Craig Martell on sat 10 jan 98
Hi:
Thanx to all the folks who sent the Wild Rose Tenmoku receipe.
This is an unusual glaze formula in several respects. The biggest concern
and mystery is: why does a high expansion glaze shiver on some clays? The
info that we have and common knowledge says that high expansion glazes
craze, which is the opposite of shivering.
I talked to Jim Robinson today, an Oregon potter with an intense interest in
clays, glazes, and all sorts of technical stuff. He agrees that the problem
is being caused by the lithium carbonate in the glaze. Jim thought that the
idea of the problem arising in the area of the clay/glaze interface was
right on the money. Thanks to Tom Buck for that idea. Anyway, lithium has
a "high coefficient of diffusion", and is very mobile and also a very potent
low expansion oxide. What is probably happening is lithium ions are
penetrating into the clay body and causing a low expansion situation at the
interface which will cause a shivering-shelling kind of fault. The low
expansion reaction is formed by lithium interacting with the alumina and
silica in the body. Jim says that it's not like the usual type of shivering
that we see when glazes are under mega-compression from having a much lower
expansion than the claybody. There is more to this but I won't bore you
guys with it unless I'm requested to do so.
Possible remedies are: Exchange the lithium carb for a naturally occuring
form of lithium alumino silicate such as, spodumene or petalite. There is
..37 moles of lithium in wild rose so you can only sub in so much spod or
petalite before you supply too much alumina and silica. So, use as much
spod or petalite as possible and supply the rest with lithium carb. The
idea is to use a type of lithium that has a lower coefficient of diffusion
and spod and petalite will probably not penetrate as deeply into the
interface being somewhat linked to the silica and alumina in the parent
mineral. The amount of lithium can be reduced and some of it replaced with
boron. The lithium can be lowered a bit and replaced with a small amount of
soda ash which will MAYBE counter the low expansion problem at the
interface. This last one is real iffy, but maybe worth a shot. Try the wild
rose glaze on several types of claybodies, stoneware, porcelain, even
flameware and observe the results. This is the easiest-don't use the glaze!!
That's it for now. I hope others will post some ideas about this if there is
an interest. I think lithium carb is a good material but it should be used
with caution and discretion. It is a very light material on the molecular
level and small percentages will supply a lot of lithium molecules to a
glaze. Wild Rose has 9.5% lithium and this supplies .37 moles to the glaze
making it the major fluxing oxide in wild rose.
tired of lithium in Oregon, Craig Martell
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