pat lester on sat 28 jul 01
Has anyone experienced glaze firing problems with Potclays St Thomas'
Reduction Body 1102, bought prior to February 2001? We bought a
tonne in July 1998
The problem is severe pimpling on the clay surface when fired in a
reduction atmosphere. Mostly the clay has been fired in a 13 cu ft
Lazer gas kiln, biscuit fired to 1000 degrees C. on the pyro, glaze
fired at a rate of 2 degrees c/min using standard Orton cones. I
have used the clay to make plant pots & planters up to 24" high x
22"wide, casseroles, pie dishes and vases.
At cone 8 reduction; severe pimpling, with the pimples bursting to
look like miniature volcanoes on both the interior and exterior faces
of all work, rendering any domestic ware totally useless.
At cone 7 reduction, severe pimpling but no bursting
At cone 6 reduction, still pimpling though nowhere near so badly.
In oxidised firings the pimpling does not occur, though maintaining a
clear oxidised atmosphere in a gas kiln has its own difficulties,
with the slightest touch of reduction producing the pimpling.
As a pottery we use earthenware, porcelain and stoneware clays, the
stoneware & porcelain being fired to cone 9-10 reduction, so to come
up against this problem with the St Thomas' has created real
difficulties. For three years I have been working on this, finding &
adapting glaze recipes, adjusting firing schedules & temperatures
etc. not easy to conduct special test firings when you need to fill a
kiln and are trying to make a living.
In my talks with Potclays technical directors it would appear there
is a granular iron (Spegmite) in the clay which produces the
speckling in the fired clay body. I have felt all along that this is
where the problem lies. They say they have heard from no-one else
with the same problem. Has anybody else found problems with the
clay or is it just me? I would be very interested to hear. I
propose to send Potclays a pimpled sample for them to examine.
Interestingly, Potclays have changed the recipe for St Thomas' body
which is now being sold as 1102(m), M is for modified . Having used
a bag it gives a completely different fired colour, very heavy iron
speckles but does not pimple.
Jonathan Hancock
Steam Pottery, Pendeen. Cornwall England
--
Iveragh Ceramics on sun 29 jul 01
----- Original Message -----
From: "pat lester"
To:
Sent: 28 July 2001 18:18
Subject: potclays St Thomas body
> Has anyone experienced glaze firing problems with Potclays St Thomas'
> Reduction Body 1102, bought prior to February 2001? We bought a
> tonne in July 1998
Jonathan,
This problem with Potclays has been ongoing for years and
they have reams of complaints. The subject was raised here on Clayart a
couple of years ago by Ailsa in Denmark. She stated that she bought a clay
body imported from the U.K. , but did not name the company; lots of us on
Clayart did that,and guess what! We all came up with the same name
"Potclays".
I mistrust and will not deal with any company which always starts of their
correspondence to me with the words "without prejudice" (this means the
letter CANNOT be used in any court proceedings), Potclays always use this
wording. You live quite close to one of the best clay companies in the
world, IMNSHO, perhaps you should talk with them, Dobles is the name.
Regards,
Bob Hollis.
>
> In my talks with Potclays technical directors it would appear there
> is a granular iron (Spegmite) in the clay which produces the
> speckling in the fired clay body. I have felt all along that this is
> where the problem lies. They say they have heard from no-one else
> with the same problem. Has anybody else found problems with the
> clay or is it just me? I would be very interested to hear. I
> propose to send Potclays a pimpled sample for them to examine.
>
> Interestingly, Potclays have changed the recipe for St Thomas' body
> which is now being sold as 1102(m), M is for modified . Having used
> a bag it gives a completely different fired colour, very heavy iron
> speckles but does not pimple.
>
> Jonathan Hancock
> Steam Pottery, Pendeen. Cornwall England
> --
>
>
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