I am currently searching for a deep red/orange clay body that flashes this color in atmospheric firings to Cone 8 and above. I have mixed roughly 20 tests and made cups out of them and fired them in Soda and Wood/Salt kilns to Cone 8 and Cone 10 respectively. Here are my findings.
It seems that red/orange flashing is promoted by small amounts of iron chemically incorporated in high temperature clay and dry material ingredients introduced in the clay body.
What Parmelee suggests in his book Ceramic Glazes that clay bodies higher in silica will need higher percentages of iron compared to clay bodies higher in alumina, stays true. Though, iron percentages stay below 3% on a whole.
Also, it seems that the silica/alumina ratio should stay below 4. The current wood kiln has tests that are below 2 (showing a high alumina content)
My tests have 2 different realms. One direction is a grolleg/PV body and the other is a Neph Sy/silica body. Both directions are currently using high alumina materials.
These 3 tests were fired in a first firing on a clay slab to ensure they would not melt down with higher than normal percentages of flux I was a little worried. I believe the firing had too much reduction to promote the flashing and too much soda to allow flashing in some areas. Cup number 6 has engulfed in soda, so no visible flashing is seen except on the foot of the cup.
My belief is that too thick a layer of flux (could be wood ash, soda in this case or salt) will stop the development or cover teh development of flashing. Also, I think that an oxidation atmosphere during 700 degree to 900 degree Celcius is neccesary for red/orange flashing, though I’m unsure if this needs to happen in firing or cooling or both.
This cup is blah blah blay
These 2 tests have 35% Mica with different types of silica mesh in them. Notice the excess iron in the mica has colored them brown as if they were made from EPK or were a dark stoneware, even though they are mainly grolleg kaolin. The silica mesh made no difference in the color. They were fired in different parts of the kiln.
These two tests have lower flux, but more silica and less mica and the color is very light almost a pastel pink. I believe the iron should have been increased to counter the added silica. It seems that 4-5% iron material needs to be added to get a deeper color.