Celia Hirsh on sat 25 jun 11
First, a big thank you to everyone who gave me advice. It helped me
figure out what to do.
Second, here's the report of what transpired. I fired my kiln, set at
15 oxygen, at 150 degrees per hour, and it all went well. The kiln was
able to keep pace with the temperature rise, and only occasionally,
for a few minutes, did it go up to 20 oxygen, reaching Cone 10 in 10
hours. The atmosphere stayed clear and no flames came out the flue or
peepholes. And, the glazes on the test tiles - I'm searching for a
beautiful turquoise - proved that it was indeed an oxidation firing.
Which leads me to my final point. Since I was primarily firing test
tiles, which hardly take up any room, I filled the kiln with pots
that are 'seconds', in order to duplicate the packing of a regular
kiln load. Many of my 'seconds' are finished with a copper red glaze
and, at this point, have gone through several cone 10 reduction
firings. When I opened the kiln they were still red. I thought that
going through an oxidation firing would alter this. After all, if I
use this same copper red glaze in oxidation, it turns a lovely pale
I'm really curious. Can someone explain what happened? Why did the red
stay constant on re-firing in a different atmosphere?
John Hesselberth on sat 25 jun 11
Once a glaze seals over it is very difficult for reducing or oxidizing =3D
gases to get to the source of your red color which is probably buried =3D
thousands of "molecules" deep in the glassy matrix. The reason potters =3D
start reduction early to get good copper reds is that the reducing gas =3D
can penetrate a relatively open structure of an unmelted glaze and do =3D
their thing with those copper oxide molecules. Once the glaze has sealed =
over, the party is over except for some modest surface effects--unless =3D
you held it at high temperature for maybe a year or two in an oxidizing =3D
On Jun 25, 2011, at 9:23 AM, Celia Hirsh wrote:
> I'm really curious. Can someone explain what happened? Why did the red
> stay constant on re-firing in a different atmosphere?