Gail Dapogny on thu 12 dec 02
I may have missed this (and if so apologize) but what is an ideal
percentage quantity or range of clay content in glazes? I seem to recall
that under 10% causes settling problems, but would like to know more. At
about what percentage should you not have to worry about adding epsom salts
or bentonite?
---Gail
Gail Dapogny
1154 Olden Road
Ann Arbor, MI 48103-3005
(734) 665-9816
gdapogny@umich.edu
http://www.silverhawk.com/ex99/dapogny (single historical photo - no longer
registered with Silverhawk)
Ron Roy on tue 17 dec 02
Hi Gail,
It depends on the materials in the glaze as well as the percent.
If there are solubles like Neph Sy and Lithium carb and many of the high
alkaline frits then you need more clay and/or perhaps bentonite and Epsom
salts.
If a glaze has less than 10 kaolin I would switch to ball clay right away -
if less than 5% clay I would think some bentonite - maybe 1 or even 2% and
even salts.
It really depends on how well the glaze suspends - I don't like adding
bentonite unless it's absolutely necessary and I don't like adding solubles
unless the bentonite does not work.
The best glazes - for suspension - have 15%+ clay in them and no solubles -
they keep forever and never settle out hard. It's a good reason to use a
frit like 3134 because it has no alumina in it - this means you can add
more clay.
My usual advice is don't try to make rules about this - if a glaze starts
acting up do whatever is necessary to tame it - ball clay for kaolin first
- then bentonite then salts. If that fails reformulate to get more raw clay
in (feldspar out) and minimize the solubles.
I usually start buy minimizing the solubles right off the bat - and finding
ways to get more clay in glazes with less than 10% - when you get enough
experience you learn to spot the potential troubles just by looking at the
recipe - lots of Neph Sy is going to mean problems - sooner or later for
instance.
RR
>I may have missed this (and if so apologize) but what is an ideal
>percentage quantity or range of clay content in glazes? I seem to recall
>that under 10% causes settling problems, but would like to know more. At
>about what percentage should you not have to worry about adding epsom salts
>or bentonite?
>---Gail
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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