A couple of years ago i tried to get some clay from a nearby creek, but i found it extremely difficult to replicate his way of making things from clay. Apparently the region he is doing his stuff got some perfect clay everywhere.
most soils contain some clay; if the problem is that all your clay is impure, you can mix it into a slurry and 'decant' it (let it settle, then pour off the water) and let it mostly dry
the clay is far finer than the sand and silt, so it will settle most slowly, and the topmost layer will be mostly clay
some soils deflocculate the clay, so you may need to add a flocculant to get it to decant properly
if your clay is very expansive you may need to add a lot of temper to keep your work from cracking while drying; the good news is that very expansive clays like bentonite are also highly plastic and so can tolerate a higher temper fraction. you probably want the shrinkage on drying (from the fully plastic state to the bone-dry state, though leather-hard is usually a good enough approximation) to be about ⅛. if you have a lot of kaolin you may be able to go lower than that but if it's much more than that you're going to have a really hard time with cracks during the final stage of drying (from leather hard to bone dry)
making things from clay requires a lot of knowledge but fortunately the humans have been doing it for twelve thousand years and so have accumulated that knowledge already
i'm mostly just repeating what i've been told in pottery classes, digitalfire.com, pottery groups on fecebutt, and classes and workshops i've attended on adobe construction
i guess i've also spent a few hundred hours reading about clay (archaeology papers, composite materials papers, textbooks on functional fillers for plastics) and playing with clay, but what i wrote above goes well beyond what i can justify from my own direct observations
Before the sedimentation pit video he just spent a lot of time manually extracting sticks and rocks from his mined clay. The videos often compress the work of a month into about 10 minutes, so it definitely ends up looking easier than it really is. Usually though it's obvious how much time is invested, simply because he shows the failures as well as the successes and when time skips ahead it's clear how much repetitive work he must have done.
I had that experience over and over as a kid reading about early human development, going out and trying to find my area's equivalent of some material and ending up with nothing but a mess.
In Art History class they mentioned off hand that the people who knew where to find random rare stuff were very valued in early societies. No citation was given though.
I imagine in a pre tech world people would develop a sharp eye for “that looks unusual, let’s take some of that and mess with it and see if it’s useful somehow”.
Well that and they were literally just the same as we are now.
Imagine if you had 80 years to live and nothing but rocks to play with. You'd get pretty good with rocks and you would be pretty desperate to find something new and interesting to play with.
