Ray Carlton on wed 5 may 99
hi all here is a little puzzle that has me stumped and I am wondering if
any body else has seen this phenomenon.
I get the pots out of the kiln and wax them as per usual. (The pots are at
room temperature) I glaze them soon after. The pots that go through my
celadon glaze pop. I am not kidding they go pop and a little piece of clay
literally spits of the surface of the piece and leaves a small hole hole.
sometimes a piece will split off as big as a 20c piece (1inch diam). more
often than not the pot is ruined. This is limited to one body [i use 3] and
1 glaze celadon. I use the body with other glazes and no problems. It seems
to be worse when the piece is very thinly thrown. The recipe for the glaze is
wollastonite 18
Dolomite 4
feldspar 35
kaolin 14
silica 33
iron oxide 1
gerstsley borate 1.5
*CaO -------- .68
*MgO -------- .08
*K2O -------- .17
*Na2O -------- .06
*Fe2O3 -------- .00
Al2O3 -------- .44
SiO2 -------- 4.34
L.O.I.
RATIO 9.76
EXPAN 6.70
WEIGHT 367.75
i would be gratefull for any ideas you may have on this one
cheers Ray Carlton
McMahons Creek Victoria Australia
Erin Hayes on thu 6 may 99
Ray,
Do you think this is lime-popping from gypsum contamination in your clay?
Have you changed clay suppliers recently or have they changed their own
supplier?
This happened to me in grad school - I'd have pops all over my work from the
time it came out of bisque to after glazing. We had a bad bag of fire clay
that had gypsum flakes in it. If this is what you're seeing, you ought to
see a little tiny flake of white at the center of each pop.
Maybe someone else will have a different explanation. But it does sound
woefully familiar...
Good Luck!
Erin.
Pierre Brayford on thu 6 may 99
-----Original Message-----
From: Ray Carlton
To: CLAYART@LSV.UKY.EDU
Date: 05 May 1999 17:33
Subject: popping bisque???????
Sounds like lime/plaster contamination.
If it is you should be able to see the remains of the lime at the bottom of
the crater.
I have not had this happen at the glazing stage, BUT much worse after the
pot has been fired and is in the show room sometimes after several weeks. I
guess the time it takes atmospheric moisture to penetrate stoneware clay.
Keep a watch on the pots that survive glazing to see if any pop later!
Pierre Brayford
Shropshire UK
----------------------------Original message----------------------------
hi all here is a little puzzle that has me stumped and I am wondering if
any body else has seen this phenomenon.
pop and a little piece of clay
literally spits of the surface of the piece and leaves a small hole hole.
sometimes a piece will split off as big as a 20c piece (1inch diam). more
Michael Banks on fri 7 may 99
Sounds very like lime-popping to me, but why it should only happen when one
specific glaze is applied, beats me.
Lime popping is due to coarse calcium carbonate or other calcium salt such
as plaster, contaminating the clay body. The offending particles lose their
volatile matter during bisc firing, are reduced to calcium oxide and
rehydrate explosively at a later date, especially in contact with water. If
the particles are fine-grained (say, minus 100 microns) there is often no
problem because they sinter with surrounding silicates. Too coarse, and a
lump of CaO is created, lying in wait for a bit of atmospheric or liquid
water to turn it into a little bomb.
Michael Banks,
Nelson,
New Zealand
----------------------------Original message----------------------------
hi all here is a little puzzle that has me stumped and I am wondering if
any body else has seen this phenomenon.
I get the pots out of the kiln and wax them as per usual. (The pots are at
room temperature) I glaze them soon after. The pots that go through my
celadon glaze pop. I am not kidding they go pop and a little piece of clay
literally spits of the surface of the piece and leaves a small hole hole.
sometimes a piece will split off as big as a 20c piece (1inch diam). more
often than not the pot is ruined. This is limited to one body [i use 3] and
1 glaze celadon. I use the body with other glazes and no problems. It seems
to be worse when the piece is very thinly thrown. The recipe for the glaze
is
wollastonite 18
Dolomite 4
feldspar 35
kaolin 14
silica 33
iron oxide 1
gerstsley borate 1.5
*CaO -------- .68
*MgO -------- .08
*K2O -------- .17
*Na2O -------- .06
*Fe2O3 -------- .00
Al2O3 -------- .44
SiO2 -------- 4.34
L.O.I.
RATIO 9.76
EXPAN 6.70
WEIGHT 367.75
i would be gratefull for any ideas you may have on this one
cheers Ray Carlton
McMahons Creek Victoria Australia
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