Tom Buck on thu 27 nov 03
For Iron-3 Oxide to become Iron-2 Oxide, the Fe2O3 molecule
must emit an Oxygen atom. Original query: why is claybody red?
so we are discussing claybody colour, not glaze colour. the
claybodies most of us shape never melt at maturity (the odd one
comes close); the body sinters, ie some molecular/particular components
do highly localized go to liquid and cement the other body components
together to make strong ceramic piece.
since the iron oxide is well dispersed in the clay and other
ingredients, and likely will be firmly linked chemically to
alumino-silicate components, the red iron oxide will probably be hindered
in its decomposition at high cones to black iron oxide. but recall that
red-coloured bodies do go greyish when reduction is carried out
strongly....reducing agents (mostly gaseous) will penetrate deep enough to
change red to black (check the unglazed bottom of your pot).
in a glaze, however, red iron oxide is physically blended
in (not bound chemically as in a clay), so it will dissolve (not melt)
in the Liquidus, Cardew says 3-4.5%w is common and adds that high
temperature molten glaze (Liquidus) can dissolve higher amounts of iron
oxide and if the cooling is rapid the glaze will be supersaturated
in iron oxide (he gives as example Temmoku/Tenmoku as a glaze that holds
5%w iron oxide in a supersaturated solution).
til later. peace. Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada