With all the current discussion about global warming and conservation, I thought I should relate a recent workshop I taught at MudFire Clayworks and Gallery, a community ceramic art center in Decatur, Georgia, just outside of Atlanta. They offer studio space, monthly workshops and a beautiful gallery of contemporary ceramics artists. They also fire to cone 6 in reduction. Erik Haagensen and Luba Sharapan, the owners of MudFire, had started to fire cone 6 reduction because of a defect in the kiln they’d purchased, but after seeing the results they had no reason to change back, even after the kiln was repaired. Firing to cone 6 was cheaper, faster, and the results were almost indistinguishable from high fire.
Although they fire to mid-range reduction, Erik and Luba had read my book, “The Complete Guide to High-Fire Glazes,” and wanted me to give a workshop on the cost and time benefits of cone 6 reduction firing, as well as to explain the reasoning behind glaze recipes, firing cycles and to show them how to bring glazes from cone 10 down to cone 6.
I normally work with, and teach about, high-fire glazes (cone 9–11), approximately 2350°F (1288°C), while mid-range, (cone 5–7), is about 2200°F (1204°C). Although this is only about a 150°F (66°C) temperature difference, raising the temperature 150°F at the peak of the firing takes quite a bit more energy and puts a lot of extra wear and tear on the kiln. It could easily take two to four more hours of firing to go from cone 6 to cone 10 with the gas on high, so firing to mid-range reduction would save considerable fuel if comparable glazes could be found.
At that time, I didn’t have a lot of experience with mid-range reduction and I found it hard to believe that the results were “almost indistinguishable.” I did have a good bit of experience firing mid-range oxidation in an electric kiln and the results are far from the look of cone 10 reduction. But the idea intrigued me, and the more I thought about it, the more I realized that the same principles of high fire reduction should apply to mid-range reduction. The key question would be if the oxides and materials needed to melt the glazes at a lower temperature would negatively affect the glaze colors. So I took the challenge, reasoning that I could use the same research methods I used for the high-fire glazes to explore these mid-range glazes.
Mid-range firing in both oxidation and reduction is a well researched area dating back before the energy crisis of the 1970s. There are also several college clay programs that use mid-range reduction and have published their glazes. One notable example is Diana Pancioli, at the University of Eastern Michigan, who started her “Glaze Forward” program. (For a small shipping fee, you could send for a list of cone 6 reduction recipes and test tiles of those glazes.) There are also organizations like the Clay Studio in Philadelphia that fire cone 6 reduction and have developed a wonderful palette.
My first step is always completing an exhaustive survey of known glazes from books, internet and workshop handouts. Luba and Erik generously sent me all their recipes from MudFire [see selected recipes on page 51], and I pulled out my cone 6 glaze notebooks and began assembling recipe lists and firing instructions.
There is so much information available today that it is almost paralyzing; you don’t know what to do with it all. So in order to make it usable, I organized the recipes into types, like iron glazes (celadon, temmoku, kaki, iron saturate, etc.), shino, copper red, oribe (copper green), magnesium matt, etc. Then, after eliminating all the duplicates, I looked for similarities and differences, and from those, selected enough glaze recipes to test that would show a broad range of possibilities within a type. Then I test those recipes in a variety of firing cycles, like heavy reduction, light reduction, early reduction, late reduction and oxidation. This way, I can reveal a glaze’s full potential.
Iron glazes are a great type to start with because you can see a wide range of colors by incrementally adding one colorant; iron oxide. For example, when firing in reduction using the same base glaze, adding 1% red iron creates a blue celadon, adding 2–4% iron oxide will give green to amber celadons, adding 5–10% iron oxide makes temmokus, and 10–20% iron oxide gives iron saturates. Teadust temmokus result from additions of magnesium carbonate to temmokus with cooling soaks. Kakis, which are also part of the iron glaze type, are obtained with additions of bone ash and magnesium carbonate. Finally, oil spots result from stiff oxidized temmokus with magnesium oxide. So you can see how one glaze type can show you a world of glaze colors.
Copper red glazes are generally low alumina and high alkaline bases with small amounts of copper carbonate (0.3%) and tin oxide (1%). Oribe glazes use copper to get greens while magnesium matt glazes yield satin whites and purples with cobalt oxide. You can try to reproduce these “types” at various cones and, as always, you may then have to make adjustments after you see the results.
The final type I concentrated on was shino glazes. Shinos are generally made with varying amounts of feldspar and clay. For example, you may have somewhere between 60–90% feldspar and 10–40% clay. A typical recipe would be 70% feldspar and 30% clay. This is the most difficult glaze type to reproduce at mid-range because most feldspars melt around cone 9 and then with the added clay it is hard to melt much lower than cone 10. I started by using nepheline syenite, which is not a true feldspar but rather a feldspathoid (containing less silica than a true feldspar). It melts around cone 6. Because it is high in sodium oxide and lower in silica, the effects are not identical, but it was a good starting point and worth a try.
I loaded the kiln with these various glaze types and then filled the remainder with line blends within these glaze types and a variety of other recipes, like blues, greens, yellows, blacks, etc., to see the overall effect of varying firing cycles across the board of glaze colors.
For first firing, I started reduction at cone 010 and kept it heavy (0.65–0.72 on the oxygen probe) to cone 6 at 3 o’clock (cone melting position, not time of day). I had pretty good copper reds and iron glazes but the shinos were dull and washed out. For the next firings, I increased the firing temperature to cone 7 at 3 o’clock, which gave me about 25°F more and brightened up the glazes. I ran five more firings to this cone, including full oxidation, light reduction, medium reduction, heavy reduction and oxidation with reduction at peak temperature. I also tested glazes with flux variations too numerous to mention, exploring mid-range fluxes like boron oxide, sodium oxide, lithium oxide, calcium oxide and zinc oxide [see sidebar on page 50]. Adding fluxes and reducing alumina and silica affects the response of coloring oxides in glazes, so the trick was finding suitable colors in properly melted glazes.
The results were great for copper reds and iron glazes, as well as greens, blacks, blues and carbon-trap shinos, which were very nice in heavy reduction. The carbon trap shinos worked because they contain soda ash, which melts very early, and with early reduction the carbon is already “trapped” below the soda layer so the peak temperature is not a factor. The only glaze type I could not achieve was traditional shinos, as I had expected. And I only had limited success with oil spots in the gas oxidation trials. This was also to be expected as iron oxide only starts to self reduce at 2250°F (1232°C) and that is about the peak temperature we reached. Soaking at cone 7 helped, but they were not as spectacular as a cone 13 oil-spot firing. Nevertheless, we did get spotting and some promising oil-spot recipes.
From all this testing, I came to the inescapable conclusion that Erik and Luba were correct. Ninety percent of the mid-range glazes were indistinguishable from their high-fire twins. This leads us to ask, why don’t more potters fire to cone 6/7 in reduction?
Making the Switch
There seem to be a few obstacles in getting potters to convert to the idea of mid-range firing. First, there is the inertia of their current practice. Change is hard in spite of the obvious benefits, especially if you have been doing the same thing for 20 years and it is working.
Also, there is an underlying belief, although it is completely incorrect, that cone 10 is superior to mid-range or low-fire, and changing this mind set is an educational challenge. I think that this comes from the long historical European search to imitate Chinese high-fire porcelain. The goal was always to achieve high fire, so it gained the psychological high ground.
When you mention mid-range, potters immediately think, as I initially did, of mid-range electric oxidation. But this is not the only way to fire mid-range. Mid-range reduction has a completely different look, as does mid-range oxidation soda firing or mid-range reduction soda firing.
And finally, when you mention firing to mid-range, potters immediately want to change or convert their cone 10 glazes to this lower temperature. This is perceived as a significant challenge because it means that they will have to learn a glaze calculation software and unity molecular formulation. Most just want recipes that work. They know it will take time and effort to learn to convert all these recipes and they just don’t want to spend their time doing that.
I don’t recommend converting glazes to the lower temperature, because when you lower the firing temperature of a glaze you are using different fluxing oxides that have different color responses. So although it is possible to convert your glaze to the lower temperature, you will end up with a different glaze anyway. It is better to use the many tried and true mid-range glazes already in use and test them in your cycle. This is the same way potters find high-fire glazes; they get glaze recipes from books or from friends and then vary the colorants and opacifiers.
Although change is hard, potters should focus on the benefits of firing mid-range reduction. First, as stated above, it saves fuel, reduces your carbon footprint and costs less. Second, it saves time. It may take 2–4 hours to get the extra temperature of cone 10 and maybe longer depending on the size of the kiln. So rather than firing for 10–12 hours you will be out in 8–10 hours. Firing to mid-range also reduces the wear and tear on your kiln, which means that it lasts longer. Finally, and most importantly, you get great results!
After all this testing, we discovered that the methods used to test high-fire glaze types also apply to mid-range types and, as a result, we found some very nice glazes. Erik, Luba and the potters of MudFire Clayworks are proof of that. Hopefully, this will help other potters get started firing to mid-range reduction.
the author John Britt lives in Bakersville, North Carolina, and is the author of the book The Complete Guide to High-Fire Glaze: Glazing & Firing at Cone 10. For more information and to see John’s work, go to www.johnbrittpottery.com.
This article was excerpted from the October 2008 issue of Ceramics Monthly, which can be viewed here.
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