Helvi Abatiell on wed 5 apr 00
Okay, I've gotten many great responses to my earlier questions (thank you
all) regarding low and high fire glaze temps but I have one more...
What happens if you don't fire a clay to the temp suggested? What
happens to a high fire clay that is fired less then that? Does it not
vitrify? Does it contaminate if fired with a low fire glaze? Will glazes
not adheer properly? Why not?
I'm sure all these questions have been answered before. I'm hoping there
are others out there with similar questions and I'm not just feeling foolish
for asking.
Thanks for your help, you wise potting souls, you.
Helvi :)
Dave Finkelnburg on thu 6 apr 00
Helvi,
Really you are asking very good questions.
In general, you are in control of this process. If you don't fire the
clay high enough, it isn't vitrified as much as it could be. If it's going
to be a sculpture, outdoors in freezing weather, that could mean it will
absorb moisture, freeze and fall apart. That's usually not the case, of
course, but you can see why you'd want to consider the use in deciding how
high to fire a clay. If you are using stoneware for vases or pitchers or
mugs, you also want the clay vitrified, to about less than 3% absorbance.
Otherwise they will leak out the bottom. Yes, the moisture will come
through the glaze and the clay and you'll have a wet spot which can damage a
wood table! So for functional work, there is an issue. Strength is also
improved by proper firing. However, beyond these considerations, it's
pretty well up to you.
I don't think there's been much of a discussion of how underfiring
affects clay expansion and glaze fit, but I think it does make a difference.
On the other hand, underfired clay seems to be more resistant to thermal
shock, so ovenware is commonly an underfired body.
You have asked a very good question!
Good potting!
Dave Finkelnburg in sunny, springtime Idaho
-----Original Message-----
From: Helvi Abatiell
To: CLAYART@LSV.UKY.EDU
Date: Wednesday, April 05, 2000 2:12 PM
Subject: High fire vs. Low?
>----------------------------Original message----------------------------
>Okay, I've gotten many great responses to my earlier questions (thank you
>all) regarding low and high fire glaze temps but I have one more...
> What happens if you don't fire a clay to the temp suggested? What
>happens to a high fire clay that is fired less then that? Does it not
>vitrify? Does it contaminate if fired with a low fire glaze? Will glazes
>not adheer properly? Why not?
> I'm sure all these questions have been answered before. I'm hoping
there
>are others out there with similar questions and I'm not just feeling
foolish
>for asking.
> Thanks for your help, you wise potting souls, you.
> Helvi :)
>
Smart on fri 7 apr 00
------------------
Dear Helvi
A clay can be fired at all temperatures which are possible. There are clays
used for earthenware (1050=B0C-1100=B0C) and the same clays used for =
stoneware
ou vitreous china at hight temperature. The properties of the clay depend on
the firing conditions (temperature and time of firing). If you fire a clay
with a low temperature its thermal coef of expansion (TEC) will be a certain
value and the body will be porous, if you increase the firing temperature
TEC grows up and porosity decrease but then after a limit of temperature TEC
falls down and goes on to fall after to a very low limit. At this limit your
clay will be vitrified with no porosity like stoneware.
Mecanical properties are reversed compared to porosity.
You can used a clay at each temperature you like, you have just to choice a
glaze according to the TEC you get.
I hope to help you, my english is not perfect but I try to explain as
possible my experience.
I have a french ceramic site on : http://perso.wanadoo.fr/smart2000/
all is written in french, sorry if you don't understand french, but there is
also pictures...
You can aks me all sort of ceramic questions
Have a good time with ceramics, good fire
Smart
----- Original Message -----
From: Helvi Abatiell =3CCreatvSpce=40aol.com=3E
To: =3CCLAYART=40LSV.UKY.EDU=3E
Sent: Wednesday, April 05, 2000 10:06 PM
Subject: High fire vs. Low?
=3E ----------------------------Original message----------------------------
=3E Okay, I've gotten many great responses to my earlier questions (thank =
you
=3E all) regarding low and high fire glaze temps but I have one more...
=3E What happens if you don't fire a clay to the temp suggested? What
=3E happens to a high fire clay that is fired less then that? Does it not
=3E vitrify? Does it contaminate if fired with a low fire glaze? Will =
glazes
=3E not adheer properly? Why not?
=3E I'm sure all these questions have been answered before. I'm hoping
there
=3E are others out there with similar questions and I'm not just feeling
foolish
=3E for asking.
=3E Thanks for your help, you wise potting souls, you.
=3E Helvi :)
Cindy Strnad on fri 7 apr 00
Helvi,
When you fire high-fire clay to a lower temperature, it does not vitrify.
This means that it will take in water or whatever liquid it comes in contact
with. For functional work, it will not serve the purpose of stoneware and
may also tend to get moldy if kept in a humid environment.
Low fire glazes are not formulated to fit underfired high fire clay. This
doesn't mean you can't use them for non-functional (and I mean that word in
the non-utilitarian sense) work. You can do anything you like, as long as it
gives you the results you're looking for. The glazes will adhere. They're
just not likely to fit.
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
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