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 reduced for too long and not re-oxidized in cooling. the cup has 325 silica and the flat tray in front has 200 silica. No visible difference in the color. Another viable explanation is the color for these tests is brown due to reducing later in the firing which caused a brown type of iron to form instead of the desired red coloration. Also, too high of silica caused the flashing to go brown when fired at the same temperature and reduced at the same temperature as high alumina clay bodies that flash red, so the iron percentage my need to be higher and/or the reduction temperature may need to be adjusted. More testing is needed to identify these variables.

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.

The 2 bodies on the right are EPK/Neph Sy and the 2 on the left are Grolleg/PV Clay compositions

After repeated tests, including the recipes used in the images above, I am forced to revisit each clay body and how they relate to one another. I now know that the firing and cooling is the most important part of flashing. As seen in the above 2 images.

In my search, I also have used high temperature clays with high percentages of kaolin and low percentages of iron in them. Kaolin works as one of the main alumina components. I have used anywhere from 35% mica to 1% mica and have used 10% red clay to 1% red clay with mixed results as there are many factors. The mica works as a flux in high percentages as it is high in potassium, but in low percentages it is mostly used to replicate ancient Chinese material contents for texture and feel of the clay body. The mica and the red clay both add iron as I believe using iron already in chemical suspension with clay works better for flashing than adding iron oxide (the iron particles are smaller combined in mica and high fire red clay bodies which removes specking). So far, I have used combinations of the 2, with Newman and Carbondale as the red clay bodies.

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 is significant to the color of flashing that is created. More testing needs to be done. The amount of airborne flux and when it is introduced into the kiln are other important factors as can be imagined; there are 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, due to its high soda content. Both are slightly soluble and cause a thixotropy in the clay bodies. The Chinese clay bodies were volcanic and micaceous, but my introduction of pumice has not done anything to enhance the color, except for trials in Graduate School that I cannot seem to duplicate.

After reading Nigel Wood’s book Chinese Glazes, I have found that potassium in high percantages can produce flashing as well. It is potassium in the mica that provides a flux to the old Southern Chinese clay bodies in Longquan porcelain. The mica also produces a thixotropy to the clay, making it slump easily. The workabilty of a high clay body has a narrow window. Creating a micaceous throwing body has not been successful.

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 of clay. 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. Adding paper to your clay promotes bacterial, rot and plasticity.

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. High alumina open or porous bodies were recreated using kyanite and mullite to raise the alumina content of the clay in recent years.

Translucent clay bodies will be my next avenue to test for flashing. So far, translucent bodies have not been successful. The high silica bodies I have tested dunted in the cooling cycle (I am firing in other peoples wood kilns, so the cooling is out of my control).

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. I am chainging the alumina/silica ratio and the iron percentage as well as the amount of flux. So far, Silica over 15% flashes brown in a cone 9-12 wood firing when reduction is interduced at cone 010 and left to fire for 30min to 2hrs. Neutral firing the rest of the way up resulted in more even surface flashing around the entire piece, but it is unclear why this happened in the 3/1/2022 firing. the force of the chimney pull may also cause the flashing to be one sided (streaking).

5
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 could be reduced/oxidized or fired to the incorrect temperature for the color I want. It is not apparent how the wood 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.

With this in mind, I now only reduce at cone 010 for 1-2hrs and my best results have been to keep the flame neutral to reducing the rest of the way up to cone 10. As indicated earlier, higher silica bodies flash brown and higher alumina bodies flash more red when this firing schedule is adhered to, but the variables are countless, so more research is needed.

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 was not able to replicate that firing schedule in the soda firing shown in the image below.

At the very bottom of the article I have added images of my last 3/1/2022 firing in a wood kiln under my control (kind of), so look there for new information on flashing.

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

One of the first books I read that helped with insight into flashing and identified proven techniques based on a life’s work was the above book. In it Furutani Michio accounts for flashing on clay bodies in anagama kilns. He calls this flashing Hi iro, a Japanese term. Below are the main identifying aspects of flashing in anagama kilns.

  • 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)
The orange flashing on this clay body is caused by re-oxidation of the iron in cooling after reaching glaze maturity temperatures. 2% iron or lower, flux percentages and possibly silica/alumina ratio are also important factors, but more testing is required.

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). Hank stated in correspondence that he has held the kiln temp in and around 1900 degrees F for up to 6hrs.

Another must read which backs up Hank Murrow’s research in downfiring a kiln which enables one to re-oxidize the iron in ones clay body is “downfiring- Mel Jacobson”. Please see below

A must read for glaze chemistry that goes way beyond flashing clay bodies is Ian Currie’s book “Stoneware Glazes”. But, in this book there is relevant information on iron and how it interacts with the silica/alumina ratio and different fluxes. It is through this book that a comprehensive understanding of how clay materials react to one another can be understood. It helped me immensely. His newer book “The Grid Method” helped me too.

It is by Pete Pinnell’s guidance and reading the books and articles mentioned within that I was able to piece together the way to create a flashing claybody. Now I am on may way to create one that is low in cost.

Jim Robinson’s article on Shino’s

https://studiopotter.org/digital-issue/240

Hank Murrow’s article on Shino downfiring

Mel Jacobson’s article on downfiring

Ian Currie’s book “Stoneware glazes” is free on Glazy. Just follow this link.

3/1/2022 Wood Firing in Woodstock, VA

This is one of the redder pieces
speckling in front is wood ash
The foot shows red area near the wadding.

These 3 examples were of a high silica version of my flashing clay body. They flashed more brown than red and I believe it is the silica that has made the difference. My next firing will be with a higher alumina clay body fired in a similar way (I hope, since it isn’t my kiln).

Back of bottle flashed red while front was white due to wood ash
Close to bag wall, but sheltered by it produced great flashing
Top of bottle white due to wood ash, but bottom flashed orange

Here are the different factors:

  • alumina/silica ratio
  • iron percentage (should be under 2%)
  • how iron is chemically bound when added to clay body
  • amount of flux and type of flux
  • total temperature of kiln
  • rate of cooling temperature
  • time/temp when reduction is begun to create red flashing
  • how the flame interacts with the ceramics or rate of pull of kiln
  • type of kaolin used and amount
  • Atmospheric material introduced into kiln

Breakdown on above factors:

  • alumina/silica ratio: The higher the alumina and lower the silica the redder the clay body flashes, but the more porous and underfired the clay body gets when only in wood. This may change in soda or salt firings. At cone 010 reduction for 1-2hrs with steady rate of temp. climb there after to cone 9-12 in a fast fired wood kiln (12-14hrs) the higher the alumina the redder the clay body. High silica bodies fire more brown in this same kiln firing schedule. My guess is the cone 010 reduction may need to shift to get the high silica body to flash red, for brown iron is created above 1400 degrees F (as indicated in Greek black/red polychrome firings). Therefore, reduction may need to be earlier (which may not be possible as cone 010 is pretty early in the firing). Or, the cooling of the high silica bodies may need to be slowed down or sped up, which is something I have not tested. Also, as indicated above, high silica bodies may need the iron percentage to change. Most of my clay bodies have .75% to 1% of iron in them. Maybe this or even the flux percentage may need to be adjusted for red flashing. Many variations.
  • Iron percentage should be under 2%: Pete Pinnell as well as several books including Nigel Wood’s and Parmelee all suggest a low percentage of iron in a porcelaneous clay body. This seems to be correct, but how the variation of the iron effects the flashing is still unknow. I have tested .5 to 1%, so I need to test some lower percentages and percentages higher than 1% on high alumina and high silica bodies to find out more. Iron is reoxidized in the cooling and iron likes to be cooled slowly, but when the key temps are in cooling are unknown. Hank Marrow likes 1900 degrees F for his holds during cooling, so that is a starting point, but my current kiln mates don’t want to do any cooling holds at the present time. it makes since that iron color is dictated by when the clay is reduced and when it is reoxidized.
  • how iron is chemically bound when added to clay body: The iron being chemically bound to another material just means that it is small enough to not produce speckling. RIO would spot the ware instead of allowing it to have an even flashing (still a theory).
  • amount of flux and type of flux: I use high soda feldspars due to it being my only source of flux. Even the potash feldspars on the market have high soda content. My first attempts were with 35% mica which worked as a flux as it is high in potassium. Though my reading on flashing in Chinese ware shows that potassium works better for flashing, I have not noticed a difference. I moved to soda feldspars as they are much cheaper than mica, so mica will continue to be a small additive until I have much more money to test that material. So far, my best red flashing was been the highest alumina clay body I have tested. My next clay body will have even more alumina by way of 15% kyanite added to it.
  • total temperature of kiln: This doesn’t seem to matter, but it is a factor to keep in mind. I have had red flashing at cone 8 and cone 13.
  • rate of cooling temperature: I think this may be the most important step in getting flashing, but it is impossible for me to know as I am not at the kiln to guard against individual fast cooling it by looking into it over and over. I fire in 2 kilns consistently. The wood salt kiln is old and made of hard brick. It has so many holes in it that it cools super quick. I have had several dunting issues with only 10% silica in my clay body, just because it is drafty. This kiln fires in 30-36hrs and the ware goes in green. The other kiln is a wood kiln and is solid. It is a combination of soft and hard brick. It is the only kiln I have had reliable flashing out of. It fires in 12-14hrs and cools in 3 days, but it seems to cool faster some firings that others, so I’m unsure how this is happening. Interestingly enough, this wood kiln is similar to the one I built at UNL- fast firing and a combination of soft and hard brick. Both allowed for great flashing. I think that holding the cooling at different stages for different clay bodies my lead to different colors of flashing. I have noticed the same clay body flashing orange or red in different parts of the kiln. It is also possible that this color difference is due to oxidation/reduction at different temperatures as well. It is difficult to tell.
  • time/temp when reduction is begun to create red flashing: In reading about Greek polychrome firing where they created a firing schedule that allowed for them to get both black and red colors from the same red slip, I decided to reduce the kiln based on this technology and it worked. I think they said that red iron converts to brown iron at 1400 degrees F, so you have to reduce before that temperature for the iron to reoxidize to red in the cooling. If you reduce the iron after 1400 you get brown pots. I think this is more complex than it sounds as I think the alumina/silica ratio and the amount of flux and iron are also important (as well as the cooling).
  • how the flame interacts with the ceramics or rate of pull of kiln: So, my question here is how to get flashing around the entire piece. I think the wood kiln I fire in has so much draft to it that streaming is happening. This means essentially, that one side of the piece is being reduced while the other is not, because the flame is sucked up the chimney. the last firing we did not brick up above the kiln shelf damper system in the chimney. This cut the drawl of the kiln’s chimney and provided better overall flashing on the pottery. I’m unsure of other factors that may stop the back side of the work from being flashed. I guess, the atmosphere of the kiln may need to touch the piece in order for it to flash?
  • type of kaolin used and amount: The kaolin will have different amounts of alumina/silica and flux, but also amounts of iron and titanium. Though at first I thought Grolleg kaolin was important to use, my best high alumina test has been a clay made of EPK. This shows that I only need to use Grolleg when I want to have low titanium for glaze color maturity. Otherwise, by adding more alumina or silica I can change the color of flashing to my own needs.
  • Atmospheric material introduced into kiln: As mentioned earlier, the build up of wood ash creates a layer of glass on the work which blocks flashing. So, a little sheen of wood ash will still have red flashing under it as a solid layer of glass is not blocking the reduction/reoxidation process, but too much will stop the reoxidation and the clay body will be the color of porcelain as there is only 1-2% or iron in the body, so it cannot be too dark without the reduction/reoxidation process. At about cone 1-6 this glass becomes solid and nothing can move through it. The temperature is different for different glazes (shinos mature early, due to their high soda content). Soda firing traps in the carbon from the reduction process, but salt doesn’t seem to do that. Most of my research is in straight wood firing. I have only fired soda once in these trials and the result did not turn out as planned. The wood salt kiln that I put work in is fired by others and I have no control of the firing or where my work is put in the kiln. For all I know my work is oxidized to cone 6, so these results are not reliable.
Porcelaneous stoneware body with 1% iron
Soldate 60 clay body

Both of these bowls were fired in the same kiln. Both have wood ash on their face where the kiln atmosphere hit the clay body directly, but the porcelaneous stoneware with 1% iron is flashing orange/red while the stoneware with a higher iron content is flashing brown. Both pieces I placed in this firing that were stoneware flashed brown while all the porcelaneous stoneware pieces with .8%-1% iron flashed red to orange to brown. This shows me that reduction/reoxidation aside, the percentage of iron is very important to how these pieces flash. The amount of kaolin may also important as I’m sure the Soldate 60 clay body has a low kaolin percentage as kaolin is expensive and this brand of stoneware is cheap.