Gwyn Ace on tue 28 aug 01
Further to Paul's useful comments on the differences between Ball, Clay =
and Kaolin ...I have found that substitution of Ball clay for Kaolin in =
a glaze recipe usually lowers the firing temp by about 1 cone and the =
glaze is better suspended and 'creamier'...it is also more suitable for =
raw glazing...
The increased shrinkage will probably be no more than the addition of =
Bentonite as a suspending agent....but the question of the adhesion of =
the dry glaze to the pot surface...must always be watched of course.. I =
have two Ball Clays and one sample from Australia has a very low Iron =
content..but it is still enough to add a creamish tone to a white glaze. =
The Colour of the clay powder will normally give a strong indication of =
the Iron content and the purest sample will be almost pure white.
Good Potting GWYN in N.Z.
Reid Harvey on wed 29 aug 01
Friends on Clayart,
I have a feeling I'm about to hit squarely the issue of which clays to
use in some particular glaze here in Bangladesh. Raw materials are
pretty basic, not so much choice as in the U.S. or Europe, Australia or
N.Z. I'm about to begin training the teachers at the Bangladesh
Institute of Glass and Ceramics. There's plenty of sand and red clay
here, and a little high fire clay, but there isn't a single rock of any
kind within a couple of hundred miles. What some call feldspar they get
by crushing boulders they bring from the way north, ending up with
something that may be about 1/3 real feldspar and 2/3 unknown material.
Oh well, I think I'll work on frit production.
At the institute they'll decide within a few days what they want the
instruction to focus on, but I'm advising they lump it all in under the
broad heading of 'appropriate technology.' When I was teaching in
Africa we used a glaze that was (if I remember correctly) 60% bottle
glass, 20% kaolin and 20% fish bone ash. It fired to ^8 to a beautiful
opaque matt. I like this because it's practically a universal glaze,
something that can be put together almost anywhere. And as Gwyn said you
can just vary the clay to go up and down temperature-wise. This could
even be easier than the frit, provided everybody wants to fire at about
^4 and up. But I've got this stubborn notion that there should be more
to production than what comes out of the big factories. Bangladesh
excels at making fine china, but there's not much done with glazes by
artisans/ small producers. There is an unfortunate gap for ceramics, a
few high end producers and those working on a more primitive scale, but
nothing being done between these extremes.
For one thing, as in so many other developing countries everybody seems
to think you need a high tech brick. Village potters make a great
product here, using beehive kilns: water storage containers, statuary,
flower pots, the list of what they make goes on and on. But with their
beehive kilns they'll never do above 600C. (A year or so ago I posted
links to a tour of a pottery village here, and promised other such
digital tours. Then my camera quit. Sorry!) At the Ceramic Institute
there's a burning desire to get lots of sophisticated machinery, but I'm
telling them that for every piece of this equipment they need a
parallel, low tech process that does basically the same job. For
example, we need to begin production of low cost refractory bricks, made
out of common sand and red clays. This is what was used in the earliest
days of the ceramic industry in the U.S., and it's use here would make
bricks affordable to aspiring graduates of the institute, or village
potters. They'll be spending about 10% of the cost of the high tech
brick. To a new graduate the option is to get swallowed up in some big
factory.
I'm going to wet my toes gradually to what the teachers want, and how I
come across to them. My Bengali is not that great, and I'm already
creating a mental scenario about runaway translation. Teaching in Sudan
several years ago I had a translator who insisted on doing one sentance,
or part of a sentance at a time, then he would go off on a tangent,
given his blossoming understanding of the subject.
Teaching will be a change for me though it's really where my heart is.
For the last three or four years I've been full time into the
development of a permeable earthenware water purifier. That effort seems
about to pay off in a big way, with first field testing in a couple of
thousand homes. We're beginning a regime of bacteriological challenge
testing, making sure pathogens are filtered out and/or killed. And we're
having excellant results in beginning production of the colloidal silver
we use to saturate these filters. This material kills any bacteria not
filtered out. (Don't get me going about the virtues of colloidal silver,
health-wise. You may never hear the end of it! Anybody want a link?)
Anyhow, pray for us in our efforts at the institute. Maybe now I can
re-focus on being a regular reader/ contributor of this great
discussion.
Reid Harvey
-----------------------------
Date: Tue, 28 Aug 2001 12:14:25 +1200
From: Gwyn Ace
Subject: clays in glazes
Further to Paul's useful comments on the differences between Ball, Clay
and Kaolin ...I have found that substitution of Ball clay for Kaolin in
a glaze recipe usually lowers the firing temp by about 1 cone and the
glaze is better suspended and 'creamier'...it is also more suitable for
raw glazing... The increased shrinkage will probably be no more than
the addition of Bentonite as a suspending agent....but the question of
the adhesion of the dry glaze to the pot surface...must always be
watched of course.. I have two Ball Clays and one sample from Australia
has a very low Iron content..but it is still enough to add a creamish
tone to a white glaze. The Colour of the clay powder will normally give
a strong indication of the Iron content and the purest sample will be
almost pure white.
Good Potting GWYN in N.Z.
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