Paul Lewing on mon 23 apr 01
A while back, several months ago, I got a glaze recipe off the list
called Sea Blue. It sounded interesting, so I made a test and it looked
good- an unusual color of blue- a Ni blue, quite matte. So in my last
firing I put it on a bunch of tiles, as one of the glazes in a bunch of
landscapes. None of the tiles were just this glaze; they all had many
glazes on them. Well, every single tile that had this glaze on it
cracked!
Here's the data on it:
Sea Blue
GERSTLEY BORATE 15.65
LITHIUM CARBONATE 6.96
CUSTER FELDSPAR 42.61
EPK KAOLIN 13.04
FLINT 21.74
ZINC OXIDE 20.89
nickel oxide 1.3
CaO .11
Li2O .19
MgO .03
K2O .09
Na2O .07
ZnO .52
B2O3 .13
Al2O3 .25
SiO2 1.97
Ratio 8.03
Expan 7.16
Now, that expansion number is usually a number that fits my clay body
perfectly, but obviously this stuff doesn't. And yes, I do realize how
far outside the usual limits this is on several oxides, so we don't need
to go into that.
So here's my question. Is it just that we don't know how to calculate
the expansion of lithium when it's this big a part of the glaze? I'm
assuming it's the high level of lithium that's causing the problem. Or
is it that lithium carbonate is soluble and is soaking into the body and
weakening it? If the glaze was really a lot nicer than it is, it would
be worth recalculating it with spodumene or petalite, but I don't think
I care enough to do that.
Anyone care to speculate?
Paul Lewing, back in Seattle after a very satisfying workshop at a truly
wonderful little clay program at Yakima Valley Community College in
Yakima, Washington, run by Clayarter Erin Hayes.
Ababi on tue 24 apr 01
I think with my limit knowledge, I know what happens I have seen it before.
I believe it connected to surface tension. You can try to avoid it (incase
that the glaze is runny) by putting the test tiles on stilits. Otherwise it
play with the shelves and breaks tiles like they were small houses in
earthquake!
Ababi Sharon
ababisha@shoval.ardom.co.il
http://members4.clubphoto.com/ababi306910/
----- Original Message -----
From: "Paul Lewing"
To:
Sent: Monday, April 23, 2001 10:53 PM
Subject: glaze that breaks tile
> A while back, several months ago, I got a glaze recipe off the list
> called Sea Blue. It sounded interesting, so I made a test and it looked
> good- an unusual color of blue- a Ni blue, quite matte. So in my last
> firing I put it on a bunch of tiles, as one of the glazes in a bunch of
> landscapes. None of the tiles were just this glaze; they all had many
> glazes on them. Well, every single tile that had this glaze on it
> cracked!
> Here's the data on it:
>
> Sea Blue
> GERSTLEY BORATE 15.65
> LITHIUM CARBONATE 6.96
> CUSTER FELDSPAR 42.61
> EPK KAOLIN 13.04
> FLINT 21.74
> ZINC OXIDE 20.89
> nickel oxide 1.3
>
> CaO .11
> Li2O .19
> MgO .03
> K2O .09
> Na2O .07
> ZnO .52
> B2O3 .13
> Al2O3 .25
> SiO2 1.97
>
> Ratio 8.03
> Expan 7.16
>
> Now, that expansion number is usually a number that fits my clay body
> perfectly, but obviously this stuff doesn't. And yes, I do realize how
> far outside the usual limits this is on several oxides, so we don't need
> to go into that.
> So here's my question. Is it just that we don't know how to calculate
> the expansion of lithium when it's this big a part of the glaze? I'm
> assuming it's the high level of lithium that's causing the problem. Or
> is it that lithium carbonate is soluble and is soaking into the body and
> weakening it? If the glaze was really a lot nicer than it is, it would
> be worth recalculating it with spodumene or petalite, but I don't think
> I care enough to do that.
> Anyone care to speculate?
> Paul Lewing, back in Seattle after a very satisfying workshop at a truly
> wonderful little clay program at Yakima Valley Community College in
> Yakima, Washington, run by Clayarter Erin Hayes.
>
>
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Stephani Stephenson on tue 24 apr 01
Paul
Unfortunately I have more questions than answers!
Could you describe the break? did it split the tile down the center?
You said there were many glazes on each tile. Was the location of the
break in direct correlation to the location of the glaze?
Did the break occur in heating or cooling ? Could you tell?
Have you fired tiles with that glaze in any other firings or was the
breakage isolated to one firing / kiln load only?
What temp were you firing to?
I have used similar amounts of lithium in glazes without incident, but
not perhaps at as high of a temp as you.
Stephani
Alchemie Studio
Leucadia CA
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