Craig Dunn Clark on wed 2 apr 03
Victoria, I would start by trying out a different clay body, preferably
one that is works well in raku. From what you have described it really does
not sound like there are many other probable causes beyond a clay body
problem. The difficulty that I'm having with what I've just said is that
there are two clay bodies that you say are giving you precisely the same
problem and that doesn't sound like the most plausible thing in the world
either.
Have you by any chance changed anything in the drying process? You say
the tiles are press molded. Do you do all of the press molding? Has anything
changed in the molding end of the process? How are you determining your
bisque temp? Have you been using sight cones? I don't really think that
changing from a bisque temp of 08 to "approximately" 05 would create the
problem that you are having.
One thing you might try before you get another clay body to determine
when the tiles are cracking is to take the kiln up as you normally would and
then let the tiles cool down inside the kiln. Don't take them out. This will
let you know for sure when the cracking occurs.
One more thought/question......has anything changed in the burners or
your gas? Does the flame look any different? How are you determining the
ramp that you have described. I pretty much do all of my raku firings based
on kiln atmoshere color.I kinda get to temp at just about full tilt boogie
for the small to medium sized pieces (they are allways pre-heated and I
don't put them into a kiln that still glows unless I want to experience a
higher loss rate that is acceptable to me.) I don't do any tiles per say but
I have fired off quited a few flat pieces of varying dimensions along with
multiple platters.
Hope this helps a bit
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "victoria henricksen"
To:
Sent: Wednesday, April 02, 2003 11:23 AM
Subject: help! problems with raku
> please help....I have started having some major problems with pieces
cracking in the raku kiln. I have done about 40-50 firings in this kiln in
the last 6 months, but recently the majority of my pieces have cracked. The
pieces are about 6x6", press molded relief tiles. up until a few firings
ago, i had never lost a tile, but now my rate of loss is 75-100%. I am using
2 different kinds of clay, Laguna's raku, as well as Seattle's columbia
white stone....both of these clays are giving me the cracking problem.
When this problem had first started occuring i thought that it was due to
the fact that the tiles had been bisqued to a hotter temperature, rendering
them less resistant to thermal shock....i normally bisque my raku ware to
08, but i had a problem with the kiln and it fired to approximately 05...but
once i realized that these pieces were all cracking, i stopped using the
tiles from this batch. Since then i have been using tiles from 2 subsequent
08 bisques and have still had
> the same problem! I have always kept a firing log, and so i can
ascertain that there is not much difference in the firing 'ramp'....i have
tried going much slower, i've 'candled' the kiln on low fo
> r 15 minutes before sloooowly climbing, but with no better success rate.
I am using a kiln which i built from a 50gallon drum, lined w/ fiberfax,
with a removable lid on top (not top-hat style), and a propane burner. The
tiles seem to be cracking before they go into the reduction bins, as the
cracks are impregnated with carbon. When i first started using this kiln,
the weather was much cooler (winter, with snow on the ground)...now of
course it is warmer....this seems like the only major variation, but it
doesnt make sense to me that this would cause the cracking.
>
> ...same kiln, same clay, same tile design, same firing schedule,same
glazes....
>
> have i missed something??? i am getting desperate as i need to fill
some orders PDQ....if anyone has ANY suggestions please offer them up as at
this point i am willing to try anything!
>
> in desperation, Victoria
>
> PS oh and FYI, i dont spray with water, i just fire the pieces until the
galzes melt and then transfer them to reducing bins filled with sawdust,
newspaper and a few other combustables.
>
>
>
> ---------------------------------
> Post your free ad now! Yahoo! Canada Personals
>
>
____________________________________________________________________________
__
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melpots@pclink.com.
victoria henricksen on wed 2 apr 03
please help....I have started having some major problems with pieces cracking in the raku kiln. I have done about 40-50 firings in this kiln in the last 6 months, but recently the majority of my pieces have cracked. The pieces are about 6x6", press molded relief tiles. up until a few firings ago, i had never lost a tile, but now my rate of loss is 75-100%. I am using 2 different kinds of clay, Laguna's raku, as well as Seattle's columbia white stone....both of these clays are giving me the cracking problem. When this problem had first started occuring i thought that it was due to the fact that the tiles had been bisqued to a hotter temperature, rendering them less resistant to thermal shock....i normally bisque my raku ware to 08, but i had a problem with the kiln and it fired to approximately 05...but once i realized that these pieces were all cracking, i stopped using the tiles from this batch. Since then i have been using tiles from 2 subsequent 08 bisques and have still had the same problem! I have always kept a firing log, and so i can ascertain that there is not much difference in the firing 'ramp'....i have tried going much slower, i've 'candled' the kiln on low fo
r 15 minutes before sloooowly climbing, but with no better success rate. I am using a kiln which i built from a 50gallon drum, lined w/ fiberfax, with a removable lid on top (not top-hat style), and a propane burner. The tiles seem to be cracking before they go into the reduction bins, as the cracks are impregnated with carbon. When i first started using this kiln, the weather was much cooler (winter, with snow on the ground)...now of course it is warmer....this seems like the only major variation, but it doesnt make sense to me that this would cause the cracking.
...same kiln, same clay, same tile design, same firing schedule,same glazes....
have i missed something??? i am getting desperate as i need to fill some orders PDQ....if anyone has ANY suggestions please offer them up as at this point i am willing to try anything!
in desperation, Victoria
PS oh and FYI, i dont spray with water, i just fire the pieces until the galzes melt and then transfer them to reducing bins filled with sawdust, newspaper and a few other combustables.
---------------------------------
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Tony Ferguson on wed 2 apr 03
Victoria,
Sounds like a potential clay problem if you are not doing anything
different.
Try: Buy some tri wire supports for underneath your tiles--when you have
time, you can make better ones than anyone sells as they are intended for
electric firing where people want to glaze the bottoms of their pots. It
will prevent uneven heating and stress on your pieces as the heat will
circulate all around the piece--this has reduced my cracking to maybe 5% or
sometimes none at all. I have fired when it was 20 below 0.
How long is your first firing and then each subsequent firing?
Was there less grog in the body?
Did your supplier change the fire clay type or mesh size?
How long are you leaving them in the reduction chamber? You may be taking
them out too soon. If I take pieces out too soon in colder weather they
will crack.
If you are not doing anything different then I would talk to your supplier
about any changes in clay or its ingredients.
Thank you.
Tony Ferguson
On Lake Superior, where the sky meets the Lake
Stoneware, Porcelain, Raku and more
by Coleman, Ferguson, Winchester...
www.aquariusartgallery.com
218-727-6339
315 N. Lake Ave
Apt 312
Duluth, MN 55806
----- Original Message -----
From: "victoria henricksen"
To:
Sent: Wednesday, April 02, 2003 11:23 AM
Subject: help! problems with raku
> please help....I have started having some major problems with pieces
cracking in the raku kiln. I have done about 40-50 firings in this kiln in
the last 6 months, but recently the majority of my pieces have cracked. The
pieces are about 6x6", press molded relief tiles. up until a few firings
ago, i had never lost a tile, but now my rate of loss is 75-100%. I am using
2 different kinds of clay, Laguna's raku, as well as Seattle's columbia
white stone....both of these clays are giving me the cracking problem.
When this problem had first started occuring i thought that it was due to
the fact that the tiles had been bisqued to a hotter temperature, rendering
them less resistant to thermal shock....i normally bisque my raku ware to
08, but i had a problem with the kiln and it fired to approximately 05...but
once i realized that these pieces were all cracking, i stopped using the
tiles from this batch. Since then i have been using tiles from 2 subsequent
08 bisques and have still had
> the same problem! I have always kept a firing log, and so i can
ascertain that there is not much difference in the firing 'ramp'....i have
tried going much slower, i've 'candled' the kiln on low fo
> r 15 minutes before sloooowly climbing, but with no better success rate.
I am using a kiln which i built from a 50gallon drum, lined w/ fiberfax,
with a removable lid on top (not top-hat style), and a propane burner. The
tiles seem to be cracking before they go into the reduction bins, as the
cracks are impregnated with carbon. When i first started using this kiln,
the weather was much cooler (winter, with snow on the ground)...now of
course it is warmer....this seems like the only major variation, but it
doesnt make sense to me that this would cause the cracking.
>
> ...same kiln, same clay, same tile design, same firing schedule,same
glazes....
>
> have i missed something??? i am getting desperate as i need to fill
some orders PDQ....if anyone has ANY suggestions please offer them up as at
this point i am willing to try anything!
>
> in desperation, Victoria
>
> PS oh and FYI, i dont spray with water, i just fire the pieces until the
galzes melt and then transfer them to reducing bins filled with sawdust,
newspaper and a few other combustables.
>
>
>
> ---------------------------------
> Post your free ad now! Yahoo! Canada Personals
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
Marvpots@AOL.COM on thu 3 apr 03
Hi Victoria:
Have you asked your suppliers if they made any changes in the recipe of their
raku bodies?
These bodies need plenty of grog; I use 25% 30-45 mesh sand, make sure the
pieces are well compressed, and hope for the best. Cracking has been
minimal.
Good luck!
Marvin Flowerman
Paul Gerhold on fri 4 apr 03
Sounds like a drying problem to me. Flat pieces in Raku are really prone to
developing stress during fast drying. Do the cracks all look about the same
say running diagonally across the tile. Try coating the outside edges with
floor wax whle relatively wet and then dry the way you normally do.This will
tend to dry from the center out rather than fron the edges in.
As a side note while changing clays is a pain and I am not familiar with the
ones you use if you are not using clay with at least 12% kyanite you are
making life harder on yourself than it has to be. Kyanite is truely a magic
ingredient for reducing cracking in Raku. There are lots of good commercial
bodies out there with kyanite.
Paul
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