Alisa and Claus Clausen on tue 7 mar 00
------------------
. Shivering
=3Eoccurs when the clay body shrinks more than the glaze does, placing the
=3Eglaze under compression and causing little flakes of glass/glaze to come =
off
=3Ein someone's mouth, not a pretty thing.
=3E
=3EPeter Jones
I am trying to understand and correct a problem I am experiencing with
shivering. I cannot
acurately attack the problem, because I do not have the recipe for two of =
the
suspicious
components. These are materials being used in the community studio, and =
they
are all bought ready made. Unfortunately I have only the claybody makeup.
Thus, I am trying to understand what the most obvious problem would be for =
this
shivering. These pieces are all white bodies, painted with engobe and
transparent glazed.
I am trying to change one variable at time, to isolate the villan.
I am using a cone 6 stoneware white body. It is
SiO2 70,0
TiO2 1,6
Al2O3 24,5
Fe2O3 0,8
CaO 0,1
K2O 1,6
Na2O 0,3
The pieces are thrown, painted with engobe while they are much wetter than
leather hard,
bisqued (no problem yet) and then dipped and fired with a transparent gloss =
or
semi gloss.
My dealer told me that I was painting the engobes on too dry ware.
Not the problem because the pots are very wet, or students have painted
on clay just rolled out.
At first I saw most of the shivering around the lips of pots.
My dealer said it was too thick.
Not the problem because it shivers albeit absolutely where it is thickest on=
the
pots
and slaps (lips and edges) but also on other areas of the ware where it is =
not
particularly thick.
According to above advice from Peter, I would conclude that the glaze does =
not
fit the engobes. But, is there also any potential problem with the white =
clay
body not fitting the =22ready made all purpose stoneware=22 engobes? Should
they fit white wares as well as common stonewares with more iron? This just
may be a fantasy problem I am adding to my list, but I would like to hear if
there
could be a problem there. This body shrinks actually only 11 precent.
I am inclined as I write this to say it is the glaze not fitting the =
engobes. I
have used
both a gloss and a matt transparent, and they both shiver.
My ceramic supplier has no more answers.
If the engobe comes out of the bisque still hanging on to the pot, it is =
safe to
say that the
engobe and clay body fit each other? When it shivers off after the glaze, =
is it
definitly a
bad fit between the engobe and glaze?
Normally for engobe, I blend down my own clay body and add color. The =
world's
best
engobe base can be found in one's splash pan. I have no really good =
experience
with the ready made ones. I wonder what in fact the advantage could be to =
buy
engobes or mixing up from a recipe, risking that they do not fit your body?
In fact I wonder about this a lot, because of recipes like Bringle or =
standard
white slips, etc. But I am more confident by using my own clay bodies =
diluted.
That may be another discussion.
Thanks in advance for any help,
Alisa in Denmark
Not especially looking forward
to opening tomorrow's kiln.
Peter Jones on wed 8 mar 00
Alisa and Claus: Aaahhhh, engobes. Another whole story. Engobes (slip
clay formulas for painting underglaze colors onto clay bodies) can be
tricky. I don't think I read that when I made my first response and assumed
it was part of the problem. It may still not be, but read on:
Engobes are basically formulated to be painted onto clay bodies at different
stages of dry-ness. This is done to accommodate the shrinkage rate at
different times of application. Say for example that your clay body shrinks
12% from the time when it is first thrown til it comes out of the glaze
firing. Then, for the sake of argument, say that it shrinks about 2% from
freshly thrown to leather hard. Then say that the shrinkage rate from
leather hard to bone dry is 4 %. Shrinkage from bone dry to bisque state is
about 2%. From bisque to glaze fired is about 4%.
Now suppose that the engobe you have bought has a shrinkage rate of 10%
based on its formulation. As such, it is perfectly matched for using on
your pots if it is applied in the leather hard stage. The shrinkage of the
engobe will match the shrinkage of the clay from leather hard to glaze
fired. But what happens if you have applied the engobe at wet stage? The
engobe shrinks only 10%, but the clay body shrinks 12%. Since the engobe is
a layer of clay on the surface, chances are that it will flake off. It
would be even more pronounced if it were applied to dry or bisque ware.
As such, you may be using an engobe that is incompatible with your clay
body. It may be flaking off under the glaze layer and appear as little
flakes of clay within the glaze melt.
However, if you are actually getting little chips of glaze itself to come
off, especially at any sharp edges or lips, then you definitely have a
glaze/clay body fit problem, not an engobe problem per se. This is far more
serious. You are looking at serious liabilty problems if glaze (a layer of
glass) is coming off your pots and people are finding it in their food. A
big NO-NO.
If you are getting the problem with all your glazes, then your clay body is
the problem. If you are only getting it with one specific glaze, then your
glaze is likely the problem. If you are getting it only with the engobe,
then the engobe is the problem.
The only way to really test this is the to do a series of tests. Put the
glaze on the non-engobed clay body and see if it flakes off after the glaze
firing. Then try it on the clay with the engobe applied. Try it this way
with a bunch of different glazes. You will ultimately see a pattern. If it
is only occurring when you use the engobe over the clay body with one glaze
in particular, throw away the glaze or check your mixture. If it happens
with a lot of them, scrap the clay body. If it only happens with the
engobe, throw away the engobe.
Hope this helps.
Peter Jones
----------
>From: Alisa and Claus Clausen
>To: CLAYART@LSV.UKY.EDU
>Subject: Sv: Another shivering engobe problem
>Date: Tue, Mar 7, 2000, 1:06 PM
>
>----------------------------Original message----------------------------
>------------------
>
>. Shivering
>>occurs when the clay body shrinks more than the glaze does, placing the
>>glaze under compression and causing little flakes of glass/glaze to come off
>>in someone's mouth, not a pretty thing.
>>
>>Peter Jones
>
>
>I am trying to understand and correct a problem I am experiencing with
>shivering. I cannot
>acurately attack the problem, because I do not have the recipe for two of the
>suspicious
>components. These are materials being used in the community studio, and they
>are all bought ready made. Unfortunately I have only the claybody makeup.
>Thus, I am trying to understand what the most obvious problem would be for this
>shivering. These pieces are all white bodies, painted with engobe and
>transparent glazed.
>I am trying to change one variable at time, to isolate the villan.
>
>I am using a cone 6 stoneware white body. It is
>SiO2 70,0
>TiO2 1,6
>Al2O3 24,5
>Fe2O3 0,8
>CaO 0,1
>K2O 1,6
>Na2O 0,3
>
>
>The pieces are thrown, painted with engobe while they are much wetter than
>leather hard,
>bisqued (no problem yet) and then dipped and fired with a transparent gloss or
>semi gloss.
>
>My dealer told me that I was painting the engobes on too dry ware.
>Not the problem because the pots are very wet, or students have painted
>on clay just rolled out.
>
>At first I saw most of the shivering around the lips of pots.
>My dealer said it was too thick.
>Not the problem because it shivers albeit absolutely where it is thickest on th
>pots
>and slaps (lips and edges) but also on other areas of the ware where it is not
>particularly thick.
>
>According to above advice from Peter, I would conclude that the glaze does not
>fit the engobes. But, is there also any potential problem with the white clay
>body not fitting the "ready made all purpose stoneware" engobes? Should
>they fit white wares as well as common stonewares with more iron? This just
>may be a fantasy problem I am adding to my list, but I would like to hear if
>there
>could be a problem there. This body shrinks actually only 11 precent.
>
>I am inclined as I write this to say it is the glaze not fitting the engobes.
>have used
>both a gloss and a matt transparent, and they both shiver.
>
>My ceramic supplier has no more answers.
>
>If the engobe comes out of the bisque still hanging on to the pot, it is safe t
>say that the
>engobe and clay body fit each other? When it shivers off after the glaze, is i
>definitly a
>bad fit between the engobe and glaze?
>Normally for engobe, I blend down my own clay body and add color. The world's
>best
>engobe base can be found in one's splash pan. I have no really good experience
>with the ready made ones. I wonder what in fact the advantage could be to buy
>engobes or mixing up from a recipe, risking that they do not fit your body?
>In fact I wonder about this a lot, because of recipes like Bringle or standard
>white slips, etc. But I am more confident by using my own clay bodies diluted.
>That may be another discussion.
>
>
>Thanks in advance for any help,
>Alisa in Denmark
>Not especially looking forward
>to opening tomorrow's kiln.
>
>
Ron Roy on thu 9 mar 00
Why engobes are not tested for fit with both bodies and glazes is a mystery
to me.
I recommend making test tiles out of it and trying your glazes on them. If
they react the way your body does with your glazes then you are on the
right track.
The idea of inserting a layer of anything between clay and glaze and
expecting it to be compatible without any experimentation is simply not
understanding what fit is about.
I recommend making slips out of the same body you are throwing with - if
possible. If you are putting a light slip over a dark body then there is
work to be done and it is easier said than done.
RR
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Barney Adams on fri 10 mar 00
Ron,
I want to mix up some slips. If I can get my clay body formula's is there
things I should look for in using Insight as a guide for working up test slips?
I usr Tucker's Mid Red, Smooth White and the MCS. (You may be familiar with
them)
Do I just liquify the clays with colorant or is there other things I can add to
adjust the slips?
I would imagine the MCS as slip may not work on the others. Could I adjust slip
fit?
Barney
Ron Roy wrote:
> ----------------------------Original message----------------------------
> Why engobes are not tested for fit with both bodies and glazes is a mystery
> to me.
>
> I recommend making test tiles out of it and trying your glazes on them. If
> they react the way your body does with your glazes then you are on the
> right track.
>
> The idea of inserting a layer of anything between clay and glaze and
> expecting it to be compatible without any experimentation is simply not
> understanding what fit is about.
>
> I recommend making slips out of the same body you are throwing with - if
> possible. If you are putting a light slip over a dark body then there is
> work to be done and it is easier said than done.
>
> RR
>
> Ron Roy
> 93 Pegasus Trail
> Scarborough
> Ontario, Canada
> M1G 3N8
> Evenings 416-439-2621
> Fax 416-438-7849
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