Kit Shannon on thu 9 dec 99
Can some one help me reformulate this clay body a little
i really like the way it throws, its very smooth and plastic,
but I've had some problems with it cracking
I made about 20, 8 inch plates and one cracked all over
the bottom and two had cracks on the lip. Some of my bowls have
cracks on the sides. Some of the cracking appeared in greenware, some
in the bisque and some not till after they were glazed.
My pots tend to be pretty consistent and fairly thin
I compress a lot, but maybe this clay body just needs more.
I'm at a loss, I'm not sure if my throwing is the problem, or if there
is
something wrong with this clay body.
I was wondering if someone had a suggestion on how I could change it
so it was less likely to crack. It is also a little on the yellow side,
and i'd like to
make it whiter if possible.
Any ideas?
white cone 6
25 - tile 6
26 - Foundry hill
26 - xx saggar
17 - Neph sye
6 - Talc
(-I used RR black with 3 % cobalt on this body and it turned out
beautiful!)
-kit
Linda Arbuckle on sat 11 dec 99
The body Kit listed:
white cone 6
25 - tile 6
26 - Foundry hill
26 - xx saggar
17 - Neph sye
6 - Talc
We found that Neph Sy deflocculates clay bodies a lot, makes it like
silly putty to work with, and causes drying problems. Because the
particle orientation is parallel, like a deck of cards, the surface
closes up some as the it dries, and then the interior has trouble
drying, causing uneven shrinkage and cracking.
The indispensible Pete Pinnell said:
I don't know a distinct recipe, just some general guidelines. For a
cone 6 light body you need about 20-25% flux with the rest being clay
or a clay/filler combination. Neph sy can work but it will tend to
deflocculate the clay unless it is counteracted with either epsom salts
or calcium chloride. Soda spar or G-200 will also work, but are not as
active so the clay will tend to be a bit more porous. You can also add
just a small amount of talc and that will really tighten up the clay,
but at a slightly heightened risk of cristobalite. The strongest
spar/talc eutectic is at a ratio of five or six parts spar to one part
talc.
As for clay, it can be "to taste". It will stand up a lot better and
crack a lot less if there is some fireclay, though very much will push
the color to buff. Similarly, a little ball clay goes a long way to
promote plasticity. Kaolin will give the whiteness, but you can't use
it alone unless you also add some other filler such as flint or
pyrophyllite. If it were me i would start with the following:
Fireclay 20, ball clay 20, tile #6 kaolin 25, flint 10, kona f-4 spar
25, bentonite 1. This will be pretty off-white (cream to buff), so if
they want whiter they will have to accept the lower workability of a
high kaolin body. They may want to add grog, in which case you could
use something like Ione Grain if they want whiteness without the cost
of Molochite. Kona can also cause deflocculation, so a little
flocculant (one quarter percent) might be a good idea. Have them
dissolve it first in hot water.
This is just a starting point- they can adjust any of the components to
fit their needs.
_________________
Linda
--
Linda Arbuckle
Graduate Coordinator, Assoc. Prof.
University of Florida Dept. Of Art
P.O. Box 115801
Gainesville, FL 32611-5801
arbuck@ufl.edu
(352) 392-0201 x 219
| |
|
