Jeff Tsai on wed 15 may 02
Hi,
I have been rebisquing my pots after low-fire salt/saggar firings. I think
that orange blushes develop nicely in a reoxidation and blues become
stronger. Anyway, I have been having about a 50/50 success rate with the
rebisquing.
Sometimes I rebisque, and the pieces are great...or not great, whatever, but
the point is, they don't crack.
Sometimes I rebisque and the pieces suddenly crack for no apparent reason.
Sometimes it's just a few hairline cracks, othertimes an entire chunk, like
a
sliver, can pop off.
My saggar fired pieces are done fairly standardly. Sawdust, pot, sprinkles
of
copper, cobalt, iron around the pot. sprikles of salt around it too. Then
some seaweed, vermiculite, more sawdust, more of the same...
There is no distiguishable difference in shape or coloration from the saggar
in the pots that crack and don't in the rebisque. I always check very very
well for cracks after the saggar firing. I don't rebisque pots that have
cracks...just toss them of use them as test tiles. I know that no visual
cracks exist on the pots prior to going into the bisque, though, there are
possible cracks that I can't see with my normal sight.
Here's my only hypothesis at this time. During the saggar, the clay gets
infused somehow with sodium. This doesn't necesarily crack the pot during
the
saggar, but during a rebisque the sodium infused areas of the pot now react
differently. Maybe the areas infused by sodium have a different expansion
and
shrinkage rate than the rest of the clay now and the cracks occur from the
clay pulling itself apart. It's a longshot and a shot in the dark, but it's
my only guess as the bisque firing go slow as hell sometimes.
I've refired some saggar pieces as many as 6 times, then rebisqued them and
had them turn out okay, but sometimes, after just one saggar firing, the pot
goes into a bisque and is cracked. anyone have any clue why this is
happening...just a curiosity; honestly, I'm happy with my successes and
don't
mind the loss of work that much.
-Jeff
Ron Roy on mon 20 may 02
Hi Jeff,
I think this may be happening during the quartz inversion at 573C. can
happen on the way up or the way down.
Usually not a big problem on the way down - but bodies with a lot of free
quartz - like porcelains - can be a problem especially with big plates and
bowls.
Here is what happens. The free quartz crystals - in the body - go through
the inversion - it is quick compared to other inversions.
On the way up the rim will heat faster then the bottom - because it is
thicker and/or in contact with a shelf which holds the bottom part of the
ware back - heating wise. Rim tries to get bigger - bottom won't let it.
I don't think this is were it is happening - think of all the ware -
bisqued and glazed being fired fast by so many potters - I think it is much
more likely to happen on the way down.
Rim cools faster than the bottom - tries to get smaller - bottom won't let
it.
If your cracks are closed (not open) then quartz dunting is the problem.
The cracks are open when they happen - but as the rest of the pot cools -
they close up.
This may be happening to you because the salt is fluxing the clay and it
becomes more brittle than unfluxed bisque.
The real clue is - it does not happen in a sagger - the ware in a sagger is
protected from fast heating and cooling.
I predict - if you slow the cooling - or keep the ware off the shelf a
little - the cracking will stop.
Let us all know what happens se we can all learn.
RR
>I have been rebisquing my pots after low-fire salt/saggar firings. I think
>that orange blushes develop nicely in a reoxidation and blues become
>stronger. Anyway, I have been having about a 50/50 success rate with the
>rebisquing.
>
>Sometimes I rebisque, and the pieces are great...or not great, whatever,
but
>the point is, they don't crack.
>
>Sometimes I rebisque and the pieces suddenly crack for no apparent reason.
>Sometimes it's just a few hairline cracks, othertimes an entire chunk, like
a
>sliver, can pop off.
>
>My saggar fired pieces are done fairly standardly. Sawdust, pot, sprinkles
of
>copper, cobalt, iron around the pot. sprikles of salt around it too. Then
>some seaweed, vermiculite, more sawdust, more of the same...
>
>There is no distiguishable difference in shape or coloration from the
saggar
>in the pots that crack and don't in the rebisque. I always check very very
>well for cracks after the saggar firing. I don't rebisque pots that have
>cracks...just toss them of use them as test tiles. I know that no visual
>cracks exist on the pots prior to going into the bisque, though, there are
>possible cracks that I can't see with my normal sight.
>
>Here's my only hypothesis at this time. During the saggar, the clay gets
>infused somehow with sodium. This doesn't necesarily crack the pot during
the
>saggar, but during a rebisque the sodium infused areas of the pot now react
>differently. Maybe the areas infused by sodium have a different expansion
and
>shrinkage rate than the rest of the clay now and the cracks occur from the
>clay pulling itself apart. It's a longshot and a shot in the dark, but it's
>my only guess as the bisque firing go slow as hell sometimes.
>
>I've refired some saggar pieces as many as 6 times, then rebisqued them and
>had them turn out okay, but sometimes, after just one saggar firing, the
pot
>goes into a bisque and is cracked. anyone have any clue why this is
>happening...just a curiosity; honestly, I'm happy with my successes and
don't
>mind the loss of work that much.
>
>-Jeff
>
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Ron Roy
RR #4
15084 Little Lake Road
Brighton
Ontario
Canada - K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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