Cathie or Dave Feild on fri 31 jul 98
Hello fellow potters. I've heard of glazes getting too thick sitting in
buckets over time, but has anyone heard of them getting thinner? This
is currently happening with several of our glazes. The last time we
used them, they were fine. The next time, too thin. This time we
removed the excess water that floats on the top of the glaze after it
settles out over time, stirred them well, tested the specific gravity,
added only about half of the water back, re-tested the s.g. and found
that we had gotten them back to the right consistency by removing some
of the water. How/why are they getting thinner/too much water over
time, on their own? The recently glazed pieces have not been fired yet,
so we'll see if we obtain the results we're used to in this latest go
round. Thanks for any help.
Cathie Feild
cfeild@cvn.net
Ron Roy on sat 1 aug 98
Hi Cathie,
You probably have a source of soluble sodium in your glaze and that leads
to deflocculation. It maybe possible to replace the offending material
(usually Neph Sy) - if you want to send me the recipes I can probably fix
them - the other option is to add a flocculant like epson salts - the
better way is to get the solubles out.
>----------------------------Original message----------------------------
>Hello fellow potters. I've heard of glazes getting too thick sitting in
>buckets over time, but has anyone heard of them getting thinner? This
>is currently happening with several of our glazes. The last time we
>used them, they were fine. The next time, too thin. This time we
>removed the excess water that floats on the top of the glaze after it
>settles out over time, stirred them well, tested the specific gravity,
>added only about half of the water back, re-tested the s.g. and found
>that we had gotten them back to the right consistency by removing some
>of the water. How/why are they getting thinner/too much water over
>time, on their own? The recently glazed pieces have not been fired yet,
>so we'll see if we obtain the results we're used to in this latest go
>round. Thanks for any help.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Cathie or Dave Feild on mon 3 aug 98
Hi Ron. Thanks for offering to help me out with this problem. I
thought deflocculation was a good thing and that when glazes
"flocculated" that meant they got clumpy and lumpy and people added
epson salts to make them deflocculated! I have been a member of Clayart
off and on for about a year now and I know this subject has been
discussed many times. I even recall a discussion fairly recently about
the definition of flocculation and deflocculation. Either I didn't pay
enough attention, or I got terribly confused. My husband and I also
have many reference books, including the very popular "Hamer's" so as
soon as I finish here, I'm going to look these terms up, and more, and
read about them again! My problem is (and I know many others share
this) I got through high school and college without ever taking a
chemistry class! Every time I read about this stuff, it's almost like
I'm reading it for the first time. I think I'll have to read about and
work with glaze chemistry for about 20 years before it all starts to
come together.
Now that I've taken up all this time with background info, I'll just
start with one of the offending glaze formulas. There are others and
maybe we can talk about those later. The followiing formula is for a
glaze called "Dark Blue," a cone 6 oxidation glaze obtained from a
potters group in Montgomery County, MD:
Custer Feldspar, 48%
Dolomite, 19%
Zinc Oxide, 9.5%
EPK, 15%
Silica, 8.5%
ADD:
Cobalt Oxide, 1.5%
Copper Oxide, 1.0%
Manganese Ox., 2.0%
We've found it has to go on really thickly to obtain a blue color rather
than a green. It has a tendency to pinhole when used by itself, but we
use it over another glaze which seems to eliminate that problem. The
issue is that after sitting for a week or so, it is too thin when it is
stirred back up, so we end up skimming the water off the top and then
mixing it back to the thicker consistency. My original question was
basically, where does this "extra" water come from? Thanks again.
Cathie Feild
cfeild@cvn.net
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