Flashing Clay Bodies

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.

The center 2 clay bodies are brown as they have too much iron from 35% mica. the cup has 325 silica and the flat tray in front has 200 silica. No visible difference in the color.

My first legitimate inspiration in creating a flashing clay body was in Graduate School 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
  • 10 Silica

The Mica has iron in it and is high in silica. It also fluxes at high temperature, replacing feldspar. 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 are other important factors as well as are the many unknown factors .

Besides the fluxing capabilities in the iron and mica in these 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.

The two clay bodies on the right are almost vitrious requiring a higher percentage of iron to enhance flashing.

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 from my graduate studies 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 in recent years.

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.

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These clay bodies had the largest percentage of iron in relation to the silica/alumina ratio.

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.

Flashing Clay Body Tests

The soda kiln tests are pictured above

Clay body #4        Clay Body #5         Clay Body #6
55 Grolleg          55 Grolleg           55 Grolleg
35 PV               40 PV                50 PV
10 Mica             2.5 Mica             2.5 Mica
                    2.5 Newman Red       2.5 Newman Red
Clay body #8  
55 Grolleg
35 PV
10 Mica

Flashing (Hi-Iro) according to Furutani Michio in Anagama: Building Kilns and Firing

  • A slow draft contributes to Hi iro pottery
  • Oxidizing atmosphere between 500-1200 degrees C helps flashing
  • In oxidation the flashing is bright, in reduction the flashing becomes dark like chocolate even.
  • The small amount of iron in a porcelaneous body makes the flashing go even darker than a normal clay body in a reduction atmosphere.
  • Clay body must be porcelaneous with a little amount of iron. It may be soluble iron compounds such as iron sulfate, iron hydroxide and iron chloride that cause flashing.
  • When fired pieces are broken and the flashing is examined it shows the red color is only on the surface. The interior of the body is white, possibly indicating it is the soluble salts that cause flashing.
  • When burned, wood emits alkaline vapors of potassium and soda which volatilize around 1000- 1200 degrees C and react with the iron on the surface through evaporative migration causing flashing.
  • At 1150 degrees C reduced iron becomes red in color which lasts to 1200 degrees C. After 1200 degrees C the iron becomes black and then gray in color. However, in slow cooling in an oxidized atmosphere the iron changes back to red, becoming Hi iro (flashing).
  • Ash pits in the firebox may help by burning up embers, so ash moves through the kiln and doesn’t deposit on the pottery.
  • The more glass or ash on pottery the less flashing
  • Use wood that burns hot, but has little ash or ember buildup such as pine
  • Do not build up embers except enough to keep the temperature climbing. Use smaller amounts of firewood in quicker succession and allow it to burn out before adding more.
  • Ash that is on the pottery at 1200 degrees C will become glaze and cover the flashing effect.
  • Hardwoods and other wood sources (other than pine) are unstable and may have additional oxides that stop flashing.
  • Adding moisture to kiln will help produce flashing (Hi iro) affects. Early on in the firing this may be because moisture in an oxidation atmosphere will provide more oxygen; therefore more oxidation.
  • Flashing shadows will not exist where pieces are stacked close together.
  • It is good to stack pots asymmetrically, so the space between is different
  • Put some pots upside down and tall ones next to short ones. Always try new arrangements
  • Fast firing schedule with little ash buildup is what is desired.
  • If overfired, the flashing will burn out, so know your clay body characteristics.
  • It the kiln temperature is too high or the kiln is fired too long the flashing disappears and the body remains white.
  • Let the wood burn out completely before closing kiln up, so kiln is stopped in an oxidation atmosphere.
  • If the kiln is sealed with flames inside the flashing may become dark or dull (due to reduction)
  • Once the kiln is closed it is left to cool slowly. Do not open the kiln early as if affects the color. If kiln is cooled quickly the flashing will disappear.
  • If the clay is refractory the temp to cool slowly is lower than if the clay is less refractory. The range is from 900 degrees C to 300 degrees C, but Futurami does not know how my clay will fit in that range.
  • In winter the kiln temp rises quickly and the chimney pulls stronger, requiring less fuel to fire the kiln, so Hi iro or flashing is more easily accomplished in the winter.
  • Kiln should have a teardrop base with high ceiling and orthodox shape for Hi iro pottery
  • The clay body must minimize its ability to build up ash (high alumina?) as the ash will discourage flashing.
  • Pure white clay with little iron content must be used.
  • New kilns produce better flashing as water has seeped into the bricks and will help oxidize the atmosphere at the beginning of the firing. As the kiln ages a layer of glaze will build up on the bricks that stop the moisture from contributing to flashing.
  • A clay slip can be applied to the inside of an old kiln to encourage moisture exchange in firing (may flake)

Hank Murrow stated in his article on downfiring that the cooling of the kiln results in the red color of shino’s. This is due the the red flash of the iron resulting at specific temperatures. This research is similar to what has been stated in articles in terra sigilada firing that an oxidized cooling is needed to retain or reclaim the red color in iron once fired in high temperature kilns.

In Hank’s experience, a heavy reduction at the beginning of the firing is then fired in oxidation on the way up after cone 2 was down. This oxidation after reduction and continued oxidation on the way down is how I fired in Nebraska where I got good red flashing results. Hank’s shino research stated that firing in oxidation from 1900 degrees F resulted in deeper reds every 50 degrees F on the way down (though he did not show an ending termperature)

Jim Robinson’s article on Shino’s

Hank Murrow’s article on Shino downfiring