Ivor and Olive Lewis on tue 3 aug 04

Dear Lois,
There can be many causes of Pinholing. You seem to be assuming that
it is your clay which is at fault but you give no information about
your glazes.
Do you have any Carbonates in your recipes? Any Iron?
I think that until you isolate and define the cause of the pinholes
then it might be a waste of time changing your firing program.
I recall discussing the appearance of gas bubbles in some glaze tests
being due to the dissociation of Iron Oxides during firing, even in
the absence of reduction at high temperatures. Fractional amounts of
Iron as residuals in some materials can account for this phenomenon
and it is know to be responsible for Oil Spot effects in high iron
glazes.
Perhaps you need to reassess the cause of the problem.
Best regards,Ivor Lewis.
Redhill,
S. Australia.




----- Original Message -----
From: "Lois"
To:
Sent: Tuesday, 3 August 2004 5:41
Subject: Reduction firing -- firing down questions


I am working with 2 glazes that are driving me NUTS! both want to
pinhole,
no matter what I do. I love these colors and don=B9t want to give up on
them.
I have increased my bisque to a very slow 18 hour with a hold at 200f
400f
900f and final 05 soak.

I use porcelain - Helios from Highwater.

I fire gas reduction, takes 12-14 hours to get a flat cone 10, then
soak for
10 minutes. I have started firing down for another 30 minutes for a
controlled cool to about 2200 f. still have pinholes.

I want to try the slower cool that had been discussed so much here,
with a
soak at about 1900F.

Will this help pin holes?

Do you crash cool (peeps and damper open) from top temp down to 1900f
or
close the kiln up and wait??

Do you soak in reduction? Or just keep it neutral?

Holding at that temp for how long???? Mel at one point said 4 hours,
but
will holding for 1 or 2 help? What does 30 minutes do? Is it possible
to
over fire at that holding stage? Meaning will glazes start to run?

Would love to hear results from others that are new to this.

Also open to any ideas about the pinholes.

Thanks
Lois Sharpe
In Durham NC..... who has to re-fire an ENTIRE load!

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Ben Shelton on tue 3 aug 04

I had this problem with one glaze that I loved (when it worked).

For some obtuse reason I was convinced that the clay was to blame... One
day it occured to me that the other glazes I was using did not
blister/pinhole and this one glaze pinholed on other clays.... DUH on my
part. really duh!

Figuring out what makes the pinholes is the first step. As Ivor has said,
changing your firing may do nothing for you.


What are the glaze formulas?
Do they contain iron and rutile?? (mine did)

Very interested in your slution as this may help me resurrect my glaze.


Ben

Ron Roy on wed 4 aug 04

When I dilatometer glazes I see when they soften - I have yet to see a
glaze that is frozen at 1900F (1037C) the most refractory c 10 glaze I have
measured softenes at about 1560F (850C.) 800C would be a better number for
most cone 10 glazes - less for cone 6 glazes and even less for 04 glazes.

We have some charts in our book - chapter 5 - the average softening temp
for the series of fit testing cone 6 glazes is a little over 700C.

When you look a the chart of a glaze - if it's been taken to the softening
point - you will see a hook at the top temperature - that is because the
material getting soft from heating and is no longer pushing on the sensor
that feels the expansion - but shrinking because it is getting soft.

RR


>The soak at 1900 will help with color but I doubt that it will go
>anything for your pinholing problem. The glaze has already frozen by then.

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 4 aug 04

Hi Lois,

Send me the two glazes that are the problem - I assume there are others
that work fine?

I may be able to fix em so they will not pin hole.

Have you done any tests on that clay body - it is possible that it is
overfired at your temperature.

I doubt any soak at the lower temperatures - like 1900F would do anything
to cure pinholes.

RR



>I am working with 2 glazes that are driving me NUTS! both want to pinhole,
>no matter what I do. I love these colors and don't want to give up on them.
>I have increased my bisque to a very slow 18 hour with a hold at 200f 400f
>900f and final 05 soak.
>
>I use porcelain - Helios from Highwater.
>
>I fire gas reduction, takes 12-14 hours to get a flat cone 10, then soak for
>10 minutes. I have started firing down for another 30 minutes for a
>controlled cool to about 2200 f. still have pinholes.
>
>I want to try the slower cool that had been discussed so much here, with a
>soak at about 1900F.
>
>Will this help pin holes?
>
>Do you crash cool (peeps and damper open) from top temp down to 1900f or
>close the kiln up and wait??
>
>Do you soak in reduction? Or just keep it neutral?
>
>Holding at that temp for how long???? Mel at one point said 4 hours, but
>will holding for 1 or 2 help? What does 30 minutes do? Is it possible to
>over fire at that holding stage? Meaning will glazes start to run?
>
>Would love to hear results from others that are new to this.
>
>Also open to any ideas about the pinholes.
>
>Thanks
>Lois Sharpe
>In Durham NC..... who has to re-fire an ENTIRE load!
>
>______________________________________________________________________________
>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

Lois on thu 5 aug 04

Thanks to all who sent ideas about my pinhole problem. I am sure it is not
a bisque, clay, or even the glaze at fault. I am convinced it is my firing.
I have tried soaking at cone 10, for longer than my usual 10 or so minutes
then starting cooling... and always OVER fire, which is worse than under
firing. (Under fired pots can be refired) I have over fired so many times I
am scared of holding to long.

I feel certain the problem is my lack of understanding heat work.
How long to hold, or not to hold, at what point do glazes stay soft and
smooth out without continuing to melt off the pot????, this is the temp I
need to hold at.

All these glazes need is a little more melt, and they are wonderful.

I think my next thing is to try draw rings to figure it out.

Thanks to all, it is wonderful to be part of this family.


Lois
In Durham NC who is firing tomorrow and gonna hold it awhile!



--
Lois Sharpe

Darlene Beverlin on fri 6 aug 04

I have been there and done that. I spent over 3 years trying to correct my "firing" but a fellow potter came to visit and had me try his clay and what do you know. No more pitting. I changed the way I fired the way I bisqued I changed my glaze formulas. I ruined more pots that you can shake a stick at. Give a new clay body a try. Mine turned out to have a ball clay that caused the pitting.

Good luck.

darlene

http://www.rarearthpottery.com





>Thanks to all who sent ideas about my pinhole problem. I am sure it is not
>a bisque, clay, or even the glaze at fault. I am convinced it is my firing.
> I have tried soaking at cone 10, for longer than my usual 10 or so minutes
>then starting cooling... and always OVER fire, which is worse than under
>firing. (Under fired pots can be refired) I have over fired so many times I
>am scared of holding to long.
>
>I feel certain the problem is my lack of understanding heat work.
>How long to hold, or not to hold, at what point do glazes stay soft and
>smooth out without continuing to melt off the pot????, this is the temp I
>need to hold at.
>
>All these glazes need is a little more melt, and they are wonderful.
>
>I think my next thing is to try draw rings to figure it out.
>
>Thanks to all, it is wonderful to be part of this family.
>
>
>Lois
>In Durham NC who is firing tomorrow and gonna hold it awhile!
>
>
>
>--
>Lois Sharpe
>
>
>______________________________________________________________________________
>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.
-------------------
Email sent using Grand River Mutual's Web E-mail (http://www.grm.net)

MarjB on sat 7 aug 04

Morning Lois

In your most recent post you write "All these glazes need is a little more
melt, and they are wonderful."

May I suggest that you nudge the Al2O3/SiO2 ratio of these glazes just a
little or if the glazes contain a large percentage of MgO that you reduce
that by a "nudge". I have never done a Currie Grid (on my list) but your
glaze results might benefit from doing such a test. One other suggestion is
to allow your kiln to climb more slowly as it nears cone ten. Apologies if
these are repetitive suggestions. MarjB

----- Original Message -----
From: "Lois"
To:
Sent: Thursday, August 05, 2004 8:26 PM
Subject: Re: Reduction firing -- firing down questions

Snail Scott on sun 8 aug 04

At 08:26 PM 8/5/2004 -0400, you wrote:
>...I have tried soaking at cone 10, for longer than my usual 10 or so minutes
>then starting cooling... and always OVER fire, which is worse than under
>firing...I have over fired so many times I
>am scared of holding to long....I feel certain the problem is my lack of
understanding heat work...


If you reach ^10, then start your soak, the
final firing will be more than ^10, because you
are still adding heat-work to the system. If
^10 gives a proper glaze maturity, then you need
to start the soak BEFORE ^10, and let it soak
until it hits temperature. If you have a ^9 in
your pack, wait until the ^9 is down, then start
the soak. If you keep the temperature constant,
you will probably drop the ^10 without any
temperature increase, because the extra time
is still producing heat-work. Also, a 10 minute
soak is practically like no soak at all. Start
sooner and give it a much longer time. Minimum
30 minutes, maybe even an hour. For the first
try, level off the temperature at a high ^9
and let it coast. If it doesn't hit ^10 in an
hour, go ahead and let the temperature rise
again until it does. It probably won't take
long.

-Snail Scott

Kathi LeSueur on mon 9 aug 04

It seems to me that a lot of the people having problems, myself
included, with pinholing and pitting at cone 10 are using glazes that
have titanium or rutile as an opacifier. Perhaps Ron Roy can give us
some insight as to why this might be so.

Kathi

Ron Roy on wed 11 aug 04

Hi Kathi,

I really don't know why but I would like to find out.

If you would care to gather some info from other potters who are having the
same problems perhaps I can distill some answers.

What I need are recipes of the offending glazes and recipes of glazes that
do not in the same firing.

It might also be useful if I could know something about the firing cycles -
both bisque and glaze.

If refiring helps cure the problem or does it make it worse.

Does it happen with all clays or just some.

Is anyone using the same glaze and not having a problem.

I'll be back from Sacramento on the 19th but will get my email while I am there.

RR

>It seems to me that a lot of the people having problems, myself
>included, with pinholing and pitting at cone 10 are using glazes that
>have titanium or rutile as an opacifier. Perhaps Ron Roy can give us
>some insight as to why this might be so.
>
>Kathi

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

daniel on wed 11 aug 04

Hi Ron, Kathi,

I've seen this recently in a rutile containing glaze in a few cases. Its
happening on a stoneware and on a porcelain when layered on top of a
tenmoku. I have not really done a systematic examination yet but did try a
wollastonite sub for some of the whiting in an effort to reduce outgassing.
This based on the idea that the glaze - though fluid might not be healing
over well. Alas, results were inconclusive.

At least one case I suspect an under-firing because of the glaze surface
quality. Other cases look as though the glaze is properly fired (whatever
that means :)) . I have not tried a refiring but I have plenty of pieces to
try that with.

Ron if your interested, I can bring the recipes and some examples to
Sacramento this w/e and we could talk about it.

Thanx
D

Belmont, California, USA
(ex terra australis)

Ron Roy on wed 11 aug 04

Hi Daniel,

Yes - bring them - we will start a project.

RR


> I've seen this recently in a rutile containing glaze in a few cases. Its
>happening on a stoneware and on a porcelain when layered on top of a
>tenmoku. I have not really done a systematic examination yet but did try a
>wollastonite sub for some of the whiting in an effort to reduce outgassing.
>This based on the idea that the glaze - though fluid might not be healing
>over well. Alas, results were inconclusive.
>
> At least one case I suspect an under-firing because of the glaze surface
>quality. Other cases look as though the glaze is properly fired (whatever
>that means :)) . I have not tried a refiring but I have plenty of pieces to
>try that with.
>
> Ron if your interested, I can bring the recipes and some examples to
>Sacramento this w/e and we could talk about it.
>
>Thanx
>D

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

Ben on wed 11 aug 04

Daniel,
I wonder if your tenmoku has alot of iron clay in it. I used to use a glaze
that deliberately utilized the boiling of a high iron clay slip glaze(50-90%
albany slip) under others of various viscosity to produce textured and
mottled effects. Supposed to boil between ^2-5. I wonder if adjusting your
firing rate and atmosphere in this stage might help.

Ben
----- Original Message -----
From: "daniel"
To:
Sent: Wednesday, August 11, 2004 10:21 AM
Subject: Re: Reduction firing -- firing down questions


> Hi Ron, Kathi,
>
> I've seen this recently in a rutile containing glaze in a few cases. Its
> happening on a stoneware and on a porcelain when layered on top of a
> tenmoku. I have not really done a systematic examination yet but did try a
> wollastonite sub for some of the whiting in an effort to reduce
outgassing.
> This based on the idea that the glaze - though fluid might not be healing
> over well. Alas, results were inconclusive.
>
> At least one case I suspect an under-firing because of the glaze surface
> quality. Other cases look as though the glaze is properly fired (whatever
> that means :)) . I have not tried a refiring but I have plenty of pieces
to
> try that with.
>
> Ron if your interested, I can bring the recipes and some examples to
> Sacramento this w/e and we could talk about it.
>
> Thanx
> D
>
> Belmont, California, USA
> (ex terra australis)
>
>
____________________________________________________________________________
__
> 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.
>
>