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2. What is clay?

What is clay?

Old dirt. Chemically it is a silicate with alumina,  water, and sometimes some of the other common elements as well.  It is  re-crystallized sheet crystals that forms as the alkaline oxides leach out of the surface of common rocky silicates.  As the surfaces become atomically porous and fragile, what is left reconsolidates with a little chemically combined water into individual, mostly autonomous sheet crystals.  A good visualization might be to think of clay as rock dandruff. 

Why is clay sticky?

Size and Shape plus Water.  Most clay is so small you have to use an electron microscope to see that it has a flat sheet structure - similar to mica only much, much smaller.  

Water has an attraction to the surface of most silicates (and many other things as well).  If you increase the surface area of a boulder by crushing it into a fine sand, water will just barley have enough of an attractive force to make a sand castle.  Let it dry out though, and it crumbles under its own weight.  Grind it further into a fine silt and it is even easier for water to hold it all together.  Now, if your silt castle dries out, it will be fragile, but likely stand.

Enter CLAY. Not only is clay usually much finer still, but because of its' flat shape it interlaces with itself.  This strengthens it so that when it drys it doesn't crumble easily.  There is even some evidence that at the scale & CLOSENESS of the finest clay, the surface attractive force is enough to balance the small  weight of the particles themselves.  

Think Surface Magnets.  Imagine that all soil particles, from sand to silt to clay, are coated with an extremely thin layer of 'Magnetic Paint'.  This layer is so thin that  it has little effect on something as large as a sand particle. But, when the size of soil particles become so small as to be close to the thickness of the 'Magnetic Paint' itself, things become more and more like a big pile of magnets. 

Now imagine that water molecules are microscopic iron ball bearings,  that themselves are about half as magnetic as the 'Magnetic Paint'.  You can now imagine a sticky plastic mass that is a good model of the physical properties of clay.  Increase the 'water', (the weakly magnetic balls), and it gets softer.  'Dry it out' a little and it gets stiffer.  'Dry it out' completely and you have a hard 'dirt clod'. [Or in our analogy, a pile of interlacing flat magnets.]

Is clay essential for pottery?

Only in the sense that it makes it possible to make durable, complicated shapes out of the other silicates.  If the clay is particularly fine/sticky, it takes surprisingly little to stick a lot of sand and silt particles together.  This is why some 'dirty' sands make more durable sand castles than cleaner sands.