How to decide what temperature to fire this clay at?

This is an extreme example of firing a clay at many temperatures to get wide-angle view of it. These SHAB test bars characterize a terra cotta body, L4170B. While it has a wide firing range its "practical firing window" is much narrower than these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware warps during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, watertight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low-fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low-fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low-fire transparent base that works fine on this up to cone 2: G1916Q.

Related Pictures

These are behind the art center. A dirty secret. Or an opportunity!

Barrels of scrap clay behind the art center

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When many clays are in use there is good reason to collect like this - you will see why in a moment. Each barrel is a unique product, a mix of random clays used by students. Each could be thus considered a wild clay, inconsistent throughout and with impurities. As such, each needs to be characterized, as a whole. For example, consider the front barrel: What does it look like when fired at various temperatures? At what temperature does it mature? What is the drying shrinkage and how plastic is it? Does it fit the glazes we use? This information not only describes the clay but also points to what needs to be added to make it useful (e.g. bentonite or feldspar). An account at Insight-Live.com is a way to organize the testing and post the results to a group (the SHAB test, for example, is perfect to describe the plastic, drying and firing properties). Once the clay in a barrel becomes predictable then it becomes useful.
To do the testing it is necessary to get a representative sample from each barrel. The method depends on how processing will be done. In a wet climate, the wet contents are likely to be thrown into a pugmill (with impurities), recycled until mixed and then bagged - a sample can be taken when complete. But the dry climate option is much better: Dewater each barrel, do quartering to get a representative sample and then characterize it. Finally, slake, slurry up batches using a propeller mixer, screen in a vibrating sieve and then dewater (e.g. on a plaster table).

Fitting an engobe to have the same firing shrinkage as the body

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The fired shrinkage of a clay can be measured accurately - and compared with that an engobe. Engobe-fit on terra cotta is very important because the bond is sintered, not glassy. These test bars show how this terra cotta body, L4170B, performs from cone 8 down to 06. Using the SHAB test procedure, I measured the dry and fired lengths of each bar and recorded the data in my Insight-live group account (it displays like the chart shown here). I compiled the same data for a super-white engobe, L3685Z3. Its calculated firing shrinkage came in much lower at all practical temperatures. At the target temperature of cone 02 a 5% addition of Ferro Frit 3110 increased the firing shrinkage from 2.1% to 5.5% (while the red body is 4.3%). To better the match I adjusted to 3% frit. What about matching the thermal expansion? I don’t have a dilatometer so I slurried up the engobe and used it as a body, it fits the same glazes without crazing or shivering (demonstrating they are under some compression).

Should you expect to vitrify terra cotta?

Hexagonal terra cotta planters in an electric kiln

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These were cast by Anna Lisovskaya, they are fired at cone 03. They are supposed to fit into hexagonal welded frames, but during firing many of them warp enough to fit poorly. Why? The color differences are the giveaway: The darker ones have shrunk significantly more. Something less obvious: the sides against the elements receive direct radiant heat, so they shrink more, turning a perfect hexagon into an imperfect one. Terra cotta clays are volatile, that means their approach to maximum density during heat-up, accompanied by shrinkage, happens across a narrow temperature range. Accurate and even firing are paramount. In a radiant-heat electric kiln this can be very difficult. Two approaches could work here: Fire at a lower temperature, perhaps cone 04. Or, greatly slow rate-of-rise for the last 100F, perhaps over several hours.

In clay body testing it is about the data.

It reveals how a 20% feldspar addition affects this clay

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Foundry Hill Creme (FHC) is used in North America as a stoneware body base or addition. It fires like a ball clay but it is less plastic. How much feldspar would it take to make it into a vitreous cone 6 stoneware? We fired SHAB test bars from cone 4 to 10 (bottom to top) and 10R (the soluble salts discolor it in reduction). The data collected (in our Insight-live.com group account) produces firing shrinkage and absorption data that calculates to the red columns (and plots to the blue lines). Notice how the porosity drops steadily from cone 3 to 10 (the firing shrinkage rises steadily over the same period). A 20% nepheline syenite addition plots to the orange lines. Notice 2200F (cone 6): The porosity drops from 8% to vitreous (zero) while the firing shrinkage increases 2%. The FHC drying shrinkage (the first DSHR red column) averages 0.3% lower than the mix, this is counterintuitive but a pleasant surprise. While the mix did have a higher water content (which would increase drying shrinkage) another factor appears to be how well this clay hosts the feldspar addition without compromising its own plasticity.

Videos

Links

Articles	Formulating a body using clays native to your area Being able to mix your own clay body and glaze from native materials might seem ridiculous, yet Covid-19 taught us about the need for independence.
Glossary	Characterization In ceramics, this normally refers to the process of doing physical or chemical testing on a raw material to accurately describe it in terms of similar ones.
Glossary	Maturity A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
Glossary	Vitreous The fired state of a ceramic when sufficient heat in the kiln has produced the density and strength common, and expected, in stonewares and porcelains.
Glossary	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
Glossary	Brick Making Brick-making is surprisingly demanding. Materials blending and processing, forming, drying and firing heavy and thick objects as fast as possible are like no other ceramic manufacturing challenge.
URLs	https://insight-live.com/insight/help/It+Starts+With+a+Lump+of+Clay-433.html Case Study: Testing a Native Clay Using Insight-Live.com
Troubles	Bloating Bloating occurs when the off-gassing of decomposing particles in a body has not completed by the onset of density and impermeability associated with the vitrification process.

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