Parmelee stated in Ceramic Glazes that small amounts of iron under 3% with a higher silica to alumina ratio will promote flashing in porcelaneous clay bodies. The higher the alumina, the lower the iron percentage will need to be. A good example of this is the flashing seen on kiln wash in long high temperature firings where small amounts of iron promote flashing in high alumina materials.
I have been experimenting with this since Graduate School with my first inspiration provided by Pete Pinnell. He told me that small amounts of iron in porcelaneous clay bodies seemed to promote flashing and gave me the suggestion of this recipe. 55 Grolleg, 35 Mica and 10 Silica. The Mica has iron in it and is high in silica. The Grolleg is high in alumina and all three are white burning materials (though the mica has iron in it). This is where I started, though admittedly, I have changed my recipe quite drastically.
In my search, I also have used high temperature clays with iron in them as the chemical combined clays work the same as the mica. I have used anywhere from 35% mica to 1.5% mica and have used 10% red clay to 1.5% red clay with mixed results as there are many factors.
As I see it there are several factors that contribute to flashing. Iron percentage, alumina percentage, silica percentage as well as other materials such as the type of flux. All these materials play a part in flashing. Also, when the iron is reduced/oxidized in heating and cooling in the firing flashing may be created or enhanced. The amount of airborne flux and when it is introduced into the kiln is another important factor as well as many other unknown factors .
Besides the flux in the iron in the mica and clay bodies, there are 2 distinct fluxing components I have chosen to test. PV Clay is similar to the petuntse that the Chinese used to make porcelain and Nephilene Syenite is known in the wood fired world as a flux that promotes flashing. Both are slightly soluble and cause a thixotropy in the clay bodies.
This thixotropy along with the shortness and non-plasticity of the clay does not allow for easy throwing to create pottery. Also, the shortness in the clay body promotes cracking especially when it is paddled and beaten with sticks (which is how I work), so sculpture like materials are added to the clay to help with this including paper, nylon fiber and bentonite (in my latest tests). The paper helps open up the clay bodies and the porosity stops cracking. The nylon fiber helps hold the clay together when it is stretched and compressed, which helps cracking and the bentonite adds to plasticity which helps with cracking as well. I also age my clay creating bacterial bonds that help with plasticity, but it makes the clay body mold and smell.
Another potential issue is that clay that is fired to the mature viscosity may not flash as well as underfired clay. This is a theory based on using high alumina content in the body. The only example of a bright red/orange claybody was fired in a wood kiln and the body is clearly non viscous. In that test the same clay body materials were used, but additions of molocite and pyrophyllite were added up to 10% which raised the maturing temperature of the clay body. these bodies were recreated using kyanite and mullite to raise the alumina content of the clay body.
My work will continue though I presently have no atmospheric kiln to fire my ware to the cone 8-10 temperatures I need. This causes me to concentrate on the clay body itself instead of how the kiln is reduced/oxidized as I have no control of the kilns that have been graciously provided for me.
The soda kiln I used I over reduced and the wood kiln I have my current work in was fired without my control, so it is important to start with the clay recipe and alter it to see how the firings will affect it. There is the potential that I have found the body I want, but it was reduced/oxidized or fired to the incorrect temperature for the color I want to be apparent once the firing was concluded, but this is acceptable in my current testing process. Once I have an atmospheric kiln of my own I will alter the firing to test different results.
Another assumption I have is the firing schedule that Pete Pinnell told us about affects how the clay body is reduced/oxidized. It is Pete’s experience that the first lower temperature reduction done anywhere from cone 010 to cone 1 is really a body and glaze reduction. Most individuals are taught in school that the first early reduction is a body reduction and the second reduction is the glaze reduction. In Pete’s and my experience the 2nd reduction at cone 8-10 is really only a body reduction as most glazes have already become mature and have become a glass, so reduction no longer affects them. This is contrary to most schools of thought.
My successful firing of reduced clay bodies that flashed had a lower body/glaze reduction and then a neutral to oxidizing atmosphere to finish off the firing to cone 8-10 (so no late reduction). I have not been able to replicate that firing schedule in my current circumstance.
Soon I will post results of these clay bodies on Glazy.org with images of Soda and Wood/Salt fired tests. The soda kiln tests pictured above are already in a post on my block with more information.