glazes. I will explain about this in my talk at NCECA in
March by the way - a crucial factor when you are looking for certain
effects in a Shino glaze.
Yes Shino glaze are out of balance with any set of limit formulas I have
ever seen - I don't use limit formulas that way any more - when working on
specific kinds of glazes I use the limits that apply to those types of
glazes.
Calculate a bunch of Tenmoku glazes and you now have limits for Tenmoku
glazes - and you can explore the ramifications of stretching those limits
for instance.
To illustrate from your example - if you replace all the EPK with OM#4 you
would upset the alumina silica ratio so much the glaze would not look the
same - Aussi Spod only has about 8% lithium and Neph Sy has over 14% KNaO.
This is why the empirical method is so difficult to use effectively - and
why using good analysis of real materials is so useful.
RR
>It certainly is the spodumene (lithium) that causes shivering. If the body
>is black cored then the clay/glaze interface is affected and a glaze that
>previously did not shiver will then shiver. So variations in kiln
>atmosphere can cause glaze problems with the same glaze batch that did not
>exhibit problems before.
>
>Shinos are the simplest glazes (feldspar and clay) and do not need to be
>re-formulated. They do not fit into the limits for cone 10 glazes so why
>do you want to keep all the other oxides the same? They are purposely out
>of balance. Carbon trap shinos are high alumina /high sodium glazes. You
>can simply exchange half of the lithium feldspar for a soda feldspar. You
>can use NC-4, F-4, , Minspar, even the feldspathoid, Nepheline Syenite
>etc. All the feldspars will work interchangeably. The clays are also
>interchangeable, you can use OM-4 for EPK if you want more iron or add
>some Redart for even more iron.
>
>John Britt
>www.johnbrittpottery.com
>
>______________________________________________________________________________
>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
jrmnr on tue 26 oct 04
Hi Ron,
thanks for your advice. My supplier tells me that for the last eight
years he has been getting the Aussi spod.So one potential culprit is
out. As for the other materials I did not buy anything new for a year
with the exception of soda ash.
One thing that I changed during my firings, I stopped overnight
kindling, but I am at the same temperature within an hour and a half
as with kindling.
I'll try your suggestions on test tiles and I won't double dip my ware
this time, since it seemed to be happening only where the glaze was
thick, even though that gives me the best effects.
Jiri
Ron Roy on thu 28 oct 04
Hi Jiri,
I just got more tests out - up to 23% soda ash in some of them - on a high
iron body and porcelain.
I am fairly sure it has to do with process now so keep careful records of
your firing, glazing and cooling.
I suggest you put some ware in with the thick applications - we must find
out if the changes make a difference.
Make sure - when you are glazing that the glaze is well stirred - the Aussi
spod I have seems to be a courser grind than all my other materials.
RR
>Hi Ron,
>thanks for your advice. My supplier tells me that for the last eight
>years he has been getting the Aussi spod.So one potential culprit is
>out. As for the other materials I did not buy anything new for a year
>with the exception of soda ash.
>One thing that I changed during my firings, I stopped overnight
>kindling, but I am at the same temperature within an hour and a half
>as with kindling.
>I'll try your suggestions on test tiles and I won't double dip my ware
>this time, since it seemed to be happening only where the glaze was
>thick, even though that gives me the best effects.
>Jiri
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
daniel on fri 29 oct 04
Hi Ron, Jiri,
>
> I suggest you put some ware in with the thick applications - we must find
> out if the changes make a difference.
>
I was also wondering about the impact of different drying. I had thought
earlier to send a note suggesting that some tests be done on the drying
procedure. For eg. dry a piece with a heat gun, dry one in a breeze and dry
another under cover away from drafts. I realise this will affect more than
just the shivering but its just a thought. Perhaps if the thickness does not
reveal more, this may be interesting.
Curious as always
D
Belmont, California, USA
(ex terra australis)
jrmnr on sat 30 oct 04
Hi Ron,
it will take me a few more days to make enough pots to fill up my big
kiln. But in the meantime I put a small, badly shivered pot into a
friend's firing and it came out completely healed over, so you would
not be able to tell that there was a problem before. I wish I kept all
the afflicted pots instead of dumping them.
By the way it is easy to peel off large areas just with my finger
nail, it almost seems that it gets worse with time.
Do you know what the rational is for crash cooling at the end of a
firing?
Jiri
--- In clayart@yahoogroups.com, Ron Roy wrote:
> Hi Jiri,
>
> I just got more tests out - up to 23% soda ash in some of them - on
a high
> iron body and porcelain.
>
> I am fairly sure it has to do with process now so keep careful
records of
> your firing, glazing and cooling.
>
> I suggest you put some ware in with the thick applications - we must
find
> out if the changes make a difference.
>
> Make sure - when you are glazing that the glaze is well stirred -
the Aussi
> spod I have seems to be a courser grind than all my other materials.
>
> RR
>
> >Hi Ron,
> >thanks for your advice. My supplier tells me that for the last eight
> >years he has been getting the Aussi spod.So one potential culprit is
> >out. As for the other materials I did not buy anything new for a year
> >with the exception of soda ash.
> >One thing that I changed during my firings, I stopped overnight
> >kindling, but I am at the same temperature within an hour and a half
> >as with kindling.
> >I'll try your suggestions on test tiles and I won't double dip my ware
> >this time, since it seemed to be happening only where the glaze was
> >thick, even though that gives me the best effects.
> >Jiri
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
______________________________________________________________________________
> Send postings to clayart@l...
>
> 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@p...
Ron Roy on sat 30 oct 04
Hi Daniel,
I concur - an excellent idea.
I heard from someone else who is having the same problems - and is
bisqueing to 05 - another reason eliminated but - she was drying with a
fan.
RR
>Hi Ron, Jiri,
>> I suggest you put some ware in with the thick applications - we must find
>> out if the changes make a difference.
>>
>
>I was also wondering about the impact of different drying. I had thought
>earlier to send a note suggesting that some tests be done on the drying
>procedure. For eg. dry a piece with a heat gun, dry one in a breeze and dry
>another under cover away from drafts. I realise this will affect more than
>just the shivering but its just a thought. Perhaps if the thickness does not
>reveal more, this may be interesting.
>
>Curious as always
>D
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sun 31 oct 04
Hi Jiri,
Some bodies develop cristobalite - and it is formed from 1100C up to end
temperature - and on the way down again till 1100C is reached. Crash
cooling down to 1100 would avoid some cristobalite formation if it is a
problem.
If you are going to hold - soak - at a lower temperature - getting there
faster would be a reason to crash cool as well - just to make the firing
time shorter.
Keep in mind - fit problems do not exist until the glaze is frozen on the
way down. Up till then it is pyroplastic and will adjust automatically to
the contraction of the body as it cools.
If the body has a lot of free quartz (under fluxed and/or lots of silica in
the mix) and the glaze is frozen before the quartz inversion - the body
suddenly goes through the quartz inversion and the glaze - is put under
compression - which can either prevent crazing - if it has not already
happened - or if the compression is too much - shivering and dunting can
occur.
Best not to have any drafts going through your kiln at this time - damper
properly closed - I would check your damper to make sure it's not cracked
for instance.
This can be tricky as well because kilns cool differently - usually faster
at the bottom because heat rises - so choosing what temperature to slow
down at gets tricky as most pyrometers are in the upper half of kilns.
If there is cristobalite in you body - the same thing happens at 200C - the
inversion again puts the glaze under compression.
Picture this - glaze crazes before the quartz inversion - glaze gets
compressed somewhat during the quartz inversion - then again during the
cristobalite inversion. So the glaze is crazed but shows all the signs of
being under too much compression.
Fast cooling after the glaze is frozen is not a good idea - best to have
everything closed up tight even slow cooling - especially when solubles are
a factor - think layers of different expanding/contracting glazes and
clays.
Remember I asked if the pots with faults were in any particular part of
your kiln? That was an attempt to figure out if the problem was due to fast
cooling in certain areas of the kiln.
If the shivered or crazed ware was near your burner ports - or in the path
off cool air being drawn through your kiln - then stopping that draft may
be the answer to the problem.
You said they were scattered all over the kiln - I say take another look
with cooling draft in mind.
RR
>Hi Ron,
>it will take me a few more days to make enough pots to fill up my big
>kiln. But in the meantime I put a small, badly shivered pot into a
>friend's firing and it came out completely healed over, so you would
>not be able to tell that there was a problem before. I wish I kept all
>the afflicted pots instead of dumping them.
>By the way it is easy to peel off large areas just with my finger
>nail, it almost seems that it gets worse with time.
>Do you know what the rational is for crash cooling at the end of a
>firing?
>Jiri
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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