Nanci Mansfield on sun 12 mar 06
I've been working with earthenware tiles for a while now and thought I had the warpage problem licked...almost. The last few kiln loads we've had were perfect until we get to the very bottom of the kiln. All of those kilns are warping. We thought it might be the fact that the temp wasn't making it, replaced the thermocouple, they wtill warp. Can anyone make suggestions? The shelves we laid on the very bottom of the kiln cracked, both of them. I wondered about that' too. The other kiln has no shelves on the bottom and the tiles still warp. I get the feeling those shelves should be there but don't know if it makes a difference with the tiles.
Also, what causes tiles to crack in the center of the tiles? How do I prevent them?
Many thanks,
Nanci
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katetiler on mon 13 mar 06
Lots of factors that you need to consider -
how big are the tiles? how thick?
how are you stacking the kiln shelves and are the props all on line
with each other (not having them in line will promote the kiln shelves
cracking.)
What gap are you leaving above the tiles between the shelves?
How close are the tiles to the elements?
Are you carving the clay for the tiles from a block or are you rolling
it out or are you using a press mould?
How wet is the clay?
Are you using any inpressed decoration and if so how close is this to
the edge of the tiles?
Are the tile cracking at biscuit stage or at glaze stage?
How fast are you cooling your kiln & taking the ware out?
Where is the thermocouple situated in the kiln?
Are you soaking the tiles at full temp to allow the bottom temp to
equalise?
Sometimes even when you know all this, tiles seem to crack for no
reason - out of the last two batches I made, I lost one from each out
of 13.
It is a right pain and I sympathise - I even tried scattering a thin
layer of sand on each kiln shelf to help promote the tile shrinkage
recently and it made no difference.
I also had to draw a line under a commission for a tiled mural and
start again after 3 months work because I had just pushed the tile
dimensions too far and I lost every panel - I went back to 6 inch
squares and drew a new design for the mural & started again, so I
really do understand how frustrating it is!
kate
Nanci Mansfield on mon 13 mar 06
Wow! You are really thorough and organized in your answer. Let me try to outline this:
Most of our tiles are 38" thick plus whatever relief design is on them, maybe 1/4 inch at the very highest but usually closer to 1/8". They are 6" tiles. Some of our tiles are flat wtih carved out design. I do notice cracking more in the center of the designs that are relief with a change in thickness right in the center. some of the tiles crack during bisque firing but recently they are cracking as they reach greenware state. We use press molds and dry thoroughly before firing them, at least a week. They are pressed in with rubber mallets. We don't have much success with the press. Could it be the way we are pounding them?
We usually have 4" posts between the bottom and the first shelf. We stacked 3 high, leaving lots of headroom. All of those tile warped at all four corners. It didn't matter is they were flat or relief tiles. We put one layer of flat, 3/8" field tiles on the bottom and they all warped. The cones show cooler temps down there. We stack our posts pretty carefully in line with each other. We use half shelves and don't always stagger them. Our shortest post is 1". We use that and the 2" posts mostly. The last firing had lots of space between the elements and the tiles.
As for firing, we fire on slo speed, soak 20 minutes (25 boiled our glaze), turn off the fan when the kiln reaches cone. It takes a LONG time to cool. I don't like to open it until it reaches 200, preferably under that. We just replaced the thermocouples in both kilns. Our kiln is computerized skutt, vented from the lower shelf of the kiln. It doesn't seem to matter which side of the lower shelf the tile is on (meaning, if the tiles are on the side it vents from)...they all warp down there. This is a new thing.
The most frustrating thing is that, after losing tile after tile warping during the glaze firing and finally figuring that part out, now this.
What a drag about your commission. I hope the next attempt is a success. After working with tile I have gained a LOT of respect for those who master the art!
Many thanks,
Nanci
katetiler wrote:
Lots of factors that you need to consider -
how big are the tiles? how thick?
how are you stacking the kiln shelves and are the props all on line
with each other (not having them in line will promote the kiln shelves
cracking.)
What gap are you leaving above the tiles between the shelves?
How close are the tiles to the elements?
Are you carving the clay for the tiles from a block or are you rolling
it out or are you using a press mould?
How wet is the clay?
Are you using any inpressed decoration and if so how close is this to
the edge of the tiles?
Are the tile cracking at biscuit stage or at glaze stage?
How fast are you cooling your kiln & taking the ware out?
Where is the thermocouple situated in the kiln?
Are you soaking the tiles at full temp to allow the bottom temp to
equalise?
Sometimes even when you know all this, tiles seem to crack for no
reason - out of the last two batches I made, I lost one from each out
of 13.
It is a right pain and I sympathise - I even tried scattering a thin
layer of sand on each kiln shelf to help promote the tile shrinkage
recently and it made no difference.
I also had to draw a line under a commission for a tiled mural and
start again after 3 months work because I had just pushed the tile
dimensions too far and I lost every panel - I went back to 6 inch
squares and drew a new design for the mural & started again, so I
really do understand how frustrating it is!
kate
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Stephani Stephenson on mon 13 mar 06
Nanci
how big are the tiles?
how thick are the tiles?
what kind of clay are you using?
what forming method are you using for the tiles?
how fast are you firing?
Stephani Stephenson
steph@revivaltileworks.com
http://www.revivaltileworks.com
Nanci wrote: I've been working with earthenware tiles for a while now
and thought I had the warpage problem licked...almost. The last few
kiln loads we've had were perfect until we get to the very bottom of
the kiln. All of those kilns are warping. We thought it might be the
fact that the temp wasn't making it, replaced the thermocouple, they
wtill warp. Can anyone make suggestions? The shelves we laid on the
very bottom of the kiln cracked, both of them. I wondered about that'
too. The other kiln has no shelves on the bottom and the tiles still
warp. I get the feeling those shelves should be there but don't know if
it makes a difference with the tiles.
Also, what causes tiles to crack in the center of the tiles? How do I
prevent them?
Many thanks, Nanci
Nanci Mansfield on tue 14 mar 06
Hi,
Thanks for answering my question. All input is highly valued.
The tiles are mostly 6X6
Most of them are 3/8" with relief designs that can double that in the high parts of designs though 1/4" is mostly the case for high spots.
The clay is earthenware called Lyman Red from Highwater Clay in Asheville. It has lots of grog.
We form using plaster molds and pund with rubber mallets.
Firing is slow speed.
We solved most of the warpage problem by bisquing a little higher but notice that the tiles on the very bottom of the kiln are warping with a very very low percentage warping on the other shelves.
The cracking has suddenly increased. Its never been more than a tile or two. Many are now cracking as they dry. The weather is more humid than it has been lately. That's the only real difference I can see. Suddenly I seem to be losing a lot. I did copy your other response abonse about compression and will try to pay attention to that.
Thanks!!
Nanci
Stephani Stephenson wrote:
Nanci
how big are the tiles?
how thick are the tiles?
what kind of clay are you using?
what forming method are you using for the tiles?
how fast are you firing?
Stephani Stephenson
steph@revivaltileworks.com
http://www.revivaltileworks.com
Nanci wrote: I've been working with earthenware tiles for a while now
and thought I had the warpage problem licked...almost. The last few
kiln loads we've had were perfect until we get to the very bottom of
the kiln. All of those kilns are warping. We thought it might be the
fact that the temp wasn't making it, replaced the thermocouple, they
wtill warp. Can anyone make suggestions? The shelves we laid on the
very bottom of the kiln cracked, both of them. I wondered about that'
too. The other kiln has no shelves on the bottom and the tiles still
warp. I get the feeling those shelves should be there but don't know if
it makes a difference with the tiles.
Also, what causes tiles to crack in the center of the tiles? How do I
prevent them?
Many thanks, Nanci
______________________________________________________________________________
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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.
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Linda Blossom on tue 14 mar 06
Nanci,
Are you using bagged clay? If so, how are you
cutting off the pug? I use a peter pugger and I
have to cut off a length of clay and then lay it on
its side and press it into the mold. If I try to put
the clay on end and press it, I will get cracks in
the center. Seems all the pressing, with a press or
mallet, does not realign the particles as they
emerged from the pugger.
I put several parallel lines of silicone on one side
of my rubber mallet. Now it doesn't stick to the
clay.
Linda
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On
Behalf Of Nanci Mansfield
Sent: Tuesday, March 14, 2006 5:32 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: warped tiles...help
Hi,
Thanks for answering my question. All input is
highly valued.
The tiles are mostly 6X6
Most of them are 3/8" with relief designs that can
double that in the high parts of designs though 1/4"
is mostly the case for high spots.
The clay is earthenware called Lyman Red from
Highwater Clay in Asheville. It has lots of grog.
We form using plaster molds and pund with rubber
mallets.
Firing is slow speed.
We solved most of the warpage problem by bisquing
a little higher but notice that the tiles on the
very bottom of the kiln are warping with a very very
low percentage warping on the other shelves.
The cracking has suddenly increased. Its never
been more than a tile or two. Many are now cracking
as they dry. The weather is more humid than it has
been lately. That's the only real difference I can
see. Suddenly I seem to be losing a lot. I did copy
your other response abonse about compression and
will try to pay attention to that.
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