Autumn Downey on sun 22 jul 01
I was going to title this someting about cone 6 clear glazes, but I didn't
want to discourage David Hendley from reading this message.
However, it's to do with fitting a clear glaze to Plainsman M370, so Ron
Roy and Tony Hansen are the experts in this area, hope you're there!
For awhile we used a clear that Ron had tweaked for us, recipe to follow.
It worked fine and then started to craze like crazy! (It did have some GB
content, so I blamed the crazing on the solubility). And, in the past
being good (cheap) potters, we have always combined the remainder of old
glazes with the new ones that we make. (Otherwise there really would be
alot of waste!)
Then we switched to a glaze that Ron had formulated for porcelain and again
all was well (recipe to follow) - for awhile. Now it crazes. Not always
right away, but sometimes in a couple of weeks after firing. Seemingly a
fresh batch is ok, but older batches or combined batches are not.
(Both these glazes are fine on other Plainsman clays - eg. M340)
The ingredients that could be soluble are the frit 3134 and maybe
wollastonite??
I've noticed that a glaze like the 5x20 of Tony's - with zircopax for
white, never crazes on M370. Could it be that the clears are too much
like a real glass and have even more fit problems on a porcelain-like
stoneware? Would the zircopax have some stablilizing effect on the glaze?
I am testing 5% of zirco in one clear now - to see if it stops the crazing
- of course it won't be clear anymore, but...
Also, all these high calcium glazes tend to form little "nodules? or
hailstones?" in the bottom of the bucket after awhile, so there must be
something going on in the solubility department?
Any thoughts would be appreciated.
Autumn Downey
Yellowknife, NWT glazes->
Clear #1
=============
F-4 FELDSPAR........ 2170.00 31.00%
SILICA.............. 1470.00 21.00%
FRIT 3134........... 350.00 5.00%
GERSTLEY BORATE..... 700.00 10.00%
EPK KAOLIN.......... 1190.00 17.00%
WHITING............. 910.00 13.00%
TALC................ 210.00 3.00%
========
7000.00
CaO 0.65* 12.15%
MgO 0.12* 1.65%
K2O 0.06* 1.74%
Na2O 0.17* 3.49%
TiO2 0.00 0.07%
Al2O3 0.42 14.24%
B2O3 0.19 4.48%
P2O5 0.00 0.05%
SiO2 3.10 61.92%
Fe2O3 0.00 0.21%
Cost/kg 1.55
Si:Al 7.38
SiB:Al 7.84
Expan 7.01
Notes:
This is a revision of BT clear. It should avoid crazing; also has more
silica. It might be foggy below cone 5 or 5 1/2.
Porcelain clear (RR's, substituted F-4 for Custer)
===============
SILICA.............. 24.00 24.00%
EPK KAOLIN.......... 27.00 27.00%
WOLLASTONITE........ 18.00 18.00%
FRIT 3134........... 27.00 27.00%
F-4 FELDSPAR........ 4.00 4.00%
========
100.00
CaO 0.82* 14.64%
MgO 0.01* 0.09%
K2O 0.01* 0.30%
Na2O 0.16* 3.20%
TiO2 0.00 0.12%
Al2O3 0.36 11.60%
B2O3 0.29 6.48%
P2O5 0.00 0.07%
SiO2 3.30 63.16%
Fe2O3 0.01 0.32%
MnO 0.00* 0.02%
Cost/kg 2.33
Si:Al 9.24
SiB:Al 10.06
Expan 6.71
Notes:
RR's clear - low expansion for porcelains.
Probably best without gum
as the kaolin swells up alot.
Ron Roy on mon 23 jul 01
Hi Autumn,
Some frits are somewhat soluble but I have never heard of 3134 being a
problem - so I am assuming that is not the problem. I am assuming the GB is
the problem - so here is that same glaze with no GB - the expansion is a
bit lower as well - it should look the same and I would doubt it would
change with age.
If you want to test for highly mineralized water - do a batch with
distilled water.
F4 - 24.0
3134 - 17.5
Whiting - 12.5
Talc - 4.5
EPK - 22.0
Silica - 19.5
Total - 100.0
Ratio - 7.37 (org is 7.35)
RR
>Then we switched to a glaze that Ron had formulated for porcelain and again
>all was well (recipe to follow) - for awhile. Now it crazes. Not always
>right away, but sometimes in a couple of weeks after firing. Seemingly a
>fresh batch is ok, but older batches or combined batches are not.
>
>(Both these glazes are fine on other Plainsman clays - eg. M340)
>
>The ingredients that could be soluble are the frit 3134 and maybe
>wollastonite??
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Ababi on tue 24 jul 01
I have two frits that crystallize (the B2O3) after a day or two. as it
has very high amounts. So what is the deal?
You can work with it better than with borax (spraying,mixing applying)
or boric acid. Just prepare what you need, kind of tricky, but this is
the cost of
37 or 38% boron in a frit
Ababi
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