Ron Roy on sat 21 feb 04
Lili asked me about shivering - and it's different forms - and thought I
should post this to the group and so did she.
Shivering is meant to describe the opposite problem from crazing. It
happens when the glaze does not contract enough during cooling compared to
the body contraction.
This leaves the glaze too "big" for the clay. In crazing the different
contraction of clay and glaze leaves the glaze too small for the clay.
When the glaze is too big for the clay we say the glaze is under
compression (squeezed) from the clay. In crazing the glaze is under tension
(pulled apart) by the clay.
Some compression of the glaze is good - it prevents crazing - and makes
pots stronger. To much and we get the faults we call shivering.
This fault shows up in three distinct ways.
1. The ware breaks or shatters either in the kiln or later at some time. I
can happen for no apparent reason or it can happen when hot water is poured
in a tea pot or when heated in the oven. The inside glaze simply pushes the
pot apart. You can blame it on the glaze or the body but it is always the
combination of the two.
2. Sometimes the glaze just cracks - usually in a spiral - it is sometimes
mistaken for crazing. What is really happening - the glaze is compressed so
much that it lifts and cracks in an attempt to relieve the pressure. If you
could magnify the crack enough you would see it open in a V shape. This is
different than crazing where there is actually a gap between the glaze
crack (pulled apart) but the sides are not lifted up.
3. Sometimes the glaze is forced off the pot - usually at sharp edges and/
or the rim - where the strain gets concentrated - little (sometimes big)
razor sharp flakes of glaze come off the ware - or remain attached for a
while. When the glaze is well bonded to the clay as in higher fired ware -
some clay will come off with the glaze. With lower fired ware - because the
bonding between clay and glaze is not well developed - only the glaze will
fall off.
The worst case I can think of (worse even than pots breaking in use) would
be someone - finding a sliver of sharp glaze in their food - or worse still
actually swallowing it.
All three situations are to be avoided - especially on any functional ware.
Any potter will do well to make sure - especially when trying new glazes -
to test for this fault very well and check carefully for any small
fractures in the glaze with a magnifying glass - especially at the rim and
any sharp glazed edges.
Freezing a pot simply continues the cooling down further and tends to
magnify this fault - pouring hot water into the ware - while it's still
frozen further aggravates the problem by heating the glaze up a fraction of
a second before the clay - expanding it a bit faster Do it in the sink -
hot water burns! Any ware that cannot pass this test should be scrapped.
Yet another reason to use calculation software - to at least get some
warning about what kind of fit to expect. Keep in mind - this works best
with shiny or semi matte glaze - the 5 glazes in our book - to be used to
ascertain clay body expansions - were all designed with calculation
software - when you look at the dilatometer charts (actual measurements of
those glazes) they are almost equally spaced. This gives some valuable
credence to just how useful calculating expansion can be for many glazes.
I should also mention - many of the clay bodies sold or made are not
measured or tested - we should all understand - most of the people making
clay bodies in North America don't have a clue about the expansion of their
cay bodies.
I just measured a cone 6 clay for somebody that changed my mind about cone
6 bodies with cristobalite - I was amazed. I have measured hundreds of
clays now and only once saw a little cristobalite at cone 6. This body had
lots - and in fact it was hard to believe that it had only been fired to
cone 6 - from the chart I would have guessed at least cone 10.
It is simply unreasonable to expect any clay body to have the same
expansion as any other - until you check it out - or your supplier shows
the evidence. It is the same with many homemade bodies - unless you are
willing to do the testing you may just get a very expensive surprise.
RR
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Helayne Friedland on sat 21 feb 04
I have a cone 6 glaze called antique white that I have been using for a few=20
years now. It is a matte glaze, beautiful soft white that gets some yellow=20
variations in it. It has always worked perfectly=E2=80=94until now. With the=
same clay=20
body I have always used with it, the last few pieces I've fired the glaze ha=
s=20
shivered, just flaking off the pot, on the rim but also on some curved areas=
of=20
the pot. This is one of my favorite glazes=E2=80=94any advise on why this ha=
s started=20
happening and how I could try to fix it would be greatly appreciated.
Thanks for your help
Helayne (from NY)
daniel on sat 21 feb 04
Hi Ron,
> This leaves the glaze too "big" for the clay. In crazing the different
> contraction of clay and glaze leaves the glaze too small for the clay.
I have been wondering about the following thought for a bit. It seems to be
unlikely to occur and yet seems to make sense that it could happen. So my
question is why does it not.
If one considers that a glaze with a higher expansion than a body crazes on
the body, why does not a body with higher expansion than the glaze craze on
the glaze ? It seems that the body is inherently stronger than the glaze and
so this never happens but when we have a thing (eg. slipcast form - the only
case I know of where a friend of mine had a glaze/body mismatch so severe as
to explode a piece) then the body explodes rather than crazing in the way
that the glaze will - ie. crazing. I guess glazes and bodies are similar but
not that similar.
This probably a lunatic way to look at it but I figured I'd throw it out
and see what people thought.
Many thanx
Daniel
Ron Roy on mon 23 feb 04
Hi Daniel,
If the glaze was on the inside and the clay on the outside then perhaps the
clay would craze. Glazes - usually being thiner - are simply not as strong
as the clay. It is interesting to note that - although the amount of
contraction of clay and glazes is very small - it is the build up of the
stresses that eventually can bring about failure - and even thin glazes can
crack a pot. I hasten to add though - thicker applications of glaze do have
a stronger effect.
One of my recommendations for glazing ovenware is to keep the inside glaze
thin rather than thick. Having no sharp corners - and rounded rims can help
as well because the stress is not consentrated in certain places as much.
RR
>> This leaves the glaze too "big" for the clay. In crazing the different
>> contraction of clay and glaze leaves the glaze too small for the clay.
>
>I have been wondering about the following thought for a bit. It seems to be
>unlikely to occur and yet seems to make sense that it could happen. So my
>question is why does it not.
>
> If one considers that a glaze with a higher expansion than a body crazes on
>the body, why does not a body with higher expansion than the glaze craze on
>the glaze ? It seems that the body is inherently stronger than the glaze and
>so this never happens but when we have a thing (eg. slipcast form - the only
>case I know of where a friend of mine had a glaze/body mismatch so severe as
>to explode a piece) then the body explodes rather than crazing in the way
>that the glaze will - ie. crazing. I guess glazes and bodies are similar but
>not that similar.
>
> This probably a lunatic way to look at it but I figured I'd throw it out
>and see what people thought.
>
>Many thanx
>Daniel
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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