Adventures In Terracotta For a Stoneware Potter

Thu, April 08, 2021 10:46 AM | Roberta Lampert (Administrator)

Nick Molatore
All photos by the Author

Some background…

I have made functional stoneware for most of my life. I started with cone 10 reduction work in the 70’s and transitioned to mid-range cone 6 in the early 2000’s. I made this transition for a few reasons. The work and expense of firing a gas kiln was a burden; the ease and low cost of cone 6 electric was attractive. Building and securing building permits for a gas kiln in a residential neighborhood was daunting. Also, the carbon footprint of gas firing is huge compared to electric firing. Gas firing uses about 1 gallon of propane per cubic foot, or more than 200 pounds of carbon dioxide production per firing in a 20 cubic foot kiln! Even if your electricity is produced by burning coal, electric firing to cone 6 produces less than half that amount. Of course, using renewable sourced electricity makes the difference even more dramatic.

Transitioning to cone 6 electric was painful and it took several years to get good results, firing hundreds of tests to find glazes I like. I missed the rich and varied colors and surfaces that cone 10 reduction glazes produce. Eventually, I developed cone 6 glazes that were successful, and I have produced cone 6 electric production pottery for about 15 years.

Cone 6 electric glazes by the author

Cone 6 electric glazes by the author

The appearance of unglazed clay is problem for me. I have never liked the color of cone 6 unglazed clay. Whether it be white, buff or brown stoneware, it is bland and boring compared to the rich surface of reduction fired clay body. I ended up using G-Mix 6 stoneware (similar to B-Mix) but I always covered the clay with glaze top to bottom. I tried many cone 6 porcelain bodies, but struggled with throwing, warping and cracking.

Through the years I have admired the beautiful brick red of terracotta (red earthenware). I love the colors and surfaces of brick buildings, terracotta floor tile, and Mexican flower pots. 

 

Work by Leño Contreras, Tecate, Mexico


 Building tile from Brazil and handmade bricks from Mexico

I am particularly enamored by the surface, detail and colors of barro bruñido, the burnished and very low fired decorative ware mastered in central Mexico.

Examples of barro bruñido
by Master Potter Angel Santo, Tonalá, Mexico

Let me digress for a moment and talk about reds. I have yet to see a cone 6 electric red that I like. High-fire reduction has spectacular copper reds and luscious iron red. Low fire yields lovely iron reds (and decent reds from stains) but there is nothing in midrange oxidation that I remotely like. Reds from inclusion stains are flat, lifeless, opaque and, in a word, awful. A very slow cooling cycle yields an almost-decent iron red, but you have to devote your entire firing to this cooling schedule just to get a barely not awful iron red!

A confluence of two events… one good, one bad 

My dear friend, Ginger Steele (a Renaissance woman if there ever was one) has worked off and on with red earthenware over the years, and returned to it after years of producing beautiful salt fired work. She encouraged me to give it a try, even giving me a box of red clay. The second event was COVID. My main source of sales, Portland Saturday Market, closed due to the pandemic. I was looking for something new to shake my quarantine funk, soI gave low fire a try, bringing red clay into my white clay studio… eee gads!

I have been more than a bit of a high fire snob when it comes to functional ware. I always believed that truly functional ware had to be at least cone 5, despite the fact that most commercial dinnerware, including high quality, high-end ware is fired to cone 1 or less. So, I had concerns about making functional terracotta. I did considerable research, with much of the information coming from digitalfire.com (See: https://digitalfire.com/glossary/terra+cotta

There is a great photo on the digitalfire website that shows changes in terracotta as it is fired from cone 06 to cone 4. As firing temperature increases the clay goes from orange red to darker red, then brownish red. It gains strength up to about cone 1, then starts deforming and bloating. The article does an excellent job of explaining why a slow firing schedule is important.

Traditionally, low fire ware is bisque fired to the same or even higher temperature as the glaze fire. The higher bisque temperature drives out gasses as materials in the body decompose and release gasses. This allows glazes to smooth out when fired and be defect-free. Glazes applied to the almost-vitrified ware require gums to thicken them and to allow them to be smoothly applied by brush to the non-absorbent ware. These commercial glazes are not suitable for dipping, because the glaze drips and runs for a long time; they are designed for brushing.                                                         

With my high fire background, where bisque is much lower than the glaze fire, the bisque ware is quite porous (un-vitrified). When a piece is dipped in glaze, water is quickly absorbed and the work can be handled in less than a minute. Dipping and pouring are the best ways to get a thick, even layer of glaze, and are the practical way to produce a large amount of ware.

I am currently bisque firing to cone 06 (1825 degrees F.) and glaze firing to cone 03 (1960 degrees F.) The cone 06 bisque leaves the ware porous enough to pour and dip glazes; the cone 03 glaze fire is a good balance between strength and color. Combining frit-based glazes and a slow glaze fire schedule (https://digitalfire.com/schedule/04dsdh) I am getting good results with my low temperature bisque fire and higher temperature glaze fire, a process similar to my high fire experience.

Yes, but is it functional?

At cone 03, the clay I use (Cherry Creek from Georgie’s in Portland, OR) has a porosity of about 5%, more than the 1 to 2% of cone 6 clays I am used to. I am impressed by the strength of the fired clay. A simple test to gauge strength is to extrude ½ inch diameter by 6-inch long rods, then support the fired rod at each end, hang a wire through the middle, adding successively more weight to the wire until the rod breaks. I was surprised that the earthenware fired to cone 03 was nearly as strong as the cone 6 fired G-mix 6 clay.

I found a couple of simple but versatile glaze recipes from digitalfire.com, my favorite being G2931K (https://digitalfire.com/recipe/g2931K), which fires beautifully at cone 03. It is smooth, not too fluid and has excellent fit with no crazing. The glaze responded well to coloring oxides and stains. G2931K is also very acid resistant, using my version of the lemon juice test. Instead of lemon juice, I use concentrated hydrochloric acid for a week. The glaze showed no trace of etching or color change.

I also tested for thermal shock resistance. A mug is heated in the oven to 400 degrees F, then plunged into cold water, repeating 5 times. A mug showed no damage from this abuse—ware remained sound and the glaze showed no hint of crazing. 

My last concern was that porosity might be a problem. If too porous, absorbed water may cause overheating in a microwave oven as absorbed water heats. The test mugs did fine, even after being soaked or run through the dishwasher. 

I have used a number of pieces daily for several months. They have held up as well as my cone 6 ware—not a single chip or crack after abusive daily use! 

I suspect that the impressive results from the microwave test may have something to do with my use of terra sigillata, which partially seals the surface. 


What finally seduced me to the dark (clay) side

One of the things that makes Ginger Steele’s terra cotta so lovely is her use of terra sigillata. (See https://en.wikipedia.org/wiki/Terra_sigillata) This ancient, simple process produces a surface that is difficult to describe: smooth, lustrous and satiny with just a hint of gloss. Color can be rich and deep. Though the process of making and applying sounded difficult, it turned out to be quite straightforward. (https://digitalfire.com/article/super-refined+terra+sigillata) My first batch of terra sig, made from Red Art clay, came out nicely, resulting in a smooth, beautiful red surface. The application of terra sig is quite forgiving, much easier than applying glaze. 

A 5-gallon batch yielded about 1 1/2 gallons of terra sig, enough to dip most items. I added 4% red iron oxide to the Red Art before the settling process to produce an even deeper red. The terra sig is applied to bone dry greenware or bisque, either by dipping or brush—both had similar results.

Terra sig retains every detail of the clay. Glaze will bury small details, but not terra sig. The mug handle in the photo clearly shows my thumb print, even though it was dipped with a fairly heavy coat of terra sig. The smallest tool mark, scratch or texture will be preserved.

What Now?

An intended short dalliance in terracotta is turning into a consuming affair. I have found a clay surface fired in an electric kiln that is beautiful and sensual in color and surface which I no longer feel a need to hide. I love the terra sig surface so much that I use only liner glazes with a splash of colorful glaze on the outside. Ware is completely functional and I use minimal energy with the low firing temperature. Meanwhile, my tubs of cone 6 glazes sit idle, gazing at me like dogs on a rainy day and I have over a ton of G-Mix 6 clay sitting in my studio. (I have made some tests, and it appears I can convert this clay to a nice terracotta-like clay with the addition of 10% frit 3110 and 4% red iron oxide!)

I don’t know where this is going. I sold a lot of cone 6 ware made with a process I tweaked for 15 years. I have had little market exposure with the terracotta, since my main market has been closed all year. The handful of people that have seen and used the terracotta appear to like it, but I don’t know how it will sell. Producing both terracotta and white stoneware in the same studio presents its own problems, mainly trying to keep the whiteware clean. By only using red low fire clay and white high fire clay I do avoid the problem of accidentally putting a low fire pot into a cone 6 glaze fire, as the color of the bisque is unmistakable. One would never want to have low and high fire white clay in the same studio, a situation that would lend itself to disaster with melted pots and shelves!

Meanwhile, this terracotta muse has helped me shake off a bit of my quarantine depression.

 

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