Dave, yeah i can only share my experiences. when you find a great clay no matter where it is ,stick to it!
FINDING CLAY:
the best clays I've found are going to be the purist clays. Color won't be much of a factor.
Road-cuts are good places to look, look under sandstone ledges, wherever coal outcrops are is often a good place to find clays.
Clay is everywhere but its going to require a lot of tests to find a clay that will be both "plastic" for rolling coils, and rigid enough not to slump.
commercial clays are usually garbage! yuck!, you don't know anything about where those ingredients came from.plus why in the world would you BUY CLAY???
MY test for a good clay is first: is it easy to access? is there an infinite supply? is it ok to dig? (am i going to be hiding when the forest service truck comes by?)
if i see some interesting clay from a road cut, i'll take out a baggie and take a sample (mark the nearest mile marker on the bag)
or if i have the time i'll crumble up some of the dry and add some water kneading it in my hand. if it can be rolled into a coil and bent around without cracking,good sign.
BUT even the siltiest river clays will do that and they are only brick material. ONE exception ( a friend sent me some wads of clay from the edge of the St.Lawrence and
it was still quality enough to be made into a vessel with minimal slumping.
When a clay looks promising,take a five gal. bucket home(make sure these buckets are MARKED!) i hate mystery buckets..."what the heck is this stuff?"
dry it in the sun on a plywood board on blocks or something above the line of aim of a male dog.
Once its dry , it can be stored forever. OR it can be processed.
Fill a five gal bucket half full of water and begin putting in the VERY dry clay till the dry clay comes to within 2" to the top of the water.
(clay with moisture in it just stays clumpy) DO NOT STIR but allow the clay to break down in clear water. If you stir it will thicken too soon it will take longer.
In a couple hours gently reach down in the clay "slurry" and feel to see if the majority of clumps are broken down. If so, stir it up. use a stick or you can use
a blunger with a drill . it will get very thick like a milk shake. and might need to be watered down till its like fresh milk. rocks go to the bottom,organic stuff to the top where it can be skimmed off with a piece of window screen. When its thin, its easier to go through the screen and won't get clogged as easily.
screening: take a piece of window screen, and cut out the middle of a plastic bucket lid and you have your screener (see picture)
have someone hold down the ring "lid" over the screen as you pour it through.
once its screened all you have to do is wait for seperation....water goes to the top which you can siphon off with a bit of tubing. or a sponge, or even draping a cloth over
the top sagging in will wick away some moisture and keep skeeters out.