Carol David on wed 25 jul 07
I'm making 6" square tiles, 5/16" thick. I very slowly bisque fired
to 04; they were fine. I glaze fired to 06 and 3 of the 20 tiles
cracked in half. I've been making tiles for awhile, albeit smaller,
and haven't had this problem. Any ideas why?
Kevin Ritter on thu 26 jul 07
Stilt the tiles. If they are laying directly on the shelf during firing,
the temp on the bottom of the tile that is in contact with the shelf is greater
than the top, it's thermal shock. Took me years and many lost tiles to
figure it out. Good luck
Kevin
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Marcia Selsor on thu 26 jul 07
Are you using tile setters? How are you firing them in the kiln? You
don't give much information. Are they sitting on the shelf?
Marcia Selsor
http://marciaselsor.com
Michael Wendt on thu 26 jul 07
Carol,
There is more than one cause for fired cracking.
Look at the cracked tiles. If you wedged the
clay for the tiles, laminar structures can cause
cracks unless they are in the same plane as the
kiln shelf. Likewise, if you simply cut them
from a pug, the auger in a pug mill lays down
spiral laminations in the extrusion combined with
a fast center core dragging the outer surface
along behind. All sources of potential cracks.
Thermal shock can also cause cracking, but in this
instance, you already fired the tiles hotter ( 04)
so if they didn't crack the first time, the other
explanation is a different firing rate (faster) that
caused the cracking. If you are firing a computer
controlled kiln, firing the glaze fire at the same
rate that you fired the bisque fire successfully can
solve a lot of cracking problems.
Finally, consider the clay glaze fit issue: Each
time you buy clay from a manufacturer, you need
to do testing prior to use to assure it is what it
says. Everyone makes mistakes once in a while.
Even if you have used the clay and glaze of the
same type in the past, pretesting new batches
before production is a kind of insurance.
Good Luck,
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
Carol David wrote:
I'm making 6" square tiles, 5/16" thick. I very slowly
bisque fired
to 04; they were fine. I glaze fired to 06 and 3 of
the 20 tiles
cracked in half. I've been making tiles for awhile,
albeit smaller,
and haven't had this problem. Any ideas why?
Angela Davis on thu 26 jul 07
How do you bisque them? I used to get edge cracks
on my larger tiles , they would start at the edge and run to the center.
Sometimes the crack would be obvious after the bisque but
some didn't show until the glaze firing.
I have not had one of those cracks since I started using Paul
Lewing's method of firing them on edge. I start the first one up against
a kiln post then place the rest against each other at and angle. They
are straight up and down just touching, not leaning on each other.
(You can probably find his better description in the archives)
You can usually get more tiles on your shelf this way too, instead of
stacking them flat 2 high.
I believe the cracks were happening during the cooling with the heat from
the shelf influencing
the rate on a large area of the tile.
Jamie in LV just asked this same question, maybe she has an answer as to
what solved
her problem. Are you there Jamie?
Let us know what fixes this for you.
Angela Davis
In warm Homosassa
----- Original Message -----
From: "Carol David"
To:
Sent: Wednesday, July 25, 2007 11:01 PM
Subject: Why do tiles crack?
> I'm making 6" square tiles, 5/16" thick. I very slowly bisque fired
> to 04; they were fine. I glaze fired to 06 and 3 of the 20 tiles
> cracked in half. I've been making tiles for awhile, albeit smaller,
> and haven't had this problem. Any ideas why?
>
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Kevin Ritter on thu 26 jul 07
The idea is to get them off the shelf. I just use small kiln posts to lift
them off the shelf. I haven't had problems with small tiles but larger ones
need a bit of space between them and the shelf, it solved the problem for me.
Kevin
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Carol David on fri 27 jul 07
In answer to various followup questions people asked:
I used a tile setter. I had bisque fired them *very* slowly, holding
a long time at 180 degrees because some tiles were slightly damp,
using a program suggested by the owner of our local ceramics store.
Then I used the computer's automatic cone 06 firing, slow speed, for
the glaze firing. As I said, all the cracking happened in the bisque
firing.
Michael Wendt on sat 28 jul 07
Carol,
I pasted your first post in:
Carol David wrote:
"I'm making 6" square tiles, 5/16" thick. I very
slowly
bisque fired
to 04; they were fine. I glaze fired to 06 and 3 of
the 20 tiles
cracked in half. I've been making tiles for awhile,
albeit smaller,
and haven't had this problem. Any ideas why?"
Now here is your second response to questions:
"In answer to various followup questions people asked:
I used a tile setter. I had bisque fired them *very*
slowly, holding
a long time at 180 degrees because some tiles were
slightly damp,
using a program suggested by the owner of our local
ceramics store.
Then I used the computer's automatic cone 06 firing,
slow speed, for
the glaze firing. As I said, all the cracking happened
in the bisque
firing."
The reasons for cracking in bisque are often
different from the glaze firing. Perhaps more
information about the clay and its method of
preparation is in order. Then more meaningful
suggestions can be made.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
stephani stephenson on mon 30 jul 07
Carol
have you tried bisque firing them on end?
stand them up , on edge, like pieces of bread in a
loaf
put a small section of brick or refractory on each
end to
hold the 'clamp' together, i.e. to hold them upright
and pressed together.
try 6-12 tiles in a clamp......
tiles should be dry prior to firing
some fine bodied clays are prone to cracking
when the reach 6" size....
a body with more grog may help
on the other hand...with thin tiles and with certain
clays....losing 3 out of 20 6 X6 might be par for the
course if you are firing horizontally in the
bisque.....
regards! nice to see you online!
Stephani Stephenson
http://www.revivaltileworks.com
CAROL wrote
I'm making 6" square tiles, 5/16" thick. I very
slowly bisque fired
to 04; they were fine. I glaze fired to 06 and 3 of
the 20 tiles
cracked in half. I've been making tiles for awhile,
albeit smaller,
and haven't had this problem. Any ideas why?
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