Ivor and Olive Lewis on sat 24 feb 07
Point 1.
Kathi asked about pinholing in glazes containing Rutile and Ben =
mentioned "out gassing" at cone 10.
Perhaps the contaminant is Ferric Oxide, which in small quantities would =
have very little effect on colour of the raw material.
Ferric Oxide is unstable once the temperature rises much beyond 1300 deg =
C, breaking down to give black iron oxide and releasing Oxygen. Fe2O3 =
with a molar weight of 160 grams will discharge 16 grams of Oxygen
Remembering that one Gram Molecule of a gas occupies the rather large =
volume of 22.4 litres, about four and a half gallons from 16 grams of =
Oxygen, it would not need so much contaminating Iron oxide to create a =
few hundred bubbles per square inch.=20
Point 2
An indication of amount of Rutile that may be successfully added to a =
glaze recipe is provided by considering the degree to which Titanium =
Dioxide will form binary compounds with Silica, that is, to create =
Titanium Silicates. I say this because such a compound would have to =
crystallise from the melt, indicating that the molten oxides were =
mutually soluble in each other. The phase diagrams ( who would doubt =
their value to an informed potter?) shows that a the eutectic =
temperature (1550 deg C) the molten mixture separates into Rutile and =
Cristobalite. Similarly there is no reaction between Titania and =
Alumina. Perhaps this is why it is such a good opacifier.
But there is hope. The Mineral Sphene is a Calcium Titanium Silicate =
(CaTiOSiO4). So it is predictable that in any high temperature glaze =
containing Calcium oxide, if you get the proportions right, some Rutile =
will dissolve and crystallise on cooling. Sounds to me like a glaze test =
program is needed. Could be a call to arms for Mel.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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