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The oven bird knows how to find and use wild clay:

If she can do it, maybe you can

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She is likely getting it from nearby. She knows what works: A mix of sand, clay and organic matter. She isn't even the best mud-nester in the bird world. Some species can stick their nests on to walls and cliffs.

If a bird can find and use a native clay, should a potter or even hobbyist be intimidated by the prospect of doing the same? She doesn't overthink it; she likely just samples different spots and uses the one that works the best. The clay appears to be a typical terracotta; if her nest were fired, it could well come out like these overlaid SHAB test bars (the two types, a sandy and smooth, were fired at cone 4 and 04).

Finding a native clay for pottery can begin just as simply: collect small samples from different spots, see how each one works, and watch how it dries. Then take the step the bird has no reason to take—fire test bars. These show how I compare drying shrinkage, firing shrinkage and fired absorption. A nest proves that mud can be built with. Test bars help determine what that mud can become in a kiln.

This bloating is a normal consequence of overfiring

This is how you know it has too much feldspar

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Example of a whiteware clay bloating at cone 10 oxidation. While it appears stable at cone 8, it isn't. This is a variation on the 25x4 porcelain recipe. It employs 200 mesh silica, kaolin, ball clay and feldspar. Whiteware bodies like this often soften and melt without ever bloating, but not this one. These fired bars appear to show that this vitrifies at cone 8. However, ware fired at cone 8 exhibits the classic evidence of overfiring: It warps. Overfire it still more, to cone 10, and we enter bloating territory by a new development: The ball clay in the recipe contains soluble salts; these become a thin glaze that glosses and seals the surface, trapping LOI gases which create internal voids (top bar). This line graph shows it as a massive spike in absorption/porosity going into cone 10. It also shows that cone 6, the bottom bar, provides the lowest porosity and highest fired shrinkage, while not being on a volatile part of the curves.

This phenomenon is a reminder that the percentage of feldspar (or feldspar-containing clays) is best tuned such that the body is just reaching (or has just reached) its minimum achievable porosity (assuming it is for functional ware). This can be done by making SHAB test bars like this and firing them at a range of temperatures up to and beyond the target to get a broad view of how maturity progresses.

Step 17: Revisiting the test bars - The data

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These screen fragment overlays are from the recipe panel at insight-live.com. The table of data shown here is from the SHAB test only. The row numbers are the specimen numbers of each bar. The first five columns are the data we collected by measuring the bars before and after firing (dry length, fired length, dry weight, fired weight). The last three red columns are the results of calculations it does on that data to produce values for drying shrinkage, firing shrinkage and fired porosity. The graph above charts the firing shrinkage (ascending line) and absorption (descending line) against temperature. These two lines are like a "fired maturity fingerprint". Finding meaning in this data enables characterizing the firing behavior of the clay. In the next step, we will compare it to a terra cotta clay body.

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