Alison Hamilton on tue 6 jan 98
I have a question that I would very much appreciate any responses to.
I work with a cone 6, mid smooth stone clay from Tuckers. Our school
has been firing it at cone 8 (electric kiln). My questions are: how
does this affect the clay? What are the benefits or the drawbacks to
doing this?
It does not seem to negatively impact on the clay i.e. everything comes
out of the kiln just fine. I'm just wondering about the effect of it.
Thanking everyone in advance! [: - }
Alison Hamilton
DONPREY on wed 7 jan 98
Alison,
A critical point here...when you say the clay is fired to cone 8 do you mean:
A. a jr. cone 8 in the kiln sitter or,
B. a large cone 8 on the kiln shelf (witness cone).
There is frequently (if not always) a one to two cone difference between the
sitter cone and the witness cone. In my experience, when a Dawson kiln sitter
is adjusted per the manufacturer's instruction a cone 8 in the sitter will
result in a cone 6 to 7 firing per the witness cones. The sitter is really a
safety device, and should not be depended upon to indicate glaze/clay
maturity.
Don Prey in Oregon
Ron Roy on thu 8 jan 98
Hi Allison,
Well I guess I am the right person to answer this one - I look after the
clays at Tuckers - my job is to make sure the shrinkage, absorption,
working qualities, colour, expansion - well everything - stay within
reasonable parameters. I try to keep the average absorption of this clay to
about 1% and the overall shrinkage about 12% at cone 6. If you fire it to
cone 8 (using a large cone 8) then I expect the average absorption to be
-none- and the overall shrinkage to be 12.5 to 13%. Our bodies are designed
to work at a single specific cone so there is some risk - in this case -
because of the natural variability in our raw materials - in firing to a
higher cone. Bowls may slump - especially if trimmed unevenly, ware will
warp easily of fired on uneven shelves, there may be some breakdown of the
body due to overfiring - I say maybe because there is some built in safe
guard for overfiring but two cones above is pressing your luck.
One of the main problems of firing a body to it's upper limit is having
brittle ware - It will be more easily broken because glassified clay cannot
absorb knocks as well as clay with some "give" to it.
If you are using a sitter and the small cone you are using is cone 8 then
you are probably only going to cone 7 - not as close to the edge.
By the way - if you have the batch # off the box Tuckers will be happy to
provide the test results for absorption and shrinkage for any batch of
clay. You can even see a fired bar of that batch or all the fired bars of
any clay going back for months and, in some cases, years.
>----------------------------Original message----------------------------
>I have a question that I would very much appreciate any responses to.
>
>I work with a cone 6, mid smooth stone clay from Tuckers. Our school
>has been firing it at cone 8 (electric kiln). My questions are: how
>does this affect the clay? What are the benefits or the drawbacks to
>doing this?
>
>It does not seem to negatively impact on the clay i.e. everything comes
>out of the kiln just fine. I'm just wondering about the effect of it.
>
>Thanking everyone in advance! [: - }
>
>Alison Hamilton
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Alison Hamilton on thu 8 jan 98
Don,
Thanks for responding to my enquiry. The kiln now has a computer
attached to it, so it's definitely firing to cone 8 ( I assume, since
it's being programmed to turn off at cone 8).
Since reading the clayart mail, I began wondering whether one of the
benefits of firing a cone 6 clay to cone 8, is that you then get to use
higher fire glazes which may be somewhat safer (or have amounts of
silica and something else that make higher glazes easier to work with?
I'm a little vague on this even tho this was just covered on the forum
since I'm still trying to figure out the throwing stuff! I confess the
glaze chemistry mail tends to make my eyes "glaze" over because I'm just
not ready for this kind of detailed info)
Would firing it higher vitrify the clay more and is that better?
Thanks for your help,
Alison
DONPREY wrote:
> ----------------------------Original
> message----------------------------
> Alison,
> A critical point here...when you say the clay is fired to cone 8 do
> you mean:
> A. a jr. cone 8 in the kiln sitter or,
> B. a large cone 8 on the kiln shelf (witness cone).
> There is frequently (if not always) a one to two cone difference
> between the
> sitter cone and the witness cone. In my experience, when a Dawson
> kiln sitter
> is adjusted per the manufacturer's instruction a cone 8 in the sitter
> will
> result in a cone 6 to 7 firing per the witness cones. The sitter is
> really a
> safety device, and should not be depended upon to indicate glaze/clay
> maturity.
> Don Prey in Oregon
DONPREY on fri 9 jan 98
Alison,
Be sure to see Ron's answer to your clay question. He has the definitive
answer. As to the kiln with the programmer attached....I would still verify
the whole setup by placing some cones (probably 6, 7 and 8) in a couple of
levels of the kiln. Only the cones give the definitive answer as to what the
clay/glaze are experiencing in any particular firing.
Don Prey in Oregon
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