David Hendley on sun 16 feb 03
Paul, you need to learn how to test the vitrification of your clay.
You do this by making test bars and firing them. Full instructions
have bee posted to Clayart several times by Ron Roy, and you
can also find instructions in several pottery textbooks.
Just judging from my experiences, I don't think you will ever
be able to use more than 50% of your earthenware clay in a stoneware
body, if that much, but you need to test. (The numbers for your
test do not add up to 100%).
Also, if the clay is already plenty plastic, I suggest you use only
fireclay and kaolin as additives, as they are more refractory than
ball clay, so their addition will raise the firing temperature more.
David Hendley
Maydelle, Texas
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> i have done some work with local clay that i dig from a pond and adding
> varied amounts of fireclay, ball clay and epk, then firing to cone 11. The
> clay was a green color when i dug it but when bisque fired it looks like
> common red earthenware. So far the clay body has held up but i have no
real
> formula to go by, and no idea if the body is overfired. The most recent
> batch was about 50% earthenware, 30% fireclay, 20% ball clay and 10% epk.
I
> think it throws better than the commercial clay i use. Does anyone have
> experience doing this, and if so, can you offer any basic formula for how
> to raise this type of common clay up to cone 10 - 11, so that it is not
> over-vitrified? Thanks,
> Paul B
Paul on sun 16 feb 03
Hello,
i have done some work with local clay that i dig from a pond and adding
varied amounts of fireclay, ball clay and epk, then firing to cone 11. The
clay was a green color when i dug it but when bisque fired it looks like
common red earthenware. So far the clay body has held up but i have no real
formula to go by, and no idea if the body is overfired. The most recent
batch was about 50% earthenware, 30% fireclay, 20% ball clay and 10% epk. I
think it throws better than the commercial clay i use. Does anyone have
experience doing this, and if so, can you offer any basic formula for how
to raise this type of common clay up to cone 10 - 11, so that it is not
over-vitrified? Thanks,
Paul B
Snail Scott on sun 16 feb 03
At 12:14 PM 2/16/03 -0500, you wrote:
>...i have done some work with local clay that i dig from a pond and adding
>varied amounts of fireclay, ball clay and epk, then firing to cone 11...i
have no real
>formula to go by, and no idea if the body is overfired...
Native clays vary widely, so I suspect that
anyone else's rule of thumb may be inapplicable
to you. But, if it hasn't bloated or slumped at
^11, it's likely not overvitrified. You could
do the basic absorption test for more info, but
how it behaves in the firing of your actual work
should probably be the defining characteristic.
-Snail
Ron Roy on mon 17 feb 03
All my experience says this is not going to work - mainly because of all
the cristobalite that will be generated.
Low fired iron bearing clays are not the right way to help melt clay at
high temperature - what you need is KNaO from feldspar to gobble up the
cristobalite as it is generated.
Iron turns into a flux in reduction - you did not say oxidation or
reduction by the way. Reduction firing of clays with lots of iron has to be
one of the most difficult problems to solve - The reduced iron fluxes so
you cannot have the necessary KNaO there to control the cristobalite - and
- to make maters worse - most low fire clays have MgO and CaO - which again
are not effective against cristobalite.
Anyway - good luck - if you need instructions on how to test for shrinkage
and absorbency - let me know.
RR
>Hello,
>i have done some work with local clay that i dig from a pond and adding
>varied amounts of fireclay, ball clay and epk, then firing to cone 11. The
>clay was a green color when i dug it but when bisque fired it looks like
>common red earthenware. So far the clay body has held up but i have no real
>formula to go by, and no idea if the body is overfired. The most recent
>batch was about 50% earthenware, 30% fireclay, 20% ball clay and 10% epk. I
>think it throws better than the commercial clay i use. Does anyone have
>experience doing this, and if so, can you offer any basic formula for how
>to raise this type of common clay up to cone 10 - 11, so that it is not
>over-vitrified? Thanks,
>Paul B
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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