Ann Brink on fri 23 feb 07
Hello Craig,
You sound like the right person to answer my question if you have time: I
was given about 10 lbs each of Georgia Kaolin and Florida Kaolin. Should I
use them for anything special, or just use them as I would EPK?
Thanks,
Ann Brink in Lompoc CA
(mostly about pottery)
----- Original Message -----
From: "Craig Martell"
................Just about all the kaolins we use that come from
> the US are Secondary Kaolins. The very clean, white, "Primary" kaolins
> that are available to us come from Cornwall in the UK. There were some
> primary kaolins in North Carolina, I think, but I don't believe any are on
> the market now.
Craig Martell on fri 23 feb 07
Hi:
Stephanie was asking about kaolin and ball clays in glazes and her ?s have
pretty much been answered but I thought I'd offer a couple of things just
for the sake of typing practice, if nothing else. 8>)
Ball clays are mainly kaolin based clays. They differ in particle size and
mineral impurities and carbonaceous stuff due to weathering and transport
from their original weathering site. Most of the US kaolins go thru the
same scenario but to a lesser degree. One term that describes this is
"secondary". Secondary clays have been traveling via water mostly and pick
up a lotta "other stuff". Just about all the kaolins we use that come from
the US are Secondary Kaolins. The very clean, white, "Primary" kaolins
that are available to us come from Cornwall in the UK. There were some
primary kaolins in North Carolina, I think, but I don't believe any are on
the market now. So it goes. EPK is the whitest US kaolin but it's still a
secondary clay.
One thing you can do with regard to analyzing kaolins and ball clays is to
look at the silica/alumina ratio. The ideal ratio for kaolin is 2/1, or
twice as much silica. You can calculate this very easily by dividing the
silica by the alumina from the clay's percentage analysis, and then
multiplying the result by 1.7 which will give you the Si/Al ratio. 1.7 is
the correction factor that takes into account the difference in molecular
weight of the two materials. The molecular wt. of Al is 102 and Si is 60.1.
When comparing the Si/Al ratios of kaolin and ball clay, you will see that
ball clays are much higher in Silica. Some are off the map, as in 11/1,
but most are not that high. I made a body that had Kentucky Stone and it
is so high in silica that I didn't have to add any quartz for glaze
fit. Thus, more clay in the body and it threw very well. So it went!
Jon was asking too if Albany could be classed as a Ball Clay. Well, that's
what I think he was saying. Anyway, the answer is that it's not a ball
clay. It's classed as an Illite type clay, which is a clay mica. Albany
is mainly very ancient shale that's been floated off by the Hudson
River. It also has some limestone and dolomite from ancient weathered down
mountains in the region. A very complex material. I still have a barrel
of the stuff.
That's it for me.
regards, Craig Martell Hopewell, Oregon
Craig Martell on sat 24 feb 07
Ann was saying:
> I was given about 10 lbs each of Georgia Kaolin and Florida Kaolin. Should I
>use them for anything special, or just use them as I would EPK?
Hello Ann:
It depends, as usual. The Florida kaolin may very well be EPK which you
know will work in bodies and glazes. The Georgia kaolin is another matter
because there are so many variations. Do you know what the Geo kaolins are
in terms of a specific type such as 6 Tile, Sapphire, Kaopaque 20? I could
give you a better answer if I knew which Geo kaolin you had.
6 tile and Sapphire will work in a glaze but they are very plastic kaolins
and are mainly used in a claybody. Anyway, my choice is to use Geo kaolins
in bodies. I have one that I use now with Sapphire and Kingsley kaolin and
they make a great throwing, handbuilding body. Depending on the types of
glazes you make the Geo kaolins may have more TiO2 than you'd like but they
could be ok too for glazes with more color etc. They ain't that great for
celadons as far as I'm concerned. Do you do any vapor glazing as in salt
or soda? Some of the Geo kaolins make very nice slips for a vapor kiln.
A good thing to do is just fire little test mice and look at the color and
then try some glaze tests, slip tests, or claybodies and see what you like.
regards, Craig Martell Hopewell, Oregon
Ann Brink on sat 24 feb 07
Hello Craig,...Thanks a lot for the response (below) . It's helpful to know
about the Florida kaolin probably being EPK. The bag of Georgia kaolin has
no other description, so I will do some testing with it, as you suggest.. I
don't do vapor firing, but I will test to see how it acts with a soda ash
wash compared to a slip using epk.
Ann Brink in Lompoc CA
----- Original Message -----
From: "Craig Martell"
To:
Sent: Saturday, February 24, 2007 9:33 AM
Subject: Re: kaolin/ball clay/albany slip
> Ann was saying:
>> I was given about 10 lbs each of Georgia Kaolin and Florida Kaolin.
>> Should I
>>use them for anything special, or just use them as I would EPK?
>
> Hello Ann:
>
> It depends, as usual. The Florida kaolin may very well be EPK which you
> know will work in bodies and glazes. The Georgia kaolin is another matter
> because there are so many variations. Do you know what the Geo kaolins
> are
> in terms of a specific type such as 6 Tile, Sapphire, Kaopaque 20? I
> could
> give you a better answer if I knew which Geo kaolin you had.
>
> 6 tile and Sapphire will work in a glaze but they are very plastic kaolins
> and are mainly used in a claybody. Anyway, my choice is to use Geo
> kaolins
> in bodies. I have one that I use now with Sapphire and Kingsley kaolin
> and
> they make a great throwing, handbuilding body. Depending on the types of
> glazes you make the Geo kaolins may have more TiO2 than you'd like but
> they
> could be ok too for glazes with more color etc. They ain't that great for
> celadons as far as I'm concerned. Do you do any vapor glazing as in salt
> or soda? Some of the Geo kaolins make very nice slips for a vapor kiln.
>
> A good thing to do is just fire little test mice and look at the color and
> then try some glaze tests, slip tests, or claybodies and see what you
> like.
>
> regards, Craig Martell Hopewell, Oregon
>
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