dave morrison on sun 2 may 99
i too have had problems with red clay in raku. when i was making pots under
12 inches tall i had relatively no problems, but when throwing larger pots i
found that i had a huge amount of loss. it always seemed to happen in the
reduction chamber as well. the only information i could get on the problem
was from an old tech friend who said that the iron in the clay does not go
through thermal shock very well. my clay was tinted red from a percentage
of red art mixed in with my normal stoneware body. since, i have left out
the red art, and substituted talc to make a cone six light colored clay for
salt, and it seems to have much fewer problems in raku as well. hope this
helps a bit. dave, morrison studios.
-----Original Message-----
From: Gary Elfring
To: CLAYART@LSV.UKY.EDU
Date: Friday, April 30, 1999 8:02 AM
Subject: Raku firing problem
----------------------------Original message----------------------------
I have done a fair number of raku firings in the past
couple of months. Tonight, however, just about everything
made with a red stoneware clay (which normally holds
up the best) literally fell apart. I'm trying to isolate
what caused this.
I'm using a gas kiln, ceramic fiber insulated, with forced
air blower. Normally, it takes 30 - 45 minutes to get up to
temperature the first time, then 15 - 20 minutes each
successive time. Then items are transfered to metal garbage
cans filled with shredded newspaper and sawdust. After
reduction the pieces go into a water bucket.
The stoneware clay did feel different when being thrown- it
was very wet and soft.
For some reason the kiln was firing very fast today (the blower
runs on a variable speed controler, which could have been set
too high.) The gas level was set about where it always was
(increase gas flow until flames come out the top of the kiln,
then back off till they don't). I could do a run in about
8 - 9 minutes. Although most runs were about 12 - 15 minutes long.
I also had problems in the past with the reduction material
making marks on the pots, so I also changed the garbage
can set up. I put fire bricks in the bottom of the can-
about half way up- so I could set pots directly on the bricks
in the can, then throw sawdust on them and seal the can up.
So the question is, was it just a batch of bad clay, am I firing
too fast, or are the fire bricks in the reduction area changing
things?
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Marvin Flowerman on sun 2 may 99
Gary:
You haven't described at what point the pieces fell apart and so it's
difficult to comment on the possible causes. Also, with the rapid fire you
may be getting too much gas into the kiln; have you checked the orifice on
your burner lately?
I'll try to help if you give more information.
Marvin Flowerman (marvpots@aol.com)
Craig Dunn Clark on sun 1 jul 01
Pam, the black dots that you are getting may be from a pin holing
effect caused by the grog in your clay. I'm not sure because I don't know
what type of clay you are using.
The speed at which I fire up my white crackle is considerably faster
than yours. I also use an electric kiln to fire a number of my pots. After
the initial firing, which usually takes about an hour and a half, I fire in
roughly twenty minute rounds.
The only problem that I encounter is a mottling of the glaze if I try to
fire too quickly and do not let the kiln cool a bit before placing the next
round of pots in the kiln.
As far as the good crackle on the inside of the pot and no crackle on
the outside I'm a bit stumped. I regularly rib the outside of my pots to get
rid of throwing rings. I also use a slip that I smooth out with a throwing
stick, rubber rib and a shammy after trimming so that there is no difference
in the surface of the pot after trimming.
The one thing that I've done to increase the "crackle" effect is to add
more flint to the glaze or spritz the pot immediately after removing it from
the kiln before putting it into the can for reduction. I just use an old
spray bottle for this. Works great. You can really localize a concentrated
area of crackling this way.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From:
To:
Sent: Thursday, July 05, 2001 5:22 AM
Subject: Raku Firing Problem
> I did a raku firing yesterday afternoon and got some disappointing
results.
> On a couple of pots I was using a white crackle. Nice crackle on the
inside,
> but no crackle on the outside. In addition to no crackling on the
outside, I
> have lots of black dots that I figure is pinholing and the blackened
claybody
> is showing through. I'm puzzled as to why this is happening. Have gotten
> good results in the past, though this phenomenon has happened before. Is
it
> my firing schedule? Does it have to do with more compression to the
claybody
> on the outside than the inside? I've been compressing a lot with a rib to
> get the throwing marks off on the outside for decoration reasons.
>
> I mentioned the firing schedule. In an electric kiln, I've been firing up
> slowly, about 4-1/2 hours to cone 06, even when my pots have been
thoroughly
> dry. Is this necessary? I know many potters fire up quickly, in a matter
of
> 1 hour or so.
>
> I'm getting frustrated losing pots for this reason. Any help with this
> problem will be greatly appreciated.
>
> Thanks Much, Pam Farmer
>
>
____________________________________________________________________________
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melpots@pclink.com.
>
PEFarm@AOL.COM on thu 5 jul 01
I did a raku firing yesterday afternoon and got some disappointing results.
On a couple of pots I was using a white crackle. Nice crackle on the inside,
but no crackle on the outside. In addition to no crackling on the outside, I
have lots of black dots that I figure is pinholing and the blackened claybody
is showing through. I'm puzzled as to why this is happening. Have gotten
good results in the past, though this phenomenon has happened before. Is it
my firing schedule? Does it have to do with more compression to the claybody
on the outside than the inside? I've been compressing a lot with a rib to
get the throwing marks off on the outside for decoration reasons.
I mentioned the firing schedule. In an electric kiln, I've been firing up
slowly, about 4-1/2 hours to cone 06, even when my pots have been thoroughly
dry. Is this necessary? I know many potters fire up quickly, in a matter of
1 hour or so.
I'm getting frustrated losing pots for this reason. Any help with this
problem will be greatly appreciated.
Thanks Much, Pam Farmer
Ababi on thu 5 jul 01
Hello Pam!
You might be surprise, but any glaze for 08 or any frit, as it is will make
nice crackles in the inside of a dish, saucer while none of the outside.
Wish it would not be, I would stop testing so many tests to win it. The
glaze gurus will explain you why, I can explain you only that it does!, I
believe , it is because of surface tension, the way the glaze, fits on the
outside of a dish rather than "breaking to small pieces" in my humble
English. I tested many glazes only for this reason. I
made a dish, pretty big, I applied the glazes , ten or so tests in each
dish, inside and outside, to see the behaving. The inside all succeeded!
Outside, perhaps tomorrow I will be smarter.
About the pinholes, I think I know what you mean, I cannot tell you why, but
perhaps, if it is repeating you can use it, like 3 white bases, on one jar,
one of them has big crackles, one none one small pine holes, between them
black frames of unglazed areas. In one of the many
raku tests I made lately I had a glaze, in firing to about 08 it had
beautiful small pinholes serounded by small crackles like they were tiny
flowers. When I fired it to 06 I got ugly ( in raku) smooth glaze.
About the test of tomorrow: I will test porcelain, by V.C. as a claybody
and colored slips plus glazes, this time try to imitate the 80/20 raku
glaze as a target, as we say, "If you cannot beat them, join them!" If
there will be any news, you (and the list), will be the first to know!
Bye bye
Ababi Sharon in Kibbutz Shoval - Israel- glaze addict
ababisha@shoval.ardom.co.il
http://members4.clubphoto.com/ababi306910/
http://www.milkywayceramics.com/cgallery/asharon.htm
http://www.israelceramics.org/
----- Original Message -----
From:
To:
Sent: Thursday, July 05, 2001 12:22 PM
Subject: Raku Firing Problem
> I did a raku firing yesterday afternoon and got some disappointing
results.
> On a couple of pots I was using a white crackle. Nice crackle on the
inside,
> but no crackle on the outside. In addition to no crackling on the
outside, I
Tommy Humphries on thu 5 jul 01
Sounds like the glaze was too thin on the outside...did you pour the inside
and brush the outside? I had the same results with a Raku firing once and
that was the cause.
Tommy
----- Original Message -----
From:
To:
Sent: Thursday, July 05, 2001 5:22 AM
Subject: Raku Firing Problem
> I did a raku firing yesterday . Nice crackle on the inside,
> but no crackle on the outside. I
> have lots of black dots that I figure is pinholing and the blackened
claybody
> is showing through. I'm puzzled as to why this is happening
Steve Mills on fri 6 jul 01
Pam,
I think you are under-firing; the pinholes are incomplete gassing. Most
Raku glazes gas towards the final stage of the melt, visible as black
dots that grow and disappear on the glazed surface. Forget the pyrometer
and cones; watch the pots, and when all gassing has ceased and the
surface looks like a golden syrup you're there.
Steve
Bath
UK
In message , PEFarm@AOL.COM writes
>I did a raku firing yesterday afternoon and got some disappointing results.
>On a couple of pots I was using a white crackle. Nice crackle on the inside,
>but no crackle on the outside. In addition to no crackling on the outside, I
>have lots of black dots that I figure is pinholing and the blackened claybody
>is showing through. I'm puzzled as to why this is happening. Have gotten
>good results in the past, though this phenomenon has happened before. Is it
>my firing schedule? Does it have to do with more compression to the claybody
>on the outside than the inside? I've been compressing a lot with a rib to
>get the throwing marks off on the outside for decoration reasons.
>
>I mentioned the firing schedule. In an electric kiln, I've been firing up
>slowly, about 4-1/2 hours to cone 06, even when my pots have been thoroughly
>dry. Is this necessary? I know many potters fire up quickly, in a matter of
>1 hour or so.
>
>I'm getting frustrated losing pots for this reason. Any help with this
>problem will be greatly appreciated.
>
>Thanks Much, Pam Farmer
--
Steve Mills
Bath
UK
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