Place: Vernon, Alabama.
Story: Potter's friend sends a picture of an outcrop of white clay in the ditch near his driveway.
Result: A DIY claybody is born.

This planet is full of accessible clay deposits. Many can be used as-is for stoneware, earthenware and even porcelain. Characterizing this clay is the first step. How plastic is it? What does it look like when fired at different temperatures? Does it contain impurities that need to be sieved out? Does it dry without cracking? Does it work with glazes? Etc.

A journey of clay discovery to a finished piece is one of the most rewarding experiences you can have as a potter. And be more self-reliant. You don’t need special gear, just curiosity, eyes that notice, a few simple tools, and a willingness to experiment and learn to characterize clays. And one more thing: An organized way to keep records of your testing. Think of an insight-live account as a commitment to building experience; it is your memory of everything that worked. And didn't.

Or, more correctly, is this one a clay? The way I found out was to test it myself. That's what I did.

The giveaway of its marine origin is the tiny shells found on the sieve. The Cretaceous Sea once connected the Arctic Ocean with the Gulf of Mexico, covering the great plains of North America. Sedimentation left this deposit of Diatomaceous earth in central Alberta, Canada. This sample contains enough clay that I was able to slurry it up, dewater it on a plaster bat and then prepare SHAB test bars to try it at five temperatures. At cone 10 (bottom right) the porosity is 62%! And the LOI is 32% (others can go as high at 50%). Why? Raw diatomaceous earth contains physically bound interlayer water, it leaves by ~100–300 °C. It also contains structural hydroxyl water (in clay minerals or hydrated silica phases). This “chemical water” burns off between ~400–700 °C. And, organic matter from ancient algae, plants, or soil contamination also burns out between ~300–800 °C (as CO₂ and other gases). Finally, the carbonates (e.g. shells shown here) decompose around 700–900 °C, releasing CO₂. That alone can cause a big weight loss.

Note the test bars under it. Where this bar was sitting there is glassy deposit. What is that? Diatomaceous earth is mostly amorphous silica, but it almost always contains alkali and alkaline-earth impurities and sometimes boron. The latter can literally drain out, as a liquid. However here, the alkalis have volatilized (vaporized) or form alkali-rich fumes. These landed on nearby surfaces to react with the other test bars to form a thin alkali-silicate glass layer (similar to what happens in soda firing).

This is G3948A (similar to the popular Amaco Ancient Copper product). To get this stunning result, it needs to be applied thickly. Therefore, it runs a lot. But the catcher glaze around the bottom of these mugs has stopped the flow. The catcher is a glossy black, G3914A (but Amaco Obsidian would also likely work). I have learned to put it on with the right height (about 2cm) and right thickness, and then apply wax emulsion to prevent the iron red glaze from sticking during dipping. The inside glaze, G2926B, is one I have tested and developed to fit Plainsman clay bodies as a liner.

Watch the G1214Z video to see me convert the G1214M cone 6 clear base into G1214Z cone 6 calcia matte using simple glaze chemistry and recipe logic. This first appeared in the Digitalfire desktop Insight instruction manual 30 years ago. It is an understatement to say that this process is interesting if you want to know more about glazes, their chemistry and recipe logic. Watch this video and see me adjust the recipe of my high-calcium transparent cone 6 glaze to convert it into a calcia matte. In an Insight-live.com account, the process is easy enough for anyone. We'll cut the Si:Al ratio, increase the CaO, maintain the thermal expansion for glaze fit and make the recipe shrinkage-adjustable using a mix of calcined kaolin and raw kaolin. We will even compare it with the High Calcium Semimatte from Mastering Glazes.

Ceramic glazes, like this GA6-B, are actually just glass. But they are not like bottle glass. The latter is formulated to work well in forming machines (harden quickly), melt and stiffen quickly, have low melt viscosity and resist milkiness and crystallization on solidification. The chemistries to accomplish this have adequate resistance to leaching and adequate durability for a few uses. A stoneware glaze melt needs to be much more viscous (to stay put on vertical surfaces). And, it must have a lower thermal expansion (to match common clay bodies). And, it must resist crystallization much more (since it cools slowly). Fortunately, meeting these needs brings along big benefits: Greater durability, hardness and resistance to leaching. Stoneware glazes and bottle glass share a common trait: They have about the same amount of SiO2. But the similarity ends there, stoneware glazes have:

-High Al2O3. Three to five times more! It is the key oxide for durable glass. And it stiffens the melt (that disqualifies high levels from bottle glass).
-The same fluxes (CaO, MgO, K2O, Na2O). But they distribute very differently (half the CaO, half to one third the KNaO, much more MgO). Other fluxes like SrO, Li2O are also common.
-Low KNaO (which they call R2O). In glazes, it produces crazing, 5% is a typical maximum. But bottle glass can have double or triple that (the high thermal expansion is not an issue, and its cheap source materials supply lots of melting power).
-B2O3 melter. It is expensive but can be justified because the glaze is just a thin layer. Glazes at the low end of the stoneware range have 5% or more boron.

Far right: A glass bottle. Left: Small test bottles made from dark and light burning stonewares. Third: A production ceramic bottle. Notice how much the dark body darkens the GA6-B glaze.

I have seven open side-by-side. There are hundreds of them, and all are well-documented with test results and photos. There are glazes, engobes, bodies, materials and special-purpose recipes. All of them are ones that have been shared over the past decade from our Insight-live.com account. These are great to open beside recipes you are evaluating or testing, it can be a real eye opener to see the chemistries and recipes compared.

These videos from Eastfork Pottery demonstrate their use of thixotropic glaze slurries. Watch them to see how effective a highly gelled glaze is. It enables a quick dip, stays fluid while draining, gives even coverage and dries in seconds. These don't hard-pan or settle out in the bucket either. They work on porous or dense bisque. Almost any glaze can be thixotropic if you take the time to learn how to do it. The fast drying enables the use of twin running (or twin belt) foot wiper machines (best shown on these Instagram and Facebook videos).

If your pottery glaze is doing on drying then it will crawl during firing. Wash it off, dry the ware. Then check the water content. If the glaze has worked fine in the past then it is likely going on too thick because the specific gravity is too high - just repeat cycles of adding a little water and dip testing (make it thixotropic if needed). But that was not the issue here. Glazes need clay to suspend and harden them, but too much clay means trouble. This was Ravenscrag Slip, a clay, being used pure as a cone 10R glaze. The glaze appeared to go in perfectly and it dried to the touch in ~20 seconds. But shrinkage continues after that, revealing after a couple of minutes. Fixing the issue was a matter of adding some roasted Ravencrag Slip to the bucket. That reduced the shrinkage and therefore the cracking. Any glaze containing excessive kaolin can be fixed the same way (trade some of the raw kaolin for calcined kaolin). Some glazes that contain plenty of clay also have bentonite - a simple fix for these is to simply remove the bentonite.

Although Nepheline Syenite and Custer Feldspar are used as effective body maturing agents and fluxes in glazes past cone 6, curiously, neither of them melt well by themselves. Thus, both of these come 6 melt fluidity tests add 20% Ferro Frit 3134 to get them flowing. This is a 2021 shipment of the feldspar and a 2022 shipment of the nepheline.

Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz particles present do the same. These are good for glaze fit but bad for ware like this. There are also sudden volume changes associated with cristobalite, but it forms (from quartz) at stoneware temperatures so should not be a concern in terra cotta or a white low fire body. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. The quartz in stonewares goes through a sudden volume change between 950-1150F on the way down. Quartz particles in low fire bodies will do the same. A simple fix is to slow down the entire cooling cycle like this potter did. Or, learn to program your kiln to approach this range more slowly, then ease down through it. No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).