Craig Martell on wed 7 jun 00
Hi:
I sent in a post about Gibby's when the server was vacationing in Tierra
del Fuego and maybe it didn't get to the list but is lost somewhere on the
beach waiting for Magellan to round Cape Horn.
I've looked at Gibby's before because there were posts saying that this
glaze shivered on some bodies but it has a very high calculated expansion
which would lead one to believe that it would craze instead. Anyway,
Gibby's has more than .3 moles of lithium in flux unity (seger form.) and
the suggested limit is .2. My feeling is that there is so much lithium in
this glaze that it is migrating into the clay glaze interface and causing a
very low expansion situation there that is producing shivering in some
folks work. Lithium is the lightest metal and gram for gram in a recipe
one gets a lot more lithium ions by weight than any other oxide so we have
to be careful about how much we use. Lithium has a "high coefficient of
diffusion" too, meaning that it is very mobile and can really travel
through a clay/glaze matrix. Michael Banks suggested that the migrating
lithium heads into the interface and claybody to form spodumene with the
other alumino silicates present there. I believe that he is correct. So
the possibility of Gibby's leaching lithium is probably high and if it were
to be used with food service pieces I would caution anyone to have it
tested on your ware at your firing temp and see what developed. If I were
thinking of using this glaze, I would lower the lithium and raise the
silica a bit to make it more durable and less likely to be giving up it's
oxides to your vinegarette or tomato sauce. Also, if your ware is prone to
shivering with Gibby's, you don't want folks to be getting those little
extras, ie shards, in their Tuna Surprise.
for what it's worth, Craig Martell in Oregon
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