Lisa Wolters on fri 12 apr 02
Hello all.
I am working on an outdoor mural and wondered if anyone has experience
with testing a claybody for thermal shock. I am firing a to cone 10 and
need to make sure the mural can withstand cold Ohio winters. Thanks.
vince pitelka on fri 12 apr 02
> I am working on an outdoor mural and wondered if anyone has experience
> with testing a claybody for thermal shock. I am firing a to cone 10 and
> need to make sure the mural can withstand cold Ohio winters. Thanks.
Lisa -
That's not thermal shock. Thermal shock is abrupt temperature change, as
when you take a casserole out of the oven and set it on a cold countertop.
Your mural is going to be subject to the rigors of hard freezing, which is
an entirely different matter. If you are using a standard cone 10
functional body, then it will not survive repeated hard freezes. Claybodies
for outdoor sculpture or architectural ornamentation need to be fairly
porous in order to allow accumulated water to escape when the clay freezes.
Too porous (like a lowfire body) and they disintegrate with the first hard
freeze. A vitreous stoneware body still has some absorption, and when the
water gets in (as it will in any outdoor clay piece) it cannot escape
easily, so the piece will disintegrate over time. Most architectural
ornamentation is terracotta, fired to very low midrange - around cone 1 to
cone 3. It is still slightly porous at that temperature. It is likely that
your cone 10 claybody would be fine at cone 5 or cone 6, but you would
obviously have to use cone 6 glazes.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Snail Scott on fri 12 apr 02
At 02:20 PM 4/12/02 -0400, you wrote:
>Hello all.
>
>I am working on an outdoor mural and wondered if anyone has experience
>with testing a claybody for thermal shock. I am firing a to cone 10 and
>need to make sure the mural can withstand cold Ohio winters. Thanks.
It's not the thermal shock you need to worry about.
Even the most sudden blizzard isn't sudden enough
to cause thermal shock to even the most sensitive
clay. What you need to consider is frostproofness.
There is a theory which says that frostproofness is
a function of vitrification. This is definitely crap,
IMHO! Unfortunately the exact mechanism by which clays
can resist the force of ice is rather poorly understood.
Some very porous terra cottas have withstood freezing
for 100 years or more, while more vitrified clays have
cracked in a single season. The best research on this
seems to have been done between 1900-1930, when the
fashion for architectural ceramics was sufficient to
support substantial research. It seems that if a clay
is sufficiently strong, full vitrification is not
necessary. However, it's very difficult to judge which
clays will perform well in advance of actual testing.
I test my clay by making test bars, boiling them, then
when the water has cooled, putting them in a ziploc
baggie while still wet. I put them in the freezer
overnight, then take them out during the day, and repeat
until I forget to do it. (Very scientific.) A more
reliable check, if you use commercial clay, is to ask
your supplier if anyone else has successfully used that
body for outdoor work in that climate. Ask to speak to
artists who do such work, and ask how old their oldest
outdoor works are. It takes a lot of freezer tests to
equal a succession of real winters. (Actually, it's the
springs and autumns that do the damage. Freezing, when
it stays frozen for a month, is still only one freeze/
thaw cycle!)
But, take special care with your techique when joining.
The best clay in the world can't survive a hairline
crack in a spot that gets wet and then freezes. Join
with scrupulous care! No clay on earth will withstand
freeze/thaw forever - even mountains erode; that's where
clay comes from, after all. But don't give erosion any
free assists. Also avoid forms that will trap pools of
water to freeze and expand. Workmanship will count for
as much as clay choice.
-Snail
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Cindi Anderson on sat 13 apr 02
Am I correct in saying that another way to achieve porosity is to have a lot
of grog? The clay that is used by the architectural ceramists I know is
extremely groggy, and the grog is coarse. But it is also somewhat
underfired stoneware as you have suggested. Do you think most of the
ability to withstand freezing comes from being underfired, or does the grog
contribute also?
Thanks
Cindi
----- Original Message -----
From: "vince pitelka"
> Your mural is going to be subject to the rigors of hard freezing, which is
> an entirely different matter. If you are using a standard cone 10
> functional body, then it will not survive repeated hard freezes.
Claybodies
> for outdoor sculpture or architectural ornamentation need to be fairly
> porous in order to allow accumulated water to escape when the clay
freezes.
> Too porous (like a lowfire body) and they disintegrate with the first hard
> freeze. A vitreous stoneware body still has some absorption, and when the
> water gets in (as it will in any outdoor clay piece) it cannot escape
> easily, so the piece will disintegrate over time. Most architectural
> ornamentation is terracotta, fired to very low midrange - around cone 1 to
> cone 3. It is still slightly porous at that temperature. It is likely
that
> your cone 10 claybody would be fine at cone 5 or cone 6, but you would
> obviously have to use cone 6 glazes.
> Good luck -
> - Vince
Snail Scott on sun 14 apr 02
At 09:04 AM 4/13/02 -0700, you wrote:
>Am I correct in saying that another way to achieve porosity is to have a
lot
>of grog? The clay that is used by the architectural ceramists I know is
>extremely groggy, and the grog is coarse. But it is also somewhat
>underfired stoneware as you have suggested. Do you think most of the
>ability to withstand freezing comes from being underfired, or does the grog
>contribute also?
I don't think that 'underfired' is the same as 'porous'.
I would expect an underfired body to have not melted
sufficiently to achieve its maximum strength. (But I'm
prepared to be informed otherwise.)
I have had fair luck with my outdoor work. I used to use a
homemade clay that was 30% grog by weight. Many pieces
made of this clay have been outside though 10 snowy winters
with no visible damage. (I do have one piece that had small
cracks open up along a coil joint which corresponded to the
end of one day's work and the beginning of the next. I'm
now much more careful of these joints.) Now I use Laguna's
^5 Buff Sculpture, which has quite a lot of large grog, and
was recommended to me as a clay which has held up over time
for other artists who do outdoor work. So far, so good.
(I know an artist who works exclusively in earthenware,
and coats her work with Thompson's Water-Seal. It seems to
be holding up well. Years of re-sealing have begun to build
up a gunky surface, though. I don't know how necessary it
actually is, or whether she's tested the clay without it.)
The recent thread on grog and the tiny star cracks that it
causes leads me to suspect that these cracks are a source
of necessary porosity in otherwise fairly well-vitrified
bodies. Anyone else have ideas on this?
-Snail
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