Sumi von Dassow on tue 4 oct 11
Vince
So I guess the consensus is that earthenware and low-fire clay are not
synonymous terms. It all makes sense now.
Sumi
> I have missed most of this discussion, but I agree with Rimas on this. I=
f
> you use the term "earthenware" in reference to utilitarian clay, you are
> referring to a porous claybody such as is used in traditional maiolica.
> Now, of course it is possible to load a low-fire claybody with glassy fri=
ts
> and get what used to be called "soft-paste porcelain" or something of the
> kind - a dense and glassy body that will in essence be vitrified after a
> cone 04 firing, but look what you really have. In order to lower the
> maturation temperature of any claybody or glaze you have to increase the
> content of fluxes, and that inherently produces a weaker, more-brittle
> glass. That's just the truth of ceramic chemistry and there is no way aro=
und
> it. Of course some fluxes are stronger than others, and terra cotta has
> been used so widely as low-fire utilitarian ware specifically because iro=
n
> and silica form a strong glassy-phase at pretty low temperatures, but if =
you
> over-fire it, the clay will bloat long before it reaches anything
> approaching vitrification. Any claybody that relies on a high concentrati=
on
> of glassy frits to achieve something close to vitrification at low-fire
> temperatures cannot be considered a practical utilitarian claybody becaus=
e
> it will be weak and brittle.
>
> It is just important to call things as they are. "Earthenware" by
> definition is never vitreous. It is possible to have a low-fire claybody
> that is vitreous, but again, it will be weak and brittle, and it is not
> earthenware.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net
> http://iweb.tntech.edu/wpitelka/
>
>
>
--
Sumi von Dassow
www.herwheel.com
sumi@herwheel.com
Rimas VisGirda on tue 4 oct 11
I thought the very definition of earthenware was that it remain porous at m=
aturity. Note I said maturity, vitrification means that a material turns in=
to a glass-like substance (aka non porous). As an aside, porcelain, by defi=
nition, is "vitrified" if it has less than 1/2 % absorptivity whereas to be=
stoneware it needs be below 3%... And here I am talking about earthenware =
that comes from the earth, the red/orange stuff that people bake chickens i=
n, not the white stuff that is infused with frits to make it glasslike at 0=
4... I think there is a certain amount of confusion with terminology since =
the formulation of white clays that vitrify at earthenware (terra cotta) te=
mps, we used to make a low-fire clay by mixing talc and kaolin in equal pro=
portions... I think ethnic chickens probably got baked in red clays that we=
re dug up from the earth not far from where the chickens were raised. I hav=
e a hunch that the ethnic pots work the way they work because of the
porosity of the earthenware. So, given that, a high fire clay taken to bis=
que (so that it's porous) might work just as well. But then again, I don't =
bake chickens in pots so I could very well be wrong... Regards, -Rimas
Vince Pitelka on tue 4 oct 11
I have missed most of this discussion, but I agree with Rimas on this. If
you use the term "earthenware" in reference to utilitarian clay, you are
referring to a porous claybody such as is used in traditional maiolica.
Now, of course it is possible to load a low-fire claybody with glassy frits
and get what used to be called "soft-paste porcelain" or something of the
kind - a dense and glassy body that will in essence be vitrified after a
cone 04 firing, but look what you really have. In order to lower the
maturation temperature of any claybody or glaze you have to increase the
content of fluxes, and that inherently produces a weaker, more-brittle
glass. That's just the truth of ceramic chemistry and there is no way aroun=
d
it. Of course some fluxes are stronger than others, and terra cotta has
been used so widely as low-fire utilitarian ware specifically because iron
and silica form a strong glassy-phase at pretty low temperatures, but if yo=
u
over-fire it, the clay will bloat long before it reaches anything
approaching vitrification. Any claybody that relies on a high concentration
of glassy frits to achieve something close to vitrification at low-fire
temperatures cannot be considered a practical utilitarian claybody because
it will be weak and brittle.
It is just important to call things as they are. "Earthenware" by
definition is never vitreous. It is possible to have a low-fire claybody
that is vitreous, but again, it will be weak and brittle, and it is not
earthenware.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
Snail Scott on wed 5 oct 11
On Oct 4, 2011, at 6:05 PM, Rimas VisGirda wrote:
> I thought the very definition of earthenware was that it remain porous =
=3D
at maturity.
Yes, I agree. Maturity is not necessarily vitrification!=3D20
I have noticed that a high-fire clay that is fired to a=3D20
lower temp is noticeably weaker and 'punkier' than=3D20
a clay body that actually matures at that temperature.=3D20
I suppose we could still call the product 'earthenware',=3D20
though, even if it's not made from an earthenware clay=3D20
body. I suspect a lot of historical claywork falls into that=3D20
category of undermatured, nominal 'earthenware'.
-Snail
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