Tom Buck on sun 5 jan 97
Sam Cuttell's request for a revision of this glaze from C6 to C10, both in
oxidizing environment, offers an opportunity to give voice to a particular
aspect of midfire and highfire stoneware (porcelain too).
Firstly, Tony Hansen correctly states that this glaze is, as
formulated, is a balanced glaze for C6 and should be fired as such if the
claybody is suited for C6 maturity. The glaze has strong fluxes in
nepheline syenite (Na2O) and Colemanite (CaO and B2O3) and these
compensate for the high alumina and silica contents that are more suited
It was also suggested that the glaze could be adjusted by lowering
the alumina and silica. If that were done without mixing with a proven C10
glaze, then the Floating Blue would likely flow off the pot.
The needed adjustment is a lowering of the B2O3 content, since it
is well above the tolerable level for C10 (usually C10 stoneware glazes
contain zero B2O3). In a glaze with a Seger Formula of 1 (fluxes):0.5
(alumina):5.0 (silica) any level of B2O3 above 0.3 moles will act more as
a flux than as a glass-former, so the glaze will become too fluid and
therefore will run badly.
Yet, the name "Floating Blue" gives a hint at what this glaze
offers, namely a streaky glossy blue that doesn't much hide the body. It
suggests a fluid glaze at C10 that is on the verge of flowing if
overfired. So, some B2O3 is beneficial, as would be a change of Na2O to
K2O via use of a potash feldspar (G200 or Custer).
Putting this all together would suggest this recipe as a first try
at getting a successful C10 Floating Blue:
Floating Blue Glaze C10 Ox. (a blue gloss with white streaks)
52.0 G200 (or Custer) feldspar
11.5 Gerstley borate
5.5 EPK (or equal)
add 4% Rutile fine mesh, 2% Iron oxide red, 1% Cobalt oxide. Screen well
to disperse the colourant oxides.
Cheers TomB Hamilton ON Canada URL http://digitalfire.com/magic/tombuck.htm