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question for those who make large sculptures--

updated tue 5 may 09


Ron Roy on mon 4 may 09

Hi Vince,
I try to keep stoneware clays at about 2% absorbency - even 3% will do - th=
water does not get all the way in Vince so it is only in voids that are not
connected to the inside clay - porcelain should be close to no absorption.

I realize that most commercial clays are not monitored continuously and
because our raw materials vary we have to be vigilant - especially when
making things we don't want to leak. I'm not surprised that you have seen
pots blow up but the real question here is - what was the absorbency of tha=

If anyone wants to test ware after it has been soaked or in use for a long
time - simply heating it in a microwave oven will show if there is water
inside. Just heat on high for 10 seconds - touch to see if it's heating -
have a cup of water in the oven and proceed - at 10 second s a time till th=
water in the cup boils. If your suspect pot gets hot - the clay is not
properly vitrified.

If anyone needs further instructions on how to test for absorbency I'll be
happy to send details.


On Sat, May 2, 2009 at 8:27 PM, Vince Pitelka wrote:

> Ron Roy wrote:
> "There could be some confusion here - properly vitrified clay does not
> absorb water and will be fine in any outside temperatures. There are ways
> to
> test clays to find out if they are vitrified enough to not absorb water -
> we
> should all know to what degree our clays are vitrified."
> Ron -
> I suppose this gets down to a definition of "properly vitrified" as it
> would
> apply to studio claybodies. As one of our foremost North American expert=
> on claybodies, I expect you would agree that very few studio claybodies
> (especially stoneware bodies) are vitrified to zero absorption, and that
> such a high degree of vitrification would decrease thermal-shock
> resistance.
> I have seen one good example of a high-fired "vitrified" piece exploding
> upon being heated excessively after having absorbed water over time, and
> have heard of many other examples. The same would be true in freezing. =
> any water is absorbed at all, then the mostly-vitrified body prevents
> escape
> of the pressure of expanding water upon freezing, and the piece cracks.
> Does this correspond to your experience? I'd be interested in your
> definition of "properly vitrified."
> - Vince
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University

Ron Roy
15084 Little Lake Road
Brighton, Ontario, Canada
K0K 1H0

Dave Finkelnburg on mon 4 may 09

Vince,=3D0A=3DA0=3DA0=3DA0 I believe it is possible to get freeze-resistant=
s by=3DA0two methods.=3DA0 One is the well-documented method of making ware=
h a carefully controlled ratio of porosity to absorption.=3DA0 The other is=
o make fully vitrified ware.=3DA0=3D0A=3DA0=3DA0=3DA0 Regarding the archive=
s, Val Cus=3D
hing's handbook=3DA0and the first method, here are clips from posts by Eva =
llagher and Stepheni Stephenson.=3DA0 Linda=3DA0Blossom has also posted=3DA=
0some =3D
of these=3DA0details.=3DA0 Tony Hansen has had a discussion on his excellen=
t Di=3D
gitalfire website with this information also.=3DA0 The most recent, and pro=
bly best discussion of this, on the list occurred in January, 2007.=3D0A=3D=
=3DA0=3DA0 All the best,=3D0A=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0=3DA0 Dave =
=3DA0=3D0AFrom: Eva Gallagher =3D0AHere is the calculation from Val's handb=
ook - =3D
he says =3D0Athat it is the =3D0Astandard test used in the ceramics industr=
y fo=3D
r =3D0Astructural clay products. =3D0AD=3D3D dry weight of test sample =3D0=
AC=3D3D we=3D
t weight of sample after soaking at room temp for =3D0A24 hours (wipe off =
surface water before weighing) =3D0AB =3D3D boiled weight after 2 hours in =
ing water =3D0A(again wipe before =3D0Aweighing) =3D0A=3D0A1. C - D / D =3D=
3D "C" val=3D
ue =3D0A2. B - D / D =3D3D "B" value =3D0A3. "C" value / "B" value =3D3D C/=
B ratio.=3D
The C/B ratio =3D0Amust be less than .78 =3D0A=3D0A=3D0AFrom: stephani ste=
phenson =3D
=3D0Athe cold water absorption divided by the boiling water =3D0Aabsorption=
es you the "Saturation" coefficient =3D0A=3D0AThe saturation coefficient mu=
st b=3D
e 0.78 in order to =3D0Apass CSA and ASTM specs for outdoor use. =3D0A=3D0A=
------------------------=3D0ADate:=3DA0 =3DA0 Sat, 2 May 2009 00:28:24 -050=
om:=3DA0 =3DA0 Vince Pitelka =3D0ASubject: Re: Questio=
n for =3D
those who make large sculptures--=3D0A=3D0AI hear that Dave Finkelnburg has=
n spreading vicious rumors about=3D0Avitrified high-fire sculpture survivin=
g =3D
outdoors in hard-freezing climates.=3D0A=3D0A...someone posted the specific=
ndards... =3D0A=3D0A=3D0A

Neon-Cat on mon 4 may 09

ASTM C373 (Glass and Ceramic Standards)

PPP: Clay Body Shrinkage & Absorption
Jeff Zamek, August 3, 2003
Ceramic Industry
(nice article for all levels of potters)