Tom Buck on fri 11 dec 98
Sylvia:
Yeah, that term, LOI, tends to emerge off the page with no one
ever defining it. The letters are short for "Loss on Ignition" that is,
the stuff that goes up the flue when the raw material is heated. A good
example is Whiting aka CaCO3 or calcium carbonate. A chemist sees this
compound as CaO.CO2 and calculates its molecular weight as: 40+16 for CaO
and 12+32 for CO2. At 1000 oC (1800 oF) the CO2 separates and since it is
a gas it leaves the scene. So the LOI for whiting is 44 parts by weight in
the original 100 parts by weight, hence the LOI is 44%, a biggie in the
glaze chem list, and one that bugs a lot of potters because they do not
soak enough to heal the bubbles in the glaze as the CO2 leaves. Result is
pinholes. Which is why Tony Hansen, me, others prefer to use costlier
Wollastonite instead of low-cost Whiting wherever it makes sense (eg, when
the Whiting amount is high).
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
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