Ron Roy on tue 15 oct 02

Hi Jose,

This crawling - and it does seem to happen more on porcelain than stoneware.

Glaze gas to get a certain amount of "grasp" on a pot to not crack and peel
back during sintering (beginning of the firing) The fine nature of clay
particles in porcelain are not as conducive to holding glaze as with
stoneware.

There are some rules to follow - first - use a magnifier to examine the
glaze after it has dried (completely) on the ware. Many glaze crack at
least a bit at this stage but not enough to show up after firing - but they
are a sign that trouble may be around the corner.

Don't try to smooth the glaze - there are much better and safer ways to
cure the problem.

Don't fire such glazes till they are dry - loading and firing wet glazes is
one way to get crawling in some glazes.

I can reformulate such glazes (you can to if you have calculation software)
to eliminate problematic materials - short list -

Gers Borate
EPK
Zinc
Magnesium carb.

Flocculated glazes need more water - shrink more - and therefore crack up
more during drying and early stages of firing - sometime pieces of glaze
fall off the ware onto the shelves.

Replacing some or all of the GB with Frit is a good solution.
Replacing kaolin with ball clay makes the surface tougher and less likely
to crack.
Defloccing the glaze will work but adding solubles to glazes should only be
a last resort.
If you would like to send me the worst offender I will send it back fixed
as an example of how to do it Jose - been there - done that - many times.

RR


>I have been using a mid range porcelain (cone 4 to 6). I bisque to 06 and
>fired with a Gerstley borate based glaze to cone 6. I have used these
>glazes successfully on stoneware, but on the porcelain the glaze is fine
>and seems to be a good fit, but it leaves some bare spots. I apply the
>glaze by dipping and the glaze seems to adhere well, but then it moves
>during firing leaving these bare spots. So the glaze is fine except for
>these bare spots. The bare spots are not very big (say a quarter to half
>dollar) and of random shapes with smooth edges. I thought in the
>beginning I had touched the bisqued pot with some masquing wax I use to
>protect the foot of the vases, but now I am more inclined to believe 06
>might be too high a temperature for bisque firing this particular
>porcelain and maybe I need to go to a lower temperature. The bisqued
>porcelain appears to be more vitrified than the stoneware (tighter
>smoother surface). I have not conducted any tests for clay/glaze fit, but
>I do not see any signs that is not a good fit.
>
>Any clues as to what might cause this and possible cures?

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

Snail Scott on wed 16 oct 02

At 07:24 PM 10/16/02 -0700, you wrote:
>Is that specifically Edgar Plastic
>Kaolin (EPK) or does that apply to any raw kaolin?...
>Tomorrow I will put in a test substituting ball clay for part of the
>koalin.


Any raw clay will exacerbate the tendency of a
glaze to crawl. I would think that ball clay
would make it worse, as most ball clays have a
higher shrinkage than most kaolins. Just calcine
a little of the kaolin instead - no need to change
the recipe.

-Snail

Marni Turkel on wed 16 oct 02

In a reply to this thread Ron mentioned that EPK can be an offender
in causing a glaze to crawl. Is that specifically Edgar Plastic
Kaolin (EPK) or does that apply to any raw kaolin? Has anyone tried
substituting other kaolins (not calcined) for EPK? I'll probably be
trying that soon.

I am working on a glaze that will crawl only on the bottom of the pot
inside of the foot ring if it is fired before completely dry. It is
21.5% EPK, but no other ingredients that I would suspect. A just
dipped piece that has cut-outs in the foot that provide air flow will
not crawl. Interestingly (to me anyway), it doesn't seem to crawl in
the traditional sense, it never leaves a bare spot, just leaves some
residue on the shelf or what I used to think was a drip. I recently
had a c5 firing abort before reaching temperature. The mat glazes in
the firing were sintered but not really fused. This glaze, which is a
high gloss, had started to get a slight sheen, but not fluid by any
means. On the inside of the foot of the vases that tend to have this
"crawling" there were bubbles about 2-3 mm in diameter, not
craters--they were as almost spheres. This bubble is what is touching
the kiln shelf and leaving residue, not something peeling off the
pot. Is it possible that the glaze coating is boiling while the
physical water is coming off in the very early stages of the firing?
From the condition of the glaze I can't believe that it was fluid
enough from the melt to boil in this manner.

Tomorrow I will put in a test substituting ball clay for part of the
koalin. Usually I hire Ron to reformulate glazes for me, but this
time I decided to use that glaze calculation software myself and give
it a whirl.

As another interesting side note, I have a variation on this base
that contains CadyCal, Laguna Borate, with clay content approx 13%
(Foundry Hill ball clay and EPK) and never crawls, even when fired 10
minutes after being dipped. You just never know...

Marni
--

Marni Turkel
Stony Point Ceramic Design
2080 Llano Rd 1B
Santa Rosa, CA 95407

Phone: 707-579-5567
Fax: 707-579-1116
http://www.marniturkel.com

Ron Roy on tue 22 oct 02

Hi Marni,

Just some thoughts of mine on the following.

Kaolins tend to have a low MOR (modulus of rupture) when compared to ball
clays. MOR is ceramic engineer talk for how hard it is to break dry clay. I
am guessing EPK has a higher MOR than most kaolins because it's particle
size is smaller than most Kaolins.

When subbing in ball clay use calculation software to make sure you get
enough alumina and cut down the SiO2. Kaolins always have more alumina and
less silica than ball clays.

Most glazes bubble at some point during melting - some do it more than once
depending on the viscosity of the glaze and when the fluxes kick in. A
glaze can bubble on the way to end temperature and do it again when over
fired. We have to time the melting so the bubbling happens before or after
our target temperature.

The reason the glaze with CadyCal and Laguna Borate does not crawl - both
materials have some soluble salts in them - these salts migrate to the
surface during drying of the raw glaze and give a tough coating which tends
to stop cracking - adding sugar to a glaze will do the same thing but will
smell up the glaze in the bucket after a short time so this is not the best
way to handle the problem. Perhaps ball clay works in somewhat the same way
- smaller particles follow the water out to the surface and give a much
tougher surface.

This thinking followed some experiments I did years ago. Jeff Ostricke (sp)
gave out a slip he used during a workshop - 95 Ball (Bell Dark) and 5 red
iron oxide. I could not get it to crack on wet, dry or bisqued ware - it is
the reason I recommend subbing in ball clay for kaolin to stop crawling -
works 99% of the time.

We think crawling is due to shrinkage of the raw glazes - it says that in
most of the texts - and it is true to some degree - but when you factor in
MOR the picture changes and presents other solutions.

I'm dying to hear how your revisions went Marni - best of luck - as if that
has anything to do with it.

RR

>In a reply to this thread Ron mentioned that EPK can be an offender
>in causing a glaze to crawl. Is that specifically Edgar Plastic
>Kaolin (EPK) or does that apply to any raw kaolin? Has anyone tried
>substituting other kaolins (not calcined) for EPK? I'll probably be
>trying that soon.
>
>I am working on a glaze that will crawl only on the bottom of the pot
>inside of the foot ring if it is fired before completely dry. It is
>21.5% EPK, but no other ingredients that I would suspect. A just
>dipped piece that has cut-outs in the foot that provide air flow will
>not crawl. Interestingly (to me anyway), it doesn't seem to crawl in
>the traditional sense, it never leaves a bare spot, just leaves some
>residue on the shelf or what I used to think was a drip. I recently
>had a c5 firing abort before reaching temperature. The mat glazes in
>the firing were sintered but not really fused. This glaze, which is a
>high gloss, had started to get a slight sheen, but not fluid by any
>means. On the inside of the foot of the vases that tend to have this
>"crawling" there were bubbles about 2-3 mm in diameter, not
>craters--they were as almost spheres. This bubble is what is touching
>the kiln shelf and leaving residue, not something peeling off the
>pot. Is it possible that the glaze coating is boiling while the
>physical water is coming off in the very early stages of the firing?
> From the condition of the glaze I can't believe that it was fluid
>enough from the melt to boil in this manner.
>
>Tomorrow I will put in a test substituting ball clay for part of the
>koalin. Usually I hire Ron to reformulate glazes for me, but this
>time I decided to use that glaze calculation software myself and give
>it a whirl.
>
>As another interesting side note, I have a variation on this base
>that contains CadyCal, Laguna Borate, with clay content approx 13%
>(Foundry Hill ball clay and EPK) and never crawls, even when fired 10
>minutes after being dipped. You just never know...

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 22 oct 02

As I have explained - Ball clay works very well - especially if you pick
one with a high MOR. I use this technique often when working on potters
problems so I am not guessing. Some times just adding bentonite will work.

the problem with calcining clay - you still have the dusty surface which
plays hell with layering other glazes on top. Calcining can also lead to
settling problems in the bucket. The ball clay solution - making the
surface tougher (way less dusty) is conducive to double and even tripe
glaze application.

I have tried the calcining solution in the past - it did not always work
and you still need to find out how much less calcined clay to add in
compared to raw clay - so you do need to change the recipe - or calcine
some of the clay after you weigh it out.

RR


>>Is that specifically Edgar Plastic
>>Kaolin (EPK) or does that apply to any raw kaolin?...
>>Tomorrow I will put in a test substituting ball clay for part of the
>>koalin.
>
>
>Any raw clay will exacerbate the tendency of a
>glaze to crawl. I would think that ball clay
>would make it worse, as most ball clays have a
>higher shrinkage than most kaolins. Just calcine
>a little of the kaolin instead - no need to change
>the recipe.

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