Carolyn Sawyer on fri 31 dec 99
This is a beautiful glaze from CM Nov. 76. I have been able to come up with
really nice runny blues, greens, yellows with flourspar but often the glaze
craters and crazes. Can someone help? Is it the cryolite?
C5 Ox
LiCo 7%
flourspar 10% or Na flouride
Cryolite 31%
Frit 25P 48%
Kaolin 4%
rutile 6%
CuO 1% or CuCO
Thank you in advance for any advice.
Carolyn
Campobello SC
Hank Murrow on sun 2 jan 00
>----------------------------Original message----------------------------
>This is a beautiful glaze from CM Nov. 76. I have been able to come up with
>really nice runny blues, greens, yellows with flourspar but often the glaze
>craters and crazes. Can someone help? Is it the cryolite?
>C5 Ox
>LiCo 7%
>flourspar 10% or Na flouride
>Cryolite 31%
>Frit 25P 48%
>Kaolin 4%
>rutile 6%
>CuO 1% or CuCO
>Thank you in advance for any advice.
>Carolyn
>Campobello SC
Good Morning Carolyn; Very likely to be the Cryolite, as it gives off
Fluorine vapors during the melt. I reccommend a soak during the cooling,
perhaps around C/1 so the runnies won't worsen. Good Luck, Hank in Eugene
Susan Goldstein on sun 2 jan 00
In a message dated 12/31/1999 7:27:43 PM Eastern Standard Time, Cerere writes:
<< i gave away flourspar when i read that it ruins the elements in the kiln.
>>
A friend sent me the above information. Is it accurate? Is there a
substitute?
Susan
Craig Martell on sun 2 jan 00
Hello Carolyn:
The glaze is cratering from the release of flourine gas from both the
fluorspar and cryolite. You would have to lower the cryolite and probably
the fluorspar too in order to stop this. This glaze crazes because it is
an extremely high expansion glaze. Actually, it's one of the highest that
I've seen. The silica content is very low too and I wouldn't use this
glaze on functional ware that is for food contact. Not a very durable glass.
My estimation is that this glaze would need some very serious adjusting and
would in all probability be a totally different glaze if it were tweaked
into something more trouble free. Calculations from Insight 5 are below
lithium carbonate... 7.00 7.00%
fluorspar........... 10.00 10.00%
cryolite............ 31.00 31.00%
frit p-25........... 48.00 48.00%
epk kaolin.......... 4.00 4.00%
========
100.00
CaO 0.20* 7.18%
Li2O 0.20* 3.86%
MgO 0.00* 0.01%
K2O 0.06* 3.53%
Na2O 0.54* 21.94%
ZnO 0.01* 0.46%
TiO2 0.00 0.02%
Al2O3 0.25 16.70%
B2O3 0.24 10.99%
P2O5 0.00 0.01%
SiO2 0.90 35.26%
Fe2O3 0.00 0.04%
Cost/kg 2.10
Si:Al 3.58
SiB:Al 4.55
Expan 13.66
Craig Martell in Oregon
Michael Banks on sun 2 jan 00
The crazing is caused by the high soda content and low silica and is
unavoidable in glazes of this type.
As for the cratering: In my opinion this glaze only has about 50% of the
silica it requires for firing to cone 5. It would be an obviously
flux-saturated composition at this temperature, even without the high
fluorine introduced by the fluorspar and cryolite in the recipe. High
fluorine to silica ratios in a glaze means that the crosslinked silica
(SiO4) tetrahedra in the glass structure are completely converted to
volitile silicon fluorides and boil off. Apart from cratering the glaze,
these gases are quite a nasty health hazard. At 41% fluorides, this glaze
is over 20% fluorine and requires a heck of a lot more silica at cone 5 to
contain it, even if it is possible at all.
Michael Banks,
nelson,
New Zealand
> ----------------------------Original message----------------------------
> This is a beautiful glaze from CM Nov. 76. I have been able to come up
with
> really nice runny blues, greens, yellows with flourspar but often the
glaze
> craters and crazes. Can someone help? Is it the cryolite?
> C5 Ox
> LiCo 7%
> flourspar 10% or Na flouride
> Cryolite 31%
> Frit 25P 48%
> Kaolin 4%
> rutile 6%
> CuO 1% or CuCO
> Thank you in advance for any advice.
> Carolyn
> Campobello SC
>
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