crawling and flocculation problem

updated fri 19 nov 99

Chris Schafale on tue 16 nov 99

Attention glaze gurus and anyone who's had a crawling problem:

Some of you might remember my struggles with crawling glazes (ongoing
over the last year, ever since I started mixing glazes). I've
managed to find some that work, but I keep being tempted by the
challenge of fixing the original culprit, because it was so beautiful
when it worked (and because, by now, with multiple attempts to fix
it, I've built up about 20 gallons of variations on this theme). To
repeat the problem, at its worst the glaze cracks as it dries and
falls off the pot, at best the glaze merely crawls in the firing.
.
Well, I thought I'd finally licked it. I mixed up a version of the
glaze that substituted calcined kaolin for all of the clay content of
the original recipe. Then I did a line blend with the original
problematic glaze, varying the percentages of each. Nothing crawled.
So I went ahead with a 50-50 blend. Worked great. No crawling.
Couldn't get it to crawl. Thick applications, multiple dips, all OK.
Tested it on a small bowl. Great. Tested it on a soup bowl. Great
again. Put it on a couple of goblets. No problem. So yesterday, I
decided to go ahead and put it on half a kiln load. Looked OK at
first, but then I had trouble when I glazed a pitcher on the inside,
let it dry, then came back to glaze the outside. Problem. Where the
glaze overlapped, it cracked, fell off in sheets. OK, so maybe the
application was a little thick. Wash it all off, try again on
another pitcher. Same thing. OK, can't overlap this glaze, I'll
just fire the bowls and other pieces without overlaps. Let the glaze
dry all day, come home, load them into the kiln, which was still a
little warm from the previous firing. Get the kiln all loaded, then
turn back to see the glaze peeling up off the edge of a bowl on the
side nearest the kiln wall. Unload the whole thing, they're all this
way. Here we go again.

Anyway, it seems to me that there must be something (soluble?) in the
glaze that is causing it to flocculate over time, so that although
the glaze seems fine when first mixed, it becomes unusable a week
later. And whatever this is, is either in the new
calcined-kaolin glaze too, or is capable of "infecting" the new glaze
material from the old. Does anyone have a clue what this could be??
And no, there is no Gerstley Borate in this glaze!! The recipe is
(or started out as) one that many of you have used, Tony Hansen's
cone 6 base, as follows.

TH Cone 6 Base
==============
Frit 3134........... 25.00
EPK................. 25.00
Flint............... 25.00
Wollastonite........ 10.00
F4 feldspar......... 15.00

I'm mixing it with distilled water, though I didn't at first, and
it's possible something in the original water caused the problem,
though a water analysis shows nothing unusual in our (city) water
supply. In addition to the calcined kaolin, other things I have
tried (and which are now in this bucket in varying quantities) have
included changing the recipe to include less kaolin, adding
Darvan 7 for deflocculation, bentonite, and CMC. I have also
"washed" the glaze a couple of times with distilled water, but this
is hard to do because it "holds on" to water -- it takes forever to
settle enough to remove the water once it's been added.

The really frustrating thing about this, and the reason I now have 20
gallons of the stuff instead of a few failed tests, is that I keep
thinking I've fixed it, get fooled into mixing up a larger batch,
then -- gotcha! I feel like Charlie Brown with the wretched
football. I know I should just quit, get rid of it (but 20 gallons
of glaze is not so easy to know what to do with), but I keep thinking
I ought to be able to fix this, or at least understand what's
happening. Help!

I would appreciate any insights any of you may have as to 1) what may
be happening, 2) how, if at all, it could be cured, 3) how to
get rid of the stuff in an environmentally responsible way, and/or 4)
how to get rid of this compulsion to try fixing it, just one more
time. Commiseration would also be appreciated. Thanks in advance.

Chris
Light One Candle Pottery
Fuquay-Varina, NC
candle@intrex.net

Michael Banks on wed 17 nov 99

Chris Schafale wrote: (snip)
> ----------------------------Original message----------------------------To
> repeat the problem, at its worst the glaze cracks as it dries and
> falls off the pot, at best the glaze merely crawls in the firing.....

> Anyway, it seems to me that there must be something (soluble?) in the
> glaze that is causing it to flocculate over time, so that although
> the glaze seems fine when first mixed, it becomes unusable a week
> later. ...

The EPK in your glaze is there (in part) Chris, to act as a binder. It also
contributes silica and alumina, but these oxides can be got from other
ingredients, so it's main role is a binder.

Now, I think you are DEFEATING THE BINDER by defloccing the glaze with
Darvan 7. Also the F4 feldspar (a soda-rich feldspar) is probably
contributing soluble Na ions which also deflocculate the EPK and also the
soda in 3134 Frit may be exacerbating this. The strength of plasticity of
EPK (the source of its binding power) is directly related to whether
deflocculant ions (Na, Li) dominate in the glaze water or flocculant ions
(H+, Ca, Mg). Deflocculant ions cause the clay particles (EPK in this case)
to repel each other, flocculants maximise the binding force. Soda feldspars
are a common (and overlooked) source of unwanted deflocculation of glaze
suspensions and this effect increases with age.

Suggested remedies to your problem are:
1. Flocculate the glaze with Epsom salts (MgSO4), calcium chloride or acid
(vinegar, HCl).
2. If (1) doesn't work, add a more powerful smectite-based binder (V-gum,
bentonite, hectorite).

Michael Banks,
Nelson,
New Zealand

Stephen Grimmer on wed 17 nov 99

Chris,
I think the problem may partially be with the way you are overlapping
the glaze:

> but then I had trouble when I glazed a pitcher on the inside,
>let it dry, then came back to glaze the outside. Problem.

When the dry glaze is rehydrated with the water from the second coat, it
expands and falls off your pitchers. Try glazing the outside right after the
inside, when the first coat has just lost its wet shine.
Also, 25% EPK may be just a bit high if you are putting on a thick glaze
coat. Try replacing 5% with the calcined stuff.
Good luck, and keep us posted.

Steve Grimmer
Bowling Green State University
Bowling Green, OH

----------
>From: Chris Schafale
>To: CLAYART@LSV.UKY.EDU
>Subject: crawling and flocculation problem
>Date: Tue, Nov 16, 1999, 11:22 AM
>

>----------------------------Original message----------------------------
>Attention glaze gurus and anyone who's had a crawling problem:
>
>Some of you might remember my struggles with crawling glazes (ongoing
>over the last year, ever since I started mixing glazes). I've
>managed to find some that work, but I keep being tempted by the
>challenge of fixing the original culprit, because it was so beautiful
>when it worked (and because, by now, with multiple attempts to fix
>it, I've built up about 20 gallons of variations on this theme). To
>repeat the problem, at its worst the glaze cracks as it dries and
>falls off the pot, at best the glaze merely crawls in the firing.
>.
>Well, I thought I'd finally licked it. I mixed up a version of the
>glaze that substituted calcined kaolin for all of the clay content of
>the original recipe. Then I did a line blend with the original
>problematic glaze, varying the percentages of each. Nothing crawled.
> So I went ahead with a 50-50 blend. Worked great. No crawling.
>Couldn't get it to crawl. Thick applications, multiple dips, all OK.
> Tested it on a small bowl. Great. Tested it on a soup bowl. Great
>again. Put it on a couple of goblets. No problem. So yesterday, I
>decided to go ahead and put it on half a kiln load. Looked OK at
>first, but then I had trouble when I glazed a pitcher on the inside,
>let it dry, then came back to glaze the outside. Problem. Where the
>glaze overlapped, it cracked, fell off in sheets. OK, so maybe the
>application was a little thick. Wash it all off, try again on
>another pitcher. Same thing. OK, can't overlap this glaze, I'll
>just fire the bowls and other pieces without overlaps. Let the glaze
>dry all day, come home, load them into the kiln, which was still a
>little warm from the previous firing. Get the kiln all loaded, then
>turn back to see the glaze peeling up off the edge of a bowl on the
>side nearest the kiln wall. Unload the whole thing, they're all this
>way. Here we go again.
>
>Anyway, it seems to me that there must be something (soluble?) in the
>glaze that is causing it to flocculate over time, so that although
>the glaze seems fine when first mixed, it becomes unusable a week
>later. And whatever this is, is either in the new
>calcined-kaolin glaze too, or is capable of "infecting" the new glaze
>material from the old. Does anyone have a clue what this could be??
>And no, there is no Gerstley Borate in this glaze!! The recipe is
>(or started out as) one that many of you have used, Tony Hansen's
>cone 6 base, as follows.
>
>TH Cone 6 Base
>==============
>Frit 3134........... 25.00
>EPK................. 25.00
>Flint............... 25.00
>Wollastonite........ 10.00
>F4 feldspar......... 15.00
>
>I'm mixing it with distilled water, though I didn't at first, and
>it's possible something in the original water caused the problem,
>though a water analysis shows nothing unusual in our (city) water
>supply. In addition to the calcined kaolin, other things I have
>tried (and which are now in this bucket in varying quantities) have
>included changing the recipe to include less kaolin, adding
>Darvan 7 for deflocculation, bentonite, and CMC. I have also
>"washed" the glaze a couple of times with distilled water, but this
>is hard to do because it "holds on" to water -- it takes forever to
>settle enough to remove the water once it's been added.
>
>The really frustrating thing about this, and the reason I now have 20
>gallons of the stuff instead of a few failed tests, is that I keep
>thinking I've fixed it, get fooled into mixing up a larger batch,
>then -- gotcha! I feel like Charlie Brown with the wretched
>football. I know I should just quit, get rid of it (but 20 gallons
>of glaze is not so easy to know what to do with), but I keep thinking
>I ought to be able to fix this, or at least understand what's
>happening. Help!
>
>I would appreciate any insights any of you may have as to 1) what may
>be happening, 2) how, if at all, it could be cured, 3) how to
>get rid of the stuff in an environmentally responsible way, and/or 4)
>how to get rid of this compulsion to try fixing it, just one more
>time. Commiseration would also be appreciated. Thanks in advance.
>
>Chris
>Light One Candle Pottery
>Fuquay-Varina, NC
>candle@intrex.net

Patsy Catsos on wed 17 nov 99

In a message dated 11/16/99 6:23:11 PM, you wrote:

<be happening, 2) how, if at all, it could be cured, 3) how to
get rid of the stuff in an environmentally responsible way, and/or 4)
how to get rid of this compulsion to try fixing it, just one more
time. Commiseration would also be appreciated. Thanks in advance.>>

Perhaps you've already considered these questions, but just in case....You
don't say much about the clay body you are putting the glaze on. What is it,
and what cone are you bisquing to? Is the bisque all fired to the same cone,
or is uneven heating in your kiln causing some of the bisque to be over or
underfired? Maybe the test pieces that worked were absorbing more or less
water from the glaze, resulting in a thicker or thinner coat. Also, is the
bisqued ware squeaky clean or is dust accumulating as is sits around. Dust,
oil, etc. on the surface of the ware could cause crawling. Are you wetting
or soaking any of the ware before glazing? That could cause different results
from piece to piece if they are getting different treatment. When you make
a bigger batch of the glaze are you using EXACTLY the same ingredients or a
slightly different mesh size, different lot, etc.? Is the specific gravity
of the new batch the same? Hope you can find a cure. I've had this problem
myself, with a gerstley borate glaze, and I never did solve it.
Patsy