Craig Martell on sat 12 jun 99
>Tom wrote some interesting notes on copper reds. I would take point with
>Tom. My history says that .5% is more than adaquet. I sometimes use
>less. The reason for the "runny" glazes is to form a glass early in the
>firing cycle and seal the copper in the glaze. Reduction is required
>from ^015 on up to ^8. I don't think it matters much after ^8. Another
>important point is slow cooling.
The main reason that copper reds tend to be "runny" is the necessity for a
glaze that is low in alumina. As one increases the alumina content of a
copper red glaze there is a tendency to develop more of a pink than a
brilliant red. Alumina is a color inhibitor in a lot of cases but not
always. Shino glazes are a good example of a high alumina glaze that has
red and orange enhanced by the alumina content. When copper reds become
glassy and melted, the copper is not sealed in as the heat weakens chemical
bonds and allows the copper to be reduced by carbon monoxide. As
temperature advances into the cone 9 and 10 range the copper becomes more
volatile and can be lost here if one lingers at this high temp for too long.
I take 2 hours to go from flat cone 8 to flat cone 10 and soak in light
reduction for 30 minutes before shut down. My good red pots are in the 70%
range and I think this is OK. I might also add that I start reduction at
cone 06 and reduce all the way to cone 10. I use med to light reduction
from cone 6 to cone 10.
regards, Craig Martell in Oregon