Charles and Tamara Schulz on sun 21 sep 97
Good morning everybody!
Please forgive this novice question, I'm fairly new to this. Lately, I've
been throwing sets of dinner plates, mugs and cereal bowls and I'm noticing
that in each batch there are one or two plates that develop cracks in the
bottom after drying. Why is this happening? What can I do about it? I've
taken extra care to use good technique and compress the bottom of the plate
but I'm still getting some cracks.I'm throwing on 1/4" pressboard bats.
Can anyone help me with this?
also, I'm having some trouble with unglazed pieces sticking to the shelves
after firing(cone 9-10 reduction, gas kiln) I sometimes use a local
Okinawan clay which is especially prone to sticking, but I have a few
problems with American made clay as well. I pull a beautiful pot from the
kiln only to find bits of the bottom stuck to the shelf-#*)"/?! I'm using a
commercial kiln wash; The local potters use alumina but I don't know how
much, mixed with what, etc.(it's a secret). Any suggestions? Thanks so much!
Tamara
in Okinawa--making my living teaching SCUBA diving--Too much time in the
training pool-- not enough in the studio! Oh well, the mud gets washed off
and the chlorine probably kills the "bad bugs."
Rick Sherman on mon 22 sep 97
----------------original message---------------------------------
Lately, I've been throwing sets of dinner plates, mugs and cereal
bowls and I'm noticing that in each batch there are one or two plates
that develop cracks in the bottom after drying. Why is this happening?
What can I do about it? I've taken extra care to use good technique
and compress the bottom of the plate but I'm still getting some
cracks.I'm throwing on 1/4" pressboard bats.
Tamara
in Okinawa
------------------------reply-------------------------------------
Tamara: If your clay is fairly fine, with little grog, you may have
the same problem I did. If the plates dry unevenly, they crack across
the bottom or, sometimes, with a spiral. I solve my problem by setting
my newly-trimmed plates on thin pieces of wood. Wood lath sanded to be
smooth. This permitted the bottom of the plates to dry at the same
rate as the top.
Good luck. Let's see what others think.
RS
Ardis Bourland on mon 22 sep 97
Tamara,
When you throw the plates, after you have almost finished refining the bottom
take a fat wooden paddle, one that won't cut into the clay and pound the
center of the plate with a few good whacks each time. THen go back over the
marks and smooth them out. This has worked for me.
I spend a lot of time in the pool, too!
Ardis
Cindy on mon 22 sep 97
Tamara,
When throwing plates, it's important to have the bottom of uniform
thickness. I throw them on masonite bats, though I'm sure plaster would be
superior. After throwing, to insure uniform thickness and to remove
throwing rings, I smooth them with a wooden rib. I do not cut them from the
bats with wire--I wait until they're dry, then remove them by spinning on
the wheel and cutting under with a (very clean) fettling knife.
My plates used to crack regularly--now they don't. Why? I have no idea
except that I did use to cut them off with wire immediately after throwing.
Really, though, I suppose I just got better at getting a nice, even bottom.
I trim them on a chuck, and I don't foot them--I just like them that way.
No foot does give the advantage of being able to throw a thin plate and not
worry about possible slumping/warping in the kiln.
Can't help you with the other questions except that it sounds as if your
clay isn't meant to fire as hot as you're firing it.
Hope this helps,
Cindy Strnad
Custer, SD
>in each batch there are one or two plates that develop cracks in the
> bottom after drying. Why is this happening? What can I do about it?
> also, I'm having some trouble with unglazed pieces sticking to the
shelves
> after firing(cone 9-10 reduction, gas kiln)
> Tamara
> in Okinawa
Vince Pitelka on mon 22 sep 97
>Please forgive this novice question, I'm fairly new to this. Lately, I've
>been throwing sets of dinner plates, mugs and cereal bowls and I'm noticing
>that in each batch there are one or two plates that develop cracks in the
>bottom after drying. Why is this happening? What can I do about it? I've
>taken extra care to use good technique and compress the bottom of the plate
>but I'm still getting some cracks.I'm throwing on 1/4" pressboard bats.
>Can anyone help me with this?
>also, I'm having some trouble with unglazed pieces sticking to the shelves
>after firing(cone 9-10 reduction, gas kiln) I sometimes use a local
>Okinawan clay which is especially prone to sticking, but I have a few
>problems with American made clay as well. I pull a beautiful pot from the
>kiln only to find bits of the bottom stuck to the shelf-#*)"/?! I'm using a
>commercial kiln wash; The local potters use alumina but I don't know how
>much, mixed with what, etc.(it's a secret). Any suggestions? Thanks so much!
Tamara -
There are several things going on here. If you are compressing your
vessel-bottoms well, if you are removing ALL traces of water from the bottom
of the vessel, if you are cutting them off the bats right away (with no
water on the bat to be pulled under the vessel by the cutoff wire), and if
you are drying them slowly, then you should not experience any cracking. If
you are neglecting any of these things it could cause the cracking.
There are a number of different kiln-wash recipes which are made of varying
combinations and proportions of alumina, flint (silica flour) and China clay
(kaolin). For standard gas or electric kiln use with ordinary shelves which
have never been fired in salt or soda, a 50-50 mix of China clay and flint
works well. It makes a crumbly, flaky kiln wash which will separate easily
from the shelf even if a glaze runs badly. For salt, soda, and wood
firings many people use a 50-50 mix of China clay and alumina, while some
boost the percentage of alumina to even seventy or eighty percent. Since
alumina is by far the most expensive material in this group it makes sense
to minimize the quantity. I know some potters who fire in normal gas or
electric who use 45% China clay, 45% flint, and 10% alumina.
Regarding the problem of pots sticking to the shelves, if you have not been
using kiln wash that explains this problem. Another consideration, however,
is whether or not you are using wax resist on the feet of your pots. If you
do not use wax resist, and simply wipe the glaze off the foot instead, some
glaze is always absorbed into the pores of the clay, and this will always
greatly increase the chances of the pot sticking to the shelf, especially
with porcelain clay, which has more/larger pores than most stoneware and
earthenware bodies.
Good luck.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
Elca Branman on mon 22 sep 97
Try wrapping plastic around the edges so thast the bottom dries out
first..I don't know what an Okinawan dry-cleaning plastic bag looks like,
but here in the usa,I scissor off about 6 inch round by cutting right
across , and wrap my edges that way
Elca, almost at home in Sarasota,Florida
Cindy on tue 23 sep 97
Oops! I didn't mean *dry* dry. I meant leather-hard dry. Sorry.
Cindy
> ----------------------------Original message----------------------------
> Tamara,
>
> When throwing plates, it's important to have the bottom of uniform
> thickness. I throw them on masonite bats, though I'm sure plaster would
be
> superior. After throwing, to insure uniform thickness and to remove
> throwing rings, I smooth them with a wooden rib. I do not cut them from
the
> bats with wire--I wait until they're dry, then remove them by spinning on
> the wheel and cutting under with a (very clean) fettling knife.
>
> My plates used to crack regularly--now they don't. Why? I have no idea
> except that I did use to cut them off with wire immediately after
throwing.
> Really, though, I suppose I just got better at getting a nice, even
bottom.
>
>
> I trim them on a chuck, and I don't foot them--I just like them that way.
> No foot does give the advantage of being able to throw a thin plate and
not
> worry about possible slumping/warping in the kiln.
>
> Can't help you with the other questions except that it sounds as if your
> clay isn't meant to fire as hot as you're firing it.
>
> Hope this helps,
>
> Cindy Strnad
> Custer, SD
>
>
> >in each batch there are one or two plates that develop cracks in the
> > bottom after drying. Why is this happening? What can I do about it?
>
> > also, I'm having some trouble with unglazed pieces sticking to the
> shelves
> > after firing(cone 9-10 reduction, gas kiln)
>
> > Tamara
> > in Okinawa
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