sibylle on sat 17 jun 00
Hi to all,
I would like to experiment with red clay (iron red). But each of my glaze
(transparent or white) turns ugly brown when covering my vessel.
Now my question: is it not possible to get a real white glaze because the iron
changes all or does somebody know what to do? I don't want to cover with slip,
that works but does not please me. Later I also would try to mix with oxides or
stains.
As I am not very firm in glaze chemistry I would like to understand what happens
to a glaze on red clay, why comes up this brown. Any help or info will be welcome.
Sibylle
(germany)
Martin Howard on sun 18 jun 00
Sybille, your glazes are too clear. Therefore the brown of the red clay
shows through.
If you use tin oxide, zirconium oxide or overdose the glaze with calcium,
then you will make an opaque glaze, which can be (almost) any colour you
want. By overdose I mean add raw materials into the glaze so that the
calcium element is way above the Cushing limits for your cones. Then the
calcium carbonate or oxide will be used to make the glaze opaque. If there
are no other colouring agents in it, it will be white.
If your glaze is too thin, then the zone of action between the raw clay and
the glaze will not have sufficient cover. You may need to thicken the glaze
by taking out some water.
Post up what cone you fire at and what your available raw materials are for
the glaze. I am sure one of us will be able to then help you further. What
is your glaze recipe or are you using a bought glaze?
Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
England
martin@webbscottage.co.uk
GURUSHAKTI@AOL.COM on sun 18 jun 00
Sibylle,
Try a white slip or engobe under your white glaze. That should help. The iron
of the red body is the cause of the darkening and the slip/engobe will act as
a buffer of sorts.
Regards,
June
Marcia Selsor on sun 18 jun 00
Dear Sibylle,
This is the precise question that began the quest by the Persians to
develop tin-glaze to become known as majolica and faience.
If you are doing low fire earthenware, your answer is to begin using
majolica. There are many good books about it or go to the Ceramics web
and look for Linda Arbuckle's site. She has much information on the web.
I have been doing some majolica and went to a conference in Toronto
about it. It has a long history and is has been is seduction of many.
Italian renaissance pottery used this glaze technique for those
historical painted platters. The lusterware of Spain was put on tin
glaze in a third firing. You may become "seduced by color" -that was the
name of the conference.
Bets wishes.
Marcia in Montana
sibylle wrote:
>
> Hi to all,
>
> I would like to experiment with red clay (iron red). But each of my glaze
> (transparent or white) turns ugly brown when covering my vessel.
> Now my question: is it not possible to get a real white glaze because the iron
> changes all or does somebody know what to do? I don't want to cover with slip,
> that works but does not please me. Later I also would try to mix with oxides or
> stains.
> As I am not very firm in glaze chemistry I would like to understand what happens
> to a glaze on red clay, why comes up this brown. Any help or info will be welcome.
>
> Sibylle
> (germany)
>
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--
Marcia Selsor
selsor@imt.net
http://www.imt.net/~mjbmls
http://www.imt.net/~mjbmls/spain99.html
http://www.silverhawk.com/ex99/selsor/welcome.html
Cindy Strnad on sun 18 jun 00
Sibylle,
I've fired white glaze on iron red clay and not had the problems you speak
of. I think it might help to know what cone temp you're firing to, and what
sort of finished effect you want. Do you only want white-glazed redware, or
would you like to paint on the surface, or decorate it in some other way? It
would be my natural assumption that you're firing to a low temp--^06 or so,
but as you don't say, I'm not certain, as it is possible to find red bodies
suitable to higher temperatures.
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
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