Paul Herman on tue 21 mar 06
Greeting Ivor,
you wrote:
> I would be interested in reading other opinions.
This is more of a report, rather than an opinion.
We've got halloysite over here too, and I can confirm it shrinks a
lot. 10% in drying and 20% in firing, about 30% total.
It's altered tuff. Hot water from below formed it, leaching almost
everything out except alumina and silica. It qualifies as a true
primary clay, and still shows the texture of the parent rock, but is
almost totally converted to halloysite. The shrinkage makes it a
problematic material, but I'm trying to figure ways to use it. It
seems to be fine as a replacement for kaolin in glazes.
best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Mar 20, 2006, at 2:33 PM, Ivor and Olive Lewis wrote:
> Dear B J Clark,
>
> As you may have gathered from recent discussions, this clay body
> has a proportion of Halloysite Clay in its composition. I am
> uncertain about the nature of Halloysite from New Zealand but
> information shows this member of the clay family does not have the
> same crystal shape as Kaolinite derived clays. There are two
> possibilities. One is an agglomeration of fibrous crystals into
> structures that look like snowballs, another is of fibrous tubes
> which can unroll as water affects them and roll up again as they
> dry out. It is not impossible that this may be the source of your
> troubles and the effects of excessive shrinking on the larger
> structures you are making.
>
> In this region of South Australia we have a lot of problems with
> moving house foundations. This is because the soil is called Bay of
> Biscay. It has a excessive drying shrinkage and a sheet of mud will
> crack like a crazy paved patio. We suspect this is because the clay
> is a Halloysite type. the clay we get from beneath the soil is
> useless for making pottery.
>
> I would be interested in reading other opinions.
>
> Best regards,
>
> Ivor Lewis.
> Redhill,
> South Australia.
Ivor and Olive Lewis on wed 22 mar 06
Thanks Paul,
Important news for people who do not have access to all of the =
information about recently introduced raw materials.
You also make us aware that some primary clays do not have their genesis =
due to the destruction of granitic rocks.
One of my students brought in a green clay from base of the sand quarry =
owned by his family. Great plasticity. Fired to wonderful pale terra =
cotta. But the shrinkage was gargantuan, similar to the figures you =
quote. But it shortened when he added a proportion of inert fillers and =
became unthrowable. Such a pity, there were several thousand tons of the =
stuff.
Best regards,
Ivor
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