Back in the late '90's I was working as a potters apprentice in the New Mexico desert and one of my jobs was digging and processing native terracotta and stoneware clays. Some of these clays were over an hour drive away. I spent many hard weeks screening and processing native clay until I figured out the method that worked best for me.
If you plan on working with native clays there is a lot of testing involved but a simple way to decide if it's worth the extra hours of processing is to begin with a small sample. Make a small moist ball of clay, roll it and see if it sticks together well. Next flatten it with your fingers, are there a few small cracks or large cracks in the ball? If on the small scale it may be worth taking the next step, testing, which I will cover in another post.
The best method to collect clay for processing is to dig it dry, if it's not, you will have to completely dry it before moving on. Assuming you are working with a large quantity of native clay you will need a few supplies to get started. 2 large plastic garbage cans with lids, 2 screen frames (2' by 2') of different mesh (expanded steel and a metal window screen) a couple of 2" X 4" about 3' long, a sturdy 1-2 quart container and lots of water. Remember to keep this whole operation in close proximity.
Fill 1 garbage can 1/2 full with water and shovel in the dry clay. Wait 1-2 days for the clay to "slake" down or completely break down into a fine slurry with the lid on. The clay will settle to the bottom leaving clean water on the top. Syphon off the top 2/3 of the water and then mix the clay with a shovel to create a uniform slurry.
Now, take the other garbage can and place the 2' by 4's on the top rim. Next place your expanded steel frame on top of the 2' by 4's. With your quart container collect the slurry and pour it onto the screen. The clay slurry will fall through but the rocks and other organic matter will get caught. Clean the screen now and again buy dumping out the rocks *If the clay you are working with is very fine with few rocks you can skip the expanded steel mesh and go to the window screen just add more water to your slurry.
After you have completely screened your clay through the expanded steel it's now time to do it all over again with the window screen. I never said it wasn't tedious or hard work. Simply clean out the first garbage can and screen into there. At this point your clay should be free of debris. Let the clay slurry settle for a day or more with the lid on and syphon off as much water as you can.
For the drying process I recommend that you build another expanded steel frame with 2' by 4's. Create a frame that is 2' X 4' X 4". Nail the expanded steel mesh to the bottom of the frame with horseshoe nails and line the bottom and sides with an old bed sheet, staple into place. Prop your frame up on 4" X 4"s or make legs. Now, using your quart container pour the slurry into the bed sheet and let the water drain out the bottom. Keep the frame covered with a piece of plywood to keep debris out and add more slurry as needed. In a few days the clay should be moist for wedging and it will peel away from the bed sheet. Now wasn't that fun!
It's been years since I processed literally thousands of pounds of native clay but it's a technique I will never soon forget. Next year Jeff and I hope to move to our land near Alfred, NY. Lucky for me it is all native terracotta clay that needs processing.