RED CLAYS

can anybody explain why some red clays will fire to stoneware
temperatures? obviously, the presence of red iron oxide is going to
bring the vitrification point down so what is it that allows some red
clays to be fired to temperatures used in, say, ash glazing as used by,
amongst others, Bernard Leach and Phil Rogers (at least the clay appears
to be red). I've also found red clays used in glaze recipes that fire up
to cone 9 and possibly beyond.

On Mon, 18 Aug 2003 20:15:18 +0100, wrote:

can anybody explain why some red clays will fire to stoneware
temperatures? obviously, the presence of red iron oxide is going to
bring the vitrification point down so what is it that allows some red
clays to be fired to temperatures used in, say, ash glazing as used by,
amongst others, Bernard Leach and Phil Rogers (at least the clay appears
to be red). I've also found red clays used in glaze recipes that fire up
to cone 9 and possibly beyond.

The vitrication point of clays in determined largely by the total
amount of fluxes. Iron oxide does act as a flux, though much more so
in a reduction atmosphere than in oxidation. Clays like Redart (see
the analysis below) contain a fair amount of other fluxs.

% 0.38 Na2O 4.15 K2O 1.59 MgO 0.26 CaO 15.51 Al2O3 64.95 SiO2
1.09 TiO2 7.05 Fe2O3 0.22 P2O5 4.80 LOI

If you could removed the Na, K, Mg, and Ca, the vitrification point
would be much higher. So, it's certainly possible to have a red clay
that fires to cone 10 - BUT you probably will need to avoid a
reduction atmosphere.

deg