Clay-Sized Particles versus Actual Clay Minerals

[Ryan M Miller]

A lot of entry level gardening books on soil health define clay particles solely by size (2 μm and smaller). This is generally fine for defining soil texture, but not useful for defining a pottery clay body since not all particles in this size range have the properties of clay minerals. In addition to regular illite, bentonites, montmorillonites, kaolin, and other aluminum silicates, there may be other particles in this size range that do not add clay plasticity or expansiveness; for example, if I have a bag of pure quartz particles (silicon dioxide) that are 2 micrometers and smaller and then I attempt to add distilled water to it to make a clay body, would the resulting paste still have any plasticity or wet strength to it that would make it useable as a clay body? I feel that I would be lucky if I at least get some kind of melted glass that leaves a mess in a kiln from such a "clay" body. What if I have a bag of dried peat or muck particles that are 2 micrometers or smaller? The resulting paste when water is added would not be suitable for firing in a kiln even if it does have suitable plasticity or wet strength since it would just burn away if it were fired in an oxidizing environment. Similarly, if I attempt to make a clay body of limestone (mostly calcium carbonate) that has been ground down to 2 micrometers or smaller, the resulting paste would just reduce in the kiln, turn into caustic quick lime, and gradually crumble apart as the fired object is explosed to atmospheric moisture.

[Kelly in AK]

Not to mention when you find honest to goodness clay minerals that are 2 μm and larger! That’s called silt, by the geologists, and it sure ain't  clay. Looks like it. Feels like it. Don’t behave like it though. You gotta squish that stuff between your fingers Ryan! Put it in your mouth to feel the grit. Make a pinch pot out of it and fire it. Then you’ll know for sure what you got.

All hillbilly talk aside, my point is there are no substitutes for field testing.

There was a great deal of data generated after WWII on Alaskan minerals, clay included, as well as detailed analysis of clay in my area (Anchorage) after the 1964 earthquake. That data has been helpful, but didn’t teach me how to differentiate the most useful clay materials from what it’s inter-layered with. It was hands on collecting, testing, and comparing that led me to being able to gather useful clay. A seam of excellent clay may be between layers of silt that, to the eye, are indistinguishable.

Further, my recent post in “Wild clay processing” illustrates another caveat on relying on data alone. It is recorded from more than one source that refractory clay is available from the Sheep Mountain site. Enterprising people in the 1950’s made firebrick from it. I got some delightful clay there, it threw  beautifully without any additions or alteration. No cracking or appreciable shrinkage. Curiously, it didn’t settle when I tried to make terra sig from it. It melted around cone 02. Not refractory at all.  I surmise I was near the source of kaolin, but not near enough. It’s also a place gypsum was mined.

Get your shovel and go have some fun. 

[glazenerd]

13 hours ago, Ryan M Miller said:

A lot of entry level gardening books on soil health define clay particles solely by size (2 μm and smaller). This is generally fine for defining soil texture, but not useful for defining a pottery clay body since not all particles in this size range have the properties of clay minerals. In addition to regular illite, bentonites, montmorillonites, kaolin, and other aluminum silicates, there may be other particles in this size range that do not add clay plasticity or expansiveness; for example, if I have a bag of pure quartz particles (silicon dioxide) that are 2 micrometers and smaller and then I attempt to add distilled water to it to make a clay body, would the resulting paste still have any plasticity or wet strength to it that would make it useable as a clay body? I feel that I would be lucky if I at least get some kind of melted glass that leaves a mess in a kiln from such a "clay" body. What if I have a bag of dried peat or muck particles that are 2 micrometers or smaller? The resulting paste when water is added would not be suitable for firing in a kiln even if it does have suitable plasticity or wet strength since it would just burn away if it were fired in an oxidizing environment. Similarly, if I attempt to make a clay body of limestone (mostly calcium carbonate) that has been ground down to 2 micrometers or smaller, the resulting paste would just reduce in the kiln, turn into caustic quick lime, and gradually crumble apart as the fired object is explosed to atmospheric moisture.

Ryan:

Particle size is not the only indicator of plasticity: although it plays a predominant role. The scientific community measures in um; but most pottery literature is given in mesh. micron, or sub-micron particle definitions. Older pottery and geology books cite "clay mineral"; which has been replaced with "montmorillites." Bentonite and hectorite both are classed as "clay minerals: which by definition means a mineral with clay properties. In addition: plastic characteristics are also determined by aluminum levels, and naturally occurring calcium, potassium, and sodium levels. Particle structure also plays a role: a 1/1 particle (kaolin) is very low plasticity; while 2:1 particles (ball clay, bentonite, hectorite) can be very high in plasticity. If you want to do a deep dive; I will send you some links.

Tom