Joe Davis on tue 11 dec 01
Dave,
Try replacing some of the clay with calcined clay, maybe
25%. Don't forget to re-calculate your glaze for loss on
ignition (LOI).
bisque fire 100 grams of your clay and weigh again. The new
weight is your LOI percentage.
calcined weight = unfired weight x (100-LOI%) / 100
Don't be surprised if the calcined clay settles into an
impervious rock at the bottom of your glaze bucket - you may
then need to add a suspending agent to the glaze.
--
good luck
Joe Davis in Eugene, OR
--
Visit Joe's web site at:
http://www.efn.org/~joedavis/
Dave Finkelnburg on tue 11 dec 01
Hi all!
I am looking for some help with flaking that occurs when I try applying
an ash glaze. It's a simple glaze, usually half clay, half pine ash.
This glaze shrinks a lot while drying and "mud cracks" whenever I get
it on thick enough to produce the effect I am after. I have no problems
with thin applications, but the results are not what I want.
Has anyone conquered this rascal of high shrinkage in a runny ash glaze?
I tried adding a small amount (1/2%) CMC without any noticeable effect.
Thanks in advance for any suggestions!
Dave Finkelnburg
Tony Ferguson on tue 11 dec 01
If it is cracking too much and flaking it is shrinking off--your glaze is
shrinking more than your clay body. Are you washing your ash or leaving it
raw? Try leaving it raw--there are more organics in the ash as well as the
potassium salts and other things--these will help the glaze stick better to
the pot as well. But first, try removing some of the clay.
Thank you.
Tony Ferguson
315 N. Lake Ave. Apt. 401
Duluth, MN 55806
USA
218.727.6339
Stoneware, Porcelain, Raku
http://www.AquariusArtGallery.com
----- Original Message -----
From: "Dave Finkelnburg"
To:
Sent: Tuesday, December 11, 2001 12:32 PM
Subject: Ash glaze application problem
> Hi all!
> I am looking for some help with flaking that occurs when I try
applying
> an ash glaze. It's a simple glaze, usually half clay, half pine ash.
> This glaze shrinks a lot while drying and "mud cracks" whenever I get
> it on thick enough to produce the effect I am after. I have no problems
> with thin applications, but the results are not what I want.
> Has anyone conquered this rascal of high shrinkage in a runny ash
glaze?
> I tried adding a small amount (1/2%) CMC without any noticeable effect.
> Thanks in advance for any suggestions!
> Dave Finkelnburg
>
>
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Dave Evans on tue 11 dec 01
Dave,
I find that glazes with this much clay in them often flake and peel =
as you describe. You may want to try substituting some calcined clay =
for part of the clay requirement. Just stick some of whatever clay you =
use into a bisqued bowl and fire in the next bisque load. The Loss On =
Ignition will change some of the analysis values, but a few line blends =
should make quick work of your problem.
Hope this helps,
John Evans in San Diego
Craig Martell on tue 11 dec 01
Hello Dave:
Woodash doesn't like to stay on anything especially if it's unwashed. You
don't say how your ash is prepared, so I just added that tidbit in case you
aren't washing the ash. Even if it's washed, if it's a very fine particle
ash it's sometimes problematic in terms of flaking.
You have a lot of clay in the glaze too which compounds the problem by
creating high plastic shrinkage in the glaze. As you've seen, thick
applications don't work well.
My advice would be to calcine at least half of the clay and you may want to
add a small % of bentonite or vee gum cer. The calcined clay will give
some tooth and decrease the wet to dry shrinkage of the glaze. If the ash
is unwashed, you might try washing part or all of the ash to remove some of
the soluble alkali. Two washings would be OK and you would still retain
some of the extra fluxing components. You could also add some 200 mesh
silica in a line blend and this would also help with the wet to dry
shrinkage and make the glaze more durable. A 50/50 clay-ash glaze is more
than likely going to be low in silica anyway so you get a double benefit
from adding some quartz.
Finally, spraying the ashglaze may be the best and least problematic way to
apply. You may not even need to calcine or change the composition of your
glaze at all. So, if you can spray, try this first if you haven't already.
regards, Craig Martell in Oregon
Snail Scott on tue 11 dec 01
At 01:32 PM 12/11/01 -0700, you wrote:
It's a simple glaze, usually half clay, half pine ash.
> This glaze shrinks a lot while drying...
suggestion 1) calcine some of the clay.
suggestion 2) glaze leatherhard.
-Snail
Paul Lewing on tue 11 dec 01
Dave, I think you need to calcine some of the clay. You don't say what clay
you're using, but most of them shrink too much in drying to comprise half of
the glaze. You shouldn't calcine all of it, but even doing one fourth of
the clay might be enough. Normally, if you're trying to replicate a glaze
and you have to calcine some of the clay, you'd have to compensate for loss
on ignition, but with a glaze as inexact as a half clay/half ash glaze, you
probably don't need to bother.
Good luck,
Paul Lewing, Seattle
Christena Schafale on wed 12 dec 01
Dave,
Calcine most of the clay content of the glaze. For my purposes, 10-15% raw
clay and the rest calcined works well.
Chris
At 01:32 PM 12/11/01 -0700, you wrote:
>Hi all!
> I am looking for some help with flaking that occurs when I try applying
>an ash glaze. It's a simple glaze, usually half clay, half pine ash.
> This glaze shrinks a lot while drying and "mud cracks" whenever I get
>it on thick enough to produce the effect I am after. I have no problems
>with thin applications, but the results are not what I want.
> Has anyone conquered this rascal of high shrinkage in a runny ash glaze?
>I tried adding a small amount (1/2%) CMC without any noticeable effect.
> Thanks in advance for any suggestions!
> Dave Finkelnburg
>
>______________________________________________________________________________
>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.
Consultation and Referral Specialist
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WHC228@AOL.COM on wed 12 dec 01
Try a little starch. From the grocery store. Or maybe some Karo syrup.
iandol on thu 13 dec 01
Dear Dave Finkelnburg,
Usual solution to this problem is to calcine your clay to bisque =
temperature and use that in proportions with raw clay and ash which =
refuse to do the crazy paving routine.
However, I offer an alternative solution. Take your measure of unwashed =
or washed ash and mix this intimately with four fifths of the measure of =
your clay. Calcine this mixture to bring it to a high degree of sinter. =
This should then be ground to pass a 100# sieve. The residue should be =
mixed with the remaining fifth of clay and applied to your pots.
Best regards,
Ivor.
Ron Roy on thu 13 dec 01
Hi Dave - I am assuming this is unwashed ash and so you have a
defloccultaed glaze - which can be a probem - particles repelling each
other and all that.
If the clay is kaolin you probably have a clay that is not very strong in
the raw state - bentonite will help as will a strong ball clay - could try
a combination of OM4 and 2% bentonite - that means less alumina so get
ready for more running.
If you sub in othe calsy with less alumina do it by calculation and add
some alumina hydrate to make up moles of alumina.
RR
> I am looking for some help with flaking that occurs when I try applying
>an ash glaze. It's a simple glaze, usually half clay, half pine ash.
> This glaze shrinks a lot while drying and "mud cracks" whenever I get
>it on thick enough to produce the effect I am after. I have no problems
>with thin applications, but the results are not what I want.
> Has anyone conquered this rascal of high shrinkage in a runny ash glaze?
>I tried adding a small amount (1/2%) CMC without any noticeable effect.
> Thanks in advance for any suggestions!
> Dave Finkelnburg
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Graeme Anderson on thu 13 dec 01
Four questions, Dave.
Is your 50/50 mix by weight or by volume? If by volume, there is more than
50% clay.
Do you bisc, or raw glaze?
Do you brush on, dip, or spray?
Do you use the same clay in the glaze as that you make the pots from?
I've had no problems with dipping on bisced pots, with a 50/50 mix by
weight, and using the same clay.
Good luck. Graeme.
Brother Augustine on thu 13 dec 01
Dave,
Try spraying the ash glaze in two or three coats to get the build up.
I spray all the ash glazes in light coatings, and would increase the CMC
to 1&1/2 % of the glaze, or make up a batch of liquid CMC of 25 grams
to one quart warm water, then add one good tablespoon per 100 grams of
glaze.
Also I find adding 10% silica to the glaze helps sometimes.
Hope this helps, br. Augustine
Clay Coordinator on fri 14 dec 01
Dave,
For a fast test you could try using Glomax (calcined kaolin) as the clay and
see if you still have a problem.
Thanks,
John Britt
Penland Clay
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