Al Pfeiffer on sun 1 feb 98
Having a problem with a porcelain body which may be related to the glaze. I
have a clear liner for cone 7 that works very well for most applications but
has a high calcium content from whiting. I've noticed that on some forms more
susceptible to warping that using this liner it is much worse. Can the
fluxing action of the calcium at C7 in the glaze also be affecting the clay
body? If so will reducing the calcium and using another oxide such as zinc
have potentially less effect on the clay body?
Thanks in advance for any help!
Al
Craig Martell on mon 2 feb 98
At 02:42 PM 2/1/98 EST, Al wrote:
>----------------------------Original message----------------------------
>Having a problem with a porcelain body which may be related to the glaze. I
>have a clear liner for cone 7 that works very well for most applications but
>has a high calcium content from whiting. I've noticed that on some forms more
>susceptible to warping that using this liner it is much worse. Can the
>fluxing action of the calcium at C7 in the glaze also be affecting the clay
>body? If so will reducing the calcium and using another oxide such as zinc
>have potentially less effect on the clay body?
Hi Al:
Calcium is not a real agressive melter at cone 7, but one needs to look at
the total composition of the glaze to really give any solid advice. It may
be that this glaze is in the low expansion range and is stressing the
claybody a bit through an expansion differential problem. Does the glaze fit
the body well and is not showing any signs of crazing? Without seeing the
glaze and putting it into the seger formula, there isn't a way to even tell
you if the calcium is over the recommended limit. Do you know the molecular
value for calcium in the glazes seger formula? The upper limit for cone 7
according to Insight is .6 moles.
I've used .9 limestone glazes at cone 10 on all kinds of porcelain forms
without any noticeable problems with warping, but that's just my experience.
The composition of your clay is undoubltedly different than what I use.
regards, Craig Martell-Oregon
paul wilmoth on mon 2 feb 98
Al,
What is probably happening is the higher calcium liner has
greater surface tension than what you are using on the outside, therefore
it is pulling more than the exterior glaze - resulting in warping. If you
were to use the liner on the outside also the tension would be equalized
resulting in little warping (of course considering that the works are set
on a flat surface and have equal heat). I doubt that the calcium is
leaching into the body and fluxing the clay more.
A good test would be to try these glazes on a flat, bisque tile
and see how much they warp from glaze on one side only ( I imagine that
your liner tests will warp the most). Then fire another set of tiles on
their side (have a bare spot where they can lean against a brick
support), glaze these tiles on both sides and compare.
I have seen this to such an extreme that the pots were actually
torn apart!
good luck- Paul
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