Stuart Altmann on wed 15 may 96
When I posted the composition of Cornwall Stone and pointed out that its
low value for L.O.I. (loss on ignition, mostly due to loss of CO2 from
carbonates), which is less than 2%, means that it is unlikely to be a source
of pinholes, compared with other common glaze ingredients. Ron Roy said
that the value was actually high compared with other feldspars. Well, this
is true but irrelevant, because the L.O.I of Cornwall is dwarfed by that
of many other common glaze ingredients.
Common feldspars have L.O.I. values around 0.1 to 0.3%. E.g., Kona F-4 is
0.2, K-200 is 0.26, Custer is 0.3, and so on. (Values from Standard
Ceramics, 1983). Very low, indeed.
Most clays have L.0.I. values of 11 to 14%. E.g., OM-4 at 11.4%, EPK at
13.8%, sapphire at 13.9%. Redart is unusually low for a clay: 4.78%.
(Values from A.R.T. Studio Clay Co. 1996). So, clays, both in our clay
bodies and our glazes, are producing gasses during firing.
The really gassers are the carbonates. Many high fire glazes have
considerable quantities of whiting, and perhaps also dolomite, barium
carbonate, strontium carbonate, or magnesium carbonate. Unfortunately, I
don't have actual values at hand, but I recall L.O.I. values in the vicinity
of 20-30% for these materials. Now, that's gas!
Stuart Altmann
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