earlk on wed 29 mar 06
Hi neighbor.
I'll take a stab at some of this for you.
On Wed, 2006-03-29 at 16:26 -0800, Susan P wrote:
> !) Am I correct in my understanding that the purpose of the bisque is to
> drive off moisture - both physical and chemical?
As one heats clay up there are a whole series of
processes that take place. First would be the
evaporation of water that exists in the spaces
between the particles that make up your clay body.
Later on, at a higher temperature the organic
matter that exists in your clay (bacteria, molds,
algae, pollen and such) breaks down and "burns"
out. Later still, the minerals which compose the
clay, feldspars and other inorganics in your clay
start decomposing.
All of these processes to this point release water.
This is the stage at which one could say that clay
transforms into a ceramic.
At still higher temperatures, the minerals themselves
start fusing together in a process known as sintering.
As the temperature rises, more and more melting
occurs until eventually the ceramic doesn't have
enough strength to support itself and slumping
occurs. I suppose one could go on to a full melt,
boiling and eventually evaporation at extremely
high temperatures (like inside the sun).
Bisquing your clay is generally done to remove all
free water, burn out any organic material and
sinter the minerals together to add strength for ease
in glazing and prevent breakdown from the water
absorbed from the glaze. The material would be
considered a ceramic at this point but is still quite
absorbant. It seems most people bisque to cone 06
to 04.
> 2) Does the type of clay body dictate at which cone to bisque fire? 04, 05,
> 06?
It is more so the amount of water you want your
bisque to absorb. Higher temperature, more
fusing of minerals, less porosity.
> 3) I have some sculptural work which I have, in the past, (and I am now just
> returning to doing more ceramics now though my clay education is limited)
> and I "bisque" fired these porcelain pieces at a very low cone 22 or 18 - so
> that I could continue to carve and sand them. So now I wonder, was this
> temperature really high enough to be considered a biscuit firing or should I
> then do a second bisque before the final firing.
These pieces are probably still pretty fragile so
they need to be handled carefully. There may also
still be some materials in the clay that when heated
to a higher tempertaure will turn into a gas which
may bubble through your glaze and cause pinholing.
>
> SAGGAR [or settter?]
>
> These pieces have very uneven undulating walls and the larger ones were
> prone to slumping and collapse. From what I have read, I am beginning to
> think that it might be best if I created a saggar for firing them and
> perhaps fill it, the saggar, with vermiculite. Hence, the questions are:
>
What cone was your clay designed for? If you
used a cone 6 clay but fired to cone 10 you might
very well have gotten some slumping as the clay
could be close to melting into a pool at that
temperature. Porcelain's have a very narrow range
between fully mature and melting so they are prone
to slumping.
The more often you heat a ceramic to it's design
temperature there is more melting that occurs
within the body. Eventually you would have an
almost glass like piece that could suddenly melt
into a pool at the design temperature of your clay.
> 1) What kind of clay body is conducive to multiple firing at possible Cone
> 10 oxidation - what body names/ what constituent ingredients (not practical
> to make my own & I am in Pacific Northwest)
>
I'll leave the rest to others to answe,r but I will ask
are you committed to cone 10? If so, why?
> 2) It would need to be handbuilt so I was assuming slab built? Any
> experience on wall thickness or other construction considerations?
>
> Thank you so much.
> Susan in Edmonds
Anyone wishing to correct me on anything, feel free.
earlk...
bothell, wa, usa
Susan P on wed 29 mar 06
---------- Forwarded message ----------
From: Susan P
Date: Mar 28, 2006 12:19 PM
Subject: Bisque Firing Questions / Saggar Questions
To: clayart@lsv.ceramics.org
Thought I sent an earlier emai, but did not see it, so here goes again with
the questions - and a bit of context.
!) Am I correct in my understanding that the purpose of the bisque is to
drive off moisture - both physical and chemical?
2) Does the type of clay body dictate at which cone to bisque fire? 04, 05,
06?
3) I have some sculptural work which I have, in the past, (and I am now jus=
t
returning to doing more ceramics now though my clay education is limited)
and I "bisque" fired these porcelain pieces at a very low cone 22 or 18 - s=
o
that I could continue to carve and sand them. So now I wonder, was this
temperature really high enough to be considered a biscuit firing or should =
I
then do a second bisque before the final firing.
SAGGAR [or settter?]
These pieces have very uneven undulating walls and the larger ones were
prone to slumping and collapse. From what I have read, I am beginning to
think that it might be best if I created a saggar for firing them and
perhaps fill it, the saggar, with vermiculite. Hence, the questions are:
1) What kind of clay body is conducive to multiple firing at possible Cone
10 oxidation - what body names/ what constituent ingredients (not practical
to make my own & I am in Pacific Northwest)
2) It would need to be handbuilt so I was assuming slab built? Any
experience on wall thickness or other construction considerations?
Thank you so much.
Susan in Edmonds
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