Jon Singer on sat 30 oct 99
Substitution of lithium minerals for some or all of the
feldspar in a glaze almost always reduces the thermal
expansion. In other words, if your glaze crazes and you
introduce some lithium, it will craze less. You can use
this effect to match the glaze very nicely to the body.
(On the other hand, if it is perfectly matched already
then you are looking for trouble, because lithium may
very well cause it to shiver.)
Unfortunately, the effect of lithium is not easy to
calculate, and programs like Richard Burkett's
_HyperGlaze_ will warn you of this. (The effect
apparently changes with concentration in a way that
is not very predictable.) This means that when you do
use lithium minerals to adjust the thermal expansion,
to a certain extent you are on your own -- your results
may not always agree precisely with what your glaze
calculation program tells you to expect.
Be extremely cautious about using lithium carbonate,
btw -- it is slightly soluble, and fairly toxic. (Because
it is soluble, if you use it in a glaze it can soak into the
bisqued body and cause fluxing. In the worst case your
pots can warp & slump or even collapse, though this
seems somewhat unlikely.)
PS: There isn't any lithium feldspar. Spodumene has
4 silica units per molecule, the alkali feldspars have 6,
and Petalite has 8. This means that if you use these
lithium minerals, you are changing the amount of
silica in your glaze in addition to the way the flux works.
It's a good idea to keep this in mind when you start
messing around. If the look or texture of your glaze
depends on a particular silica to alumina ratio, you can
mess it up very easily.