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glaze refiring - cracking "platters"

updated wed 9 nov 11

 

MJ on sun 6 nov 11


Hello - I have been making one piece chip and dip platters and that are
about 12 inches after firing. I fire in an electric kiln to cone 5/6 and
do a slow controlled cool. I am currently using cone 5 Laguna B-mix. I
throw the platters and dip bowl and join the two.

After the bisque firing, there is no visible sign of a crack. After the
glaze firing, there were no visible signs of cracks, but there was a
problem with the glaze so i decided to try a refire. After the refire, all
of the chip and dip platters had cracks across the bottom of the platters
and all i the same place - basically a wavy line across the diameter of the
platter right around the area where the dip bowl had been joined to the
platter.

My questions are two fold - any ideas what is causing the cracks on the
second firing?

Even though I cannot see (or hear) any signs of cracks after the first
firing, is it likely the crack has already started and will show up over
time with use?

I am very concerned as I have done a few of these that have only been fired
once and appear to be fine.

Thanks in advance for any help on this issue.

Cheers, MJ

Snail Scott on sun 6 nov 11


On Nov 6, 2011, at 6:26 PM, MJ wrote:
> ...After the refire, all
> of the chip and dip platters had cracks across the bottom of the =3D
platters...



Cracks can often appear when things are refired.
The stresses were there all along; it just took the=3D20
extra heating to allow the stress to release in the=3D20
form of a crack in that spot. Bisque is very forgiving=3D20
of thermal stresses, so the first firing went fine, but=3D20
ironically, already-vitrified stuff is more vulnerable.
Some of the cracking may be due to the increased=3D20
pyroplasticity due to what is effectively a lot more=3D20
time spent in the pyroplastic temperature range for=3D20
that body, (allowing the stress to manifest as a crack)=3D20
but I have always suspected that the added thermal=3D20
stresses are a contributor as well. Just my own theory,=3D20
though.=3D20

-Snail=3D

John Britt on mon 7 nov 11


My guess is that you have to slow the second (refiring) firing around 900=
=3D
- 1100 F=3D20
(quartz inversion). (Take about an hour and see if that helps.) Let me k=
=3D
now.=3D20

http://ncclayclub.blogspot.com

Britt

Michael Wendt on mon 7 nov 11


MJ,
I found that refiring even 10.5" plates after glaze firing
them made
them crack. My first clue to a solution came when I began
doing
NAIA World Series Trophy plates for the baseball tournament
with decals on them. Fired flat in the kiln cracked most of
them the way you describe except where they were on a shelf
with lots of clearance under them ( 7-8" up from the
bottom).
I built a flat kiln with elements only in the lid that could
fire a
single plat to decal temperature in about 20 minutes. No
more
cracking because the plate was heated evenly across the
entire
face at the same time.
This lead me to experiment with insulating firebrick cuts
stacked
around the edges of the plates I refired much like saggars.
End of the cracking problem. Since then, I also found taller
brick
cuts stacked around bowls prevent thermal irregularity
caused
warping.
Try refiring one with saggars around it on a shelf set above
a taller lower layer and report your results.
Regards,
Michael Wendt

My questions are two fold - any ideas what is causing the
cracks on the
second firing?

Even though I cannot see (or hear) any signs of cracks after
the first
firing, is it likely the crack has already started and will
show up over
time with use?

I am very concerned as I have done a few of these that have
only been fired
once and appear to be fine.

Thanks in advance for any help on this issue.

Cheers, MJ

C. Tullis on tue 8 nov 11


I agree with John Britt on this. Stall and crawl through the quartz inver=
=3D
sion. I=3D20
also found that shimming or wadding the pieces so they don't sit directly=
=3D
on the=3D20
shelf helped a great deal.

marci Boskie's Mama =3D^..^=3D on tue 8 nov 11


>
>From: Michael Wendt
>Subject: Re: glaze refiring - cracking "platters"
>MJ,
>I found that refiring even 10.5" plates after glaze firing
>them made
>them crack. My first clue to a solution came when I began
>doing
>NAIA World Series Trophy plates for the baseball tournament
>with decals on them. Fired flat in the kiln cracked most of
>them the way you describe except where they were on a shelf
>with lots of clearance under them ( 7-8" up from the
>bottom).


China painters refire constantly ( some painters will refire
15-20 times on one piece ) ......and rarely have breakage but when
we do, its usually because of uneven heating.
It can be problematic with pieces that are large and have uneven
thicknesses ... like vases which are bottom heavy . .. and is usually
more of an issue on porous clays ( earthenware for example ) than it
is with vitrified porcelain ..
The solution for us is to not set them directly on a shelf but on
kiln stilts so that heat can get to the bottom at the same rate
as the sides. ( the theory being that if its sitting directly on a
shelf, the shelf has to heat up too so the bottom doesnt heat as
quickly as the sides do ) ... and also as an extra precaution , bring
up the heat slowly ... although allowing air flow under the
piece seems to fix the issue.

marci the chinapainter