David Hendley on wed 17 dec 03
Generally, it is better to have many different clays in a claybody.
Two reasons: one, it gives you a wide variety of particle sizes
and characteristics, and two, if (not if, when) one clay changes
from batch to batch it will not effect your claybody as much
because you are not using a high percentage of any one clay.
I have found Goldart to be particularly variable. Kaolins, on the
other had, tend to be the most consistent from batch to batch. If
I were you I would certainly want to add the kaolin. I use EPK
kaolin in my claybody. Of course, adding another ingredient makes
buying, storing, and mixing you claybody more trouble.
You will also lose some plasticity.
You decide if it is worth it.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> Aside from whiteness, is there any particular reason for adding kaolin to
a
> stoneware body? In my case whiteness is not essential but i am considering
>.
Paul on wed 17 dec 03
Hello,
Aside from whiteness, is there any particular reason for adding kaolin to a
stoneware body? In my case whiteness is not essential but i am considering
it because the body, which includes fireclay, ball clay and goldart, was
actually too high in fire clay so i cut it back considerably. The problem is
that i don't want to have more than 25% ball clay, so the only other thing
is to increase the goldart -- but then the concern is that the high sulfur
content will cause glaze flaws, which i have already noticed in a few cases.
So i guess another way to ask the question would be is it practical to have
35 - 40% goldart in a stoneware with 25% each of ball clay and fireclay,
(not counting the non-plastics) or would it be better to sub some kaolin for
some of the goldart in a reduction body? Any advice?
thanks,
Paul B
Falmouth,KY
Jon Pacini on thu 18 dec 03
Greetings All ---Hi Paul
Paul Wrote--Aside from whiteness, is there any particular reason for adding
kaolin to a
stoneware body? In my case whiteness is not essential but I am considering
it because the body, which includes fireclay, ball clay and gold art, was
actually too high in fire clay so I cut it back considerably. The problem is
that I don't want to have more than 25% ball clay, so the only other thing
is to increase the gold art -- but then the concern is that the high sulfur
content will cause glaze flaws, which I have already noticed in a few cases.
So I guess another way to ask the question would be is it practical to have
35 - 40% gold art in a stoneware with 25% each of ball clay and fireclay,
(not counting the non-plastics) or would it be better to sub some kaolin for
some of the gold art in a reduction body? Any advice?
Kaolin is the least plastic of all the clays we use, Ball clays are the most
and Fireclays are in-between. This is a major generalization, but for
illustration purposes it will work.
Subbing for the fireclay will indeed lighten the body and will change the
plasticity the least. It will definitely help out with the impurities that
come with the fireclay.
Subbing for a portion of the Ballclay will not make much difference in the
color as most ball clays are light firing inspite of their raw color, but it
will change the plasticity the most. Kaolin will also cut down the
shrinkage.
If you find at some point you're using a lot of kaolin, or you just need
more plasticity, Bentonite will help bring the plasticity back up. I like
Bentolite L-10, it's of the non-swelling variety.
Lastly--Kaolin is also a little more refractory than the other clays, but if
you're not doing wholesale substitutions that won't come into play. Nor
should it change the glaze fit. If you do however need to get the Absorpsion
level down --add a dash of spar and of course, silica will help adjust the
glaze fit as needed.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
Ron Roy on fri 19 dec 03
Hi Paul,
Absolutely - most of the stonewares I have developed have some - in fact it
would be impossible to keep the iron containing stonewares refractory
enough without it.
EPK is one of the best - substituting it for a ball clay for instance.
I agree completely with David on his three points - and would like
reinforce his comments about Gold Art - 10% of the body would be the limit
I would include - if any.
EPK is a very stable clay as is Hawthorn and FHC. Keep the silica out or
very low for high fire stonewares - unless you are looking for cristobalite
- and keep the spar to 10% - what you are looking for is a shrinkage of 12%
overall (dry and fired) and an absorption of about 2%.
Read Peter Sohngens article in Studio Potter - Vol 28 #1- the one thing we
don't agree on is - he likes a little cristobalite to help with crazing
(that is like a pact with the devil as far as I am concerned) I say none
and adjust you glazes to counter expansion.
If the absorption needs to go up sub kaolin for ball - if the shrinkage is
too high add some fine Kyanite.
Let me know if you run into a snag - but I will need some absorbency and
shrinkage (wet to dry and fired) numbers to work with.
RR
>Aside from whiteness, is there any particular reason for adding kaolin to a
>stoneware body? In my case whiteness is not essential but i am considering
>it because the body, which includes fireclay, ball clay and goldart, was
>actually too high in fire clay so i cut it back considerably. The problem is
>that i don't want to have more than 25% ball clay, so the only other thing
>is to increase the goldart -- but then the concern is that the high sulfur
>content will cause glaze flaws, which i have already noticed in a few cases.
>So i guess another way to ask the question would be is it practical to have
>35 - 40% goldart in a stoneware with 25% each of ball clay and fireclay,
>(not counting the non-plastics) or would it be better to sub some kaolin for
>some of the goldart in a reduction body? Any advice?
>thanks,
>Paul B
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
| |
|
