Tom Buck on wed 9 dec 98
In a typical C6 oxidation glaze, substituting G-200 for Custer, or F4 for
either of those, will NOT make any significant difference to the fired
glaze on most claybodies. The level of alkali metal oxides in the spar
tells the story>
G-200 - 10.7% K2O (potassia); 3.0% Na2O (soda).
F-4 - 4.8% K2O; 6.9% Na2O.
Custer- 10.1% K2O; 3.0% Na2O. G-200 and Custer spars are almost
equal in these flux oxides; F-4 has less KNaO, more CaO.
For a C6 firing in oxidation, K2O & Na2O have equally strong
fluxing action, with CaO close behind. So if the only change in the recipe
is replacing one spar for another (same quantity), then the fired glaze
will exhibit the same characteristics (assuming matching firing cycles) on
most claybodies. Yet, since Na2O has a higher coefficient of thermal
expansion than that of K2O and CaO, on a few bodies some crazing may occur
with F-4 feldspar.
The one-for-one replacement does introduce a noticeable change in
a C10 reduction firing for copper red - K2O tends to give Cu reds a
bluish/purplish tinge whereas Na2O, if dominant, tends to impart an
orangey tone. A similar different hue might occur with rutile blue glazes,
the blue tone would vary, although the claybody could have a greater
effect than the change in flux oxides.
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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