louroess2210 on sun 18 mar 07
HI, I recently bought glaze software and have some questions.
For cone 6 glazes, I understand it is good to have the COE "around"
70. How far off can I be and still be in the ballpark? Is 65 or 75
still OK? How about 68 and 72? Any further guidelines here would be
appreciated.
The Si and Al limits for cone 6 are given as Silica, from 2.5 to 4.0
with alumina at between .25 and .50.
Is it a good idea to match the highness or lowness of the two or
doesn't it matter. In other words, if the silica is at the high end,
at 4, should the alumina be at the high end also, or could the
alumina be at.25, the lower limit, and the recipe still be OK?
Thanks for any help anyone can give on this.
All the best,
Lou
Dave Finkelnburg on mon 19 mar 07
Lou,
There is no easy answer to your first question.
The Coefficient of Expansion (COE) you need depends on
the clay body you use. At cone 5 or 10 or 3,
porcelain will be "about" one full digit lower in COE,
using your units, than stoneware. Say 6.5 for
porcelain, 7.5 for stoneware, plus or minus about 0.5
depending on the body. Remember, also, that how you
fire influences the amount of glass phase in the body,
which controls the COE of the body, so not everyone
gets the same COE from the exact same clay.
You have it right that the Silica:Alumina ratio is
what you want to watch. In general, though, at cone
5, 2.5 moles is enough silica on a UMF basis, with
between a 7 and 10:1 SiO2:Al2O3 ratio if you want a
glossy glaze. Be at or below 5:1 for a matte, more or
less.
This gets complex, though. If you have a high
level of alkalis in the glaze (Na,K,Li) or a bunch of
boron, you can melt more silica than if you have a lot
of alkaline earths (Mg,Ca,Ba,Sr) or no boron.
The bottom line here is you will get the best
information when you mix a couple of glazes from the
extremes of the limits and fire them on your clay in
your kiln. That sort of testing will quickly give you
a feel for what the calculations are telling you.
Good glazing!
Dave Finkelnburg
From: louroess2210
HI, I recently bought glaze software and have some
questions.
For cone 6 glazes, I understand it is good to have the
COE "around"
70. How far off can I be and still be in the
ballpark? Is 65 or 75
still OK? How about 68 and 72?
The Si and Al limits for cone 6 are given as Silica,
from 2.5 to 4.0
with alumina at between .25 and .50.
Is it a good idea to match the highness or lowness of
the two or
doesn't it matter. In other words, if the silica is
at the high end,
at 4, should the alumina be at the high end also, or
could the
alumina be at.25, the lower limit, and the recipe
still be OK?
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