Craig Martell on fri 9 apr 99
Hi:
At the risk of being redundant, I'd like to offer a theory or two about why
lithium bearing materials do such odd things with glazes from time to time.
Every so often a post will come in about a glaze shivering and bingo, it
contains some sort of lithium mineral. Lithium has a low coefficient of
expansion so we might expect to see problems such as shivering and
shattering in lithium bearing glazes. But, lithium's COE isn't as low as
boron and I think it is comparable to zinc and magnesia and we don't
associate these sorts of glaze faults with the other low expansion oxides.
So, why does lithium do what it does? Beats the hell out of me, I thought
you guys knew!!
I think that the problems may result from lithium's high coefficient of
diffusion, or it's ability to migrate and penetrate any kind of silicate
melt. Lithium is the lightest metal oxide, and the ions are pretty small so
it can move around pretty easily in certain types of silicate chains. This
sounds pretty heady but I don't have a real firm grasp of the chemistry
involved here and I hope some of you who do will help me out. I know that
there has to be a place in the chain that can be occupied by the lithium
ions.
One really odd thing that is reported to happen, especially with lithium
carbonate is crazing and shivering in the same piece. How can this happen
when the two faults are at opposite ends of the expansion spectrum? I
calculated the expansion of a Lithium carb glaze that was shivering on some
pots and found the expansion to be high enough that the glaze should have
crazed. So, I'm thinking that the high coefficient of diffusion has allowed
a penetration of the clay-glaze interface by lithium ions and since this
area contains more alumina and silica and will have a lower COE than the
glaze matrix, shivering will occur at the interface while there is some
crazing happening in the glaze matrix. I'm also still not sure that I
believe this is possible although many folks have said that this happens.
Sometimes people mistake shivering for crazing, if the shivering is not bad
enough to cause pieces of glaze to pop off taking some of the claybody too.
Or, there is fracturing and release of chunks of clay and the remaining
fracture lines on the pot are mistaken for crazing. One way to avoid this
error is to use a hand lens and examine the glaze closely to see if the
matrix if fractured by being pushed together, which is from compression
(shivering), or if the matrix has been pulled apart and there is a small
separation between the crack lines, which is from tension (crazing).
Another reason to be cautious with lithium carb is that it's weight to the
glaze batch is 40% lithium. Spodumene and petalite are alumino silicate
lithium spars and contribute less lithium by weight to the batch. One glaze
I calculated had about 10% lithium carb in the receipe. When this was
converted to the seger formula, it was .37 moles and was the major fluxing
oxide in the glaze. Also, lithia begins it's fluxing action between cone
015 and 014 so lithium carb is going to get pretty active and mobile whereas
spodumene and petalite with probably hang on to their lithium ions a bit
longer and retard these migrant little pests! I'm just kidding about the
pest thing. Actually, lithium is a wonderful material with very good color
response. I guess we just need to keep an eye on things. I hope some other
folks will add to this, if it's of interest. Anyone still awake?
regards, Craig Martell in Oregon
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