Linda Arbuckle on wed 5 mar 97
In response to Beverley's comment that lowfire clays should be bisqued to 04
then glaze fired to 06, I'd say not always so. Lowfire whiteware bodies,
like those for slipcasting, may be more open than terracotta bodies, and
I've know potters using plastic lowfire whiteware clay who bisqued higher
than the glaze firing, or just as high.
Those bodies may be something like 60 ball clay, 40 talc, and firing higher
helps get rid of gassing from dissociating materials without over-bisquing
the clay. Ball clay matures around cone 10, so the high amount of talc is
needed to flux it.
However, many lowfire bodies, esp. some of the commercial terracottas, are
really tight and so mature at cone 04 that it's hard to get a good glaze
coat. Earthenware clays the bodies are based on mature lower than white
clays, and many are formulated to be relatively mature for use as tableware.
I find that if I bisque as high as 05 I may have real problems with getting
edges and thin cross sections to accept glaze. So... like so much else in
clay... it all depends on the clay body you use. I'd suggest using a test
kiln if you have access and trying a higher bisque before committing to a
whole kiln load.
Linda
Linda Arbuckle
Associate Professor, Graduate Coordinator
University of Florida, Box 115801, Gainesville, FL 32611-5801
e-mail: Linda Arbuckle@ufl.edu
Tony Hansen on thu 6 mar 97
> Those bodies may be something like 60 ball clay, 40 talc...
> Ball clay matures around cone 10, so the high amount of talc is
> needed to flux it.
My experience is that cone 06-04 low fire white bodies have a porosity of
10-14% or more (stoneware is 2%) so there is very little fluxing going on.
Fire a typical cone 10 body at cone 04 and you'll find it is also 10-15%
porosity, no better or worse. The talc is added for better glaze fit, although
last month I had my ears clipped on this one so I'm not as sure as I used to
be. At least that's why most manufacturers think they are adding it.
> However, many lowfire red bodies, are really tight and so mature at cone 04
> that it's hard to get a good glaze coat.
It is true that terra cotta clays are more vitreous, but they are still in the
range of 8% porosity and even at cone 2 still higher than most stonewares.
They are still quite strong however. Redart, the most well known terra cotta
clay in North America, has a porosity of 9% at cone 04 according to the
manufacturer. Working in low fire means accepting porous ware, I know of know
way to make a workable body from Redart that will come anywhere close to
vitrifying at cone 04?
Yet I've heard enough people say it can be done? Does anyone know how?
--
=================================================================
Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
http://digitalfire.com or http://www.ceramicsoftware.com
Gavin Stairs on fri 7 mar 97
Hi All,
It is my understanding that ALL mature bodies consist of a glassy
near-eutectic phase composed of all the fluxes, most of the silica and a
portion of the alumina, and a crystaline phase consisting of the rest of the
silica and alumina. Actually, the crystals may be refractory silicates and
aluminates of many kinds. The dissolved silica causes the melted glass to
be viscous, and the undissolved crystals provide the structural framework,
like grog. This is the classic blood, muscle and bone of the chinese
porcelain potters.
In an open, low-fire body, the amount of glass is insufficient to fill the
voids between the grains of refractory. However, there is lots of flux
around. So you can't fire it hot, or the flux will dissolve more of the
refractory than you want, and the ware will be glassy and may slump. No
bone left. In this flux-rich body the flux overpowers the silica, making
the glass too runny to support anything. What you have is almost a glaze,
like Albany Slip. So you have to underfire it in order to leave lots of
bone (un-dissolved silica and alumina rich phases). This body will "mature"
at low heat, but will have a structure very different from a mature, high
fire ware.
In the case of high fire, the amount of flux is much less, and the glassy
phase is mostly silica. At around cone10, silica glass is still pretty
viscous, and it won't let the refractory particles flow over eachother
easily. So the body can have much more glassy phase, and it can close up
the pores with glass. In the extreme, this gives the classic translucent
porcelain. But a porcelain with too much flux will have a tendency to
slump, because of all the glass. With extra flux, the glass turns runny.
No muscle.
If you could find a viscous glass at cone 04, you could make a cone04
porcelain. I don't know of any. As for turning Redart into a workable body
vitrifying at cone04, it just means adding fluxes to an already flux rich
clay (iron). But with that much flux in it, it would be quite touchy about
firing temperature, I'd guess.
I'm interested that Tony says that most manufacturers add talc for glaze
fit. I've been trying to understand low-fire glazes, and it does seem that
the main problem is to keep the Cte low with all the flux in there.
Especially the NaKO's. So you have to add Boron, with all its attendant
problems. The high solubility leads to frits, and there you are. Expensive
glaze. So add talc to the body, and reduce the Boron in the glaze. Makes
sense if you don't intend to make kitchenware. So you get a good school
body, for practicing and economical firing. Makes sense.
I want to have a look at some of these questions with an electron
microscope. Any suggestions?
Gavin
Tony Hansen on sat 8 mar 97
> If you could find a viscous glass at cone 04, you could make a cone 04
> porcelain.
I've tried using boron frits. I can't do it. The plasticity was long
gone
by the time there was enough frit. It reminds me of dental porcelain. It
fires very low (cone 015?) but has absolutely no plasticity, it's like
adding
water to a frit and trying to form it. Really weird. I've used the stuff
and
it is beyond me how they can form teeth out of the stuff.
> As for turning Redart into a workable body
> vitrifying at cone 04, it just means adding fluxes to an already flux rich
> clay (iron). But with that much flux in it, it would be quite touchy about
> firing temperature, I'd guess.
Again, by the time you got enough frit into it there would be no
plasticity left.
--
=================================================================
Tony Hansen, IMC - Get INSIGHT 5 beta or The Magic of Fire II at
http://digitalfire.com or http://www.ceramicsoftware.com
Billie & Beverly Cohen on wed 12 mar 97
>----------------------------Original message----------------------------
>In response to Beverley's comment that lowfire clays should be bisqued to 04
>then glaze fired to 06, I'd say not always so. Lowfire whiteware bodies,
>like those for slipcasting, may be more open than terracotta bodies, and
>I've know potters using plastic lowfire whiteware clay who bisqued higher
>than the glaze firing, or just as high.
Dear Linda,
Thank you so much for this response. One of the reasons I love this
list is because of the knowledge that is contained here. Obviously, I
was spouting what I had learned, and I expect that within the confines
of what I learned, I was correct. But there are always perimeter
areas that contain other information, and this is one of them. As a
ceramist, I want to gain as much knowledge of my craft as possible,
and it's only because of people like you who are willing to pass on
your own knowledge that people like me can continue to grow.
You brought up information here that pulled me up short and caused me
to investigate further the differences in the clays and slips that I
wasn't aware of before. Thanks so much. :-)
Beverly
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