Dave Finkelnburg on sun 17 dec 00
Mary Lynch wrote:
I've had concerns about lithium in glazes on functional ware and have been
using strontium instead, as suggested by another potter (maybe on Clayart?).
So far the glazes have all worked fine (using 3/4 as much strontium).
If you understand glaze chemistry, you can ignore this. For those who don't
know, however, I would like to try to explain some small part of this as I
understand it. When you substitute one flux or other glaze ingredient for
another and try to maintain the same effect, you begin by trying to
substitute the same number of molecules of the new ingredient as of the
ingredient used previously.
We can't measure number of molecules directly, but can measure weight.
We relate weight of a substance to the number of molecules using a unit
called a mole.
The exact number of molecules in a mole isn't important, but knowing that
every mole of anything has the same number of molecules is a very powerful
understanding. If you know how to figure the weight of a mole of something,
and know how many grams of it you have, then you can figure how many moles
of that material you have and you can substitute the same number of moles of
whatever else you want to use.
One mole of anything is simply the same number of grams of that material
as the molecular weight of the material, For example, the atomic weight of
lithium is about 7, so a mole of lithium weighs 7 grams. The atomic weight
of strontium is about 88, so a mole of pure strontium weighs about 88 grams.
To replace a mole of lithium with a mole of strontium use 88/7 = 12.6
times as much strontium!
How can this be? Because lithium is a very small, light atom, compared
to strontium.
Lithium, of course, can behave somewhat differently in glazes than
strontium. Just making the direct substitution doesn't mean the glazes will
be the same. It does mean you've got a new place to start testing that is
as close as possible to where you were, but with the new material.
What's the point of all this? That by using a little bit of chemistry
you can save a lot of time and expense testing in the lab to use different
materials.
I hope some of you find this helpful! :-)
Dave Finkelnburg
Idaho Fire Pottery
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