Ned Ludd on mon 16 dec 02
MJ Moriarty wrote:
>I recently decided to try a different claybody.
> and got some EM343 Ohio red from Laguna.
>
>Well, the glazes are falling off in sheets. Just flaking off like there is
>absolutely no bond to the clay surface at all. It's happening with the
>commercial glazes as well as with my own glazes.
>
>The spec sheet looks like the two clays should be very similar, as far as
>absoption and shrinkage. The last bisque kiln was fired a little hotter
>than usual (she said she went to cone 02 instead of 04 to make the ware
>more durable) but the EM343 is supposed to go to cone 2 without a problem,
>so that 02 bisque should not have vitrified the clay to the point where the
>glaze wouldn't bond.
>
>Does anyone have any ideas what could have happened? I want to play
>hookey from the school of hard knocks!
>
>MJ Moriarty
Dear Moriarty,
Hmm... I wonder if you are by any chance related to the professor we
last heard of in a famous struggle at the Reichenbach Falls, on or
about the year 1880? If yes, I will answer your question, if you
answer mine. What happened was excessive vitrification of the clay,
which your glaze did not take to. You see, wet glaze passionately
wants to be sucked onto the pot. It was not. Your glaze fell off in
disgust, I'm afraid. :-/
Now, Moriarty, what _really_ happened to Sherlock Holmes?
Oh dear me.. I couldn't resist! No disrespect intended to any person,
fictional or real!
I'm fairly sure that if the maker says EM 343 is good to cone 2, it
means it will not bloat, distort or slump up to that temperature. It
does not mean you can apply glaze to it after a bisque fire that
high, when it has lost its porosity and is highly unco-operative with
a typical wet glaze. Hey, if YOU were a wet glaze wanting to go on
the pot, you wouldn't like it either!
My experience on this comes from working with a potter using EM343.
She routinely bisqued to cone 06 only. By mischance, once a kilnload
was bisqued to cone 04 and even that 'low' there were serious glaze
application problems. So my advice is stick to cone 06, tops, for
bisque firing EM 343.
Bon voyage with this excellent red clay, such a pleasure to work
with. I must've thrown tons of it. My kudos to Laguna for this one.
Unsolicited!
best,
Ned
If there are no dogs in Heaven, then when I die I want to go where they went.
- Will Rogers
MJ Moriarty on mon 16 dec 02
I recently decided to try a different claybody. I have been using EM338
red from Laguna, and my glazes were working just fine, and the commercial
glazes (Amaco Velvets and Duncan Concepts) were working fine too.
I decided to try a slightly different clay (for no good reason, just
curious) and got some EM343 Ohio red from Laguna.
Well, the glazes are falling off in sheets. Just flaking off like there is
absolutely no bond to the clay surface at all. It's happening with the
commercial glazes as well as with my own glazes.
The spec sheet looks like the two clays should be very similar, as far as
absoption and shrinkage. The last bisque kiln was fired a little hotter
than usual (she said she went to cone 02 instead of 04 to make the ware
more durable) but the EM343 is supposed to go to cone 2 without a problem,
so that 02 bisque should not have vitrified the clay to the point where the
glaze wouldn't bond.
Does anyone have any ideas what could have happened? I want to play
hookey from the school of hard knocks!
MJ Moriarty
Snail Scott on tue 17 dec 02
At 01:32 PM 12/17/02 -0500, you wrote:
>She uses a clay that vitrifies much higher, and says she was taught that
>for low fire you should always bisque high in order to make the ware
>durable, then glaze fire low to keep the colors bright. She also says that
>you must have at least one cone of difference between the bisque and
>glaze. So she bisques to 03 or 02 and glazes at 05 or 04, and it works for
>her.
This is pretty typical. With low-fire glazes, many of them
are sensitive to the outgassing of the clay, so if it's
fired higher for bisque than for glaze, no new stuff will
outgas during the glaze firing. Cadmium reds are especially
sensitive, and even having greenware in the kiln with it
can kill the color. With high-fire, it's not practical to
fire higher for bisque, since it makes it hard to glaze
afterward, but fortunately, most high-fire glazes don't care.
>...founded a town in New Mexico.
>It's right on the old Route 66, and is the pinto bean capital of the US.
Go Pintos! (Yep, the local high school team is the Moriarty
Pintos. They have spotted horses on their t-shirts, but
everyone knows what they _really_ mean.
-Snail
(grew up just over the pass, in Albuquerque.)
MJ Moriarty on tue 17 dec 02
Since I just rent space in the studio I don't have control over firing
decisions. So until I get my own place and my own kiln I have to go along
with the way the owner is accustomed to doing things.
She uses a clay that vitrifies much higher, and says she was taught that
for low fire you should always bisque high in order to make the ware
durable, then glaze fire low to keep the colors bright. She also says that
you must have at least one cone of difference between the bisque and
glaze. So she bisques to 03 or 02 and glazes at 05 or 04, and it works for
her.
I was taught that you bisque low, just enough to keep the pots from melting
when you glaze them. At school we always bisqued everything together, low,
medium or high, to 06. So we have a difference in experience and
education. Since the owner is doing what works for her, I will adapt, and
use a clay that works with her methods.
To answer your question:
My great-grandfather, Professor James, and Mr. Holmes decided that all that
squabbling was foolish and the world was big enough for both of them. So
the professor moved to the new world and founded a town in New Mexico.
It's right on the old Route 66, and is the pinto bean capital of the US.
You may also be familiar with my cousin Dean, who was a fantastic driver
and loved to travel all over the country. I am afraid I am more like
cousin Dean than great-grampa. The professor's mathematical skills never
made it down to me, and I still don't quite grasp the binary system, in
spite of having read great-grampa's monograph several times.
I can always tell a person's literary bent by whether they ask me about the
Professor as documented by Sir Arthur or cousin Dean as documented by Mr.
Kerouac.
Thanks for the help!
MJ
Jon Pacini on wed 18 dec 02
Greetings all----Ned and Snail pretty much addressed this, but I'll throw in
my two bits.
Em 343 is routinely bisque fired to ^05/04 for the purpose of resolving
gassing problems posed by organics. And as Snail stated, Low fire earthen
ware is notorious for shivering ^06 glazes when it is OVER bisqued. One
reason is the higher you bisque fire the ware, the more difference there is
between the COE of the clay and low fire glaze.
Also Lead free-Low fire glazes don't achieve the interface layer that
leaded low fire or stoneware ones do, that is, there is not a lot of
interaction between the clay and glaze.
The more porous the ware is (lower bisque), the more the glaze soaks
into the surface of the ware when being applied, the better the fired
adhesion. By dampening the bisque slightly you can also achieve a better
bond as the glaze will be drawn into the pores of the ware through capillary
action rather than dried out when it hits the bone dry surface of the ware.
Em 343 can and is used at ^2 as architectural Terra Cotta'. At ^2 it's
absorbency rate approaches 2% and it resembles some of the great Terra Cotta
facades that you see on old Columbus and Cincinnati, Ohio brick buildings.
The materials used in the body come from deposits that have been mined in
Ohio for quite some time. But then that's probably historical clutter that
could be filtered out. :)
I'm not sure about being knocked over by a feather Ned, you gave a good
explanation. I can certainly sympathize with MJ's misfortune, but as stated,
the technique did work for the studio operator and MJ followed along.
Always best to test before committing to production.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
Ron Roy on thu 19 dec 02
Before, during or after the firing?
RR
>I recently decided to try a different claybody. I have been using EM338
>red from Laguna, and my glazes were working just fine, and the commercial
>glazes (Amaco Velvets and Duncan Concepts) were working fine too.
>
>I decided to try a slightly different clay (for no good reason, just
>curious) and got some EM343 Ohio red from Laguna.
>
>Well, the glazes are falling off in sheets. Just flaking off like there is
>absolutely no bond to the clay surface at all. It's happening with the
>commercial glazes as well as with my own glazes.
>
>The spec sheet looks like the two clays should be very similar, as far as
>absoption and shrinkage. The last bisque kiln was fired a little hotter
>than usual (she said she went to cone 02 instead of 04 to make the ware
>more durable) but the EM343 is supposed to go to cone 2 without a problem,
>so that 02 bisque should not have vitrified the clay to the point where the
>glaze wouldn't bond.
>MJ Moriarty
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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