Where does the clay come from? I don’t dig my own clay in our backyard. I wish. I purchase from the Laguna Clay Company, and come in a box pre-mixed #510. They keep the content (receipe) secret, so I really do not know.
I used to mix the clay, which was back breaking operation. Yes, mixed. I purchased 50lb bag of dry materials; Hawthorne Bond Clay (Missouri) , OM-4 ball clay (Kentucky), EPK(Florida), Custer Feldspar (South Dakot) and Flint. It is not mine in single location.
In Japan, pottery clay is mined locally, and glazed with local materials (ashes) to make their product unique (one of kind), and people appreciate it. It makes the pottery more expensive . Bizen pottery was known to purchase a lot of the rice field for their clay, by digging the rice field and removing the clay means no more rice production there as well as you do not know what kind of clay you can get until mine. It is very expensive transaction.
Is it possible to do the same in US? Yes, I think some do, however I think people do not see much difference in the final product, or its value. I sometimes wonder how many people see the difference between the high fire and low fire. I think mining and making clay too much time-consuming and challenging practice , meaning finding, testing and processing clay as well as fitting the glaze, not mention storing these unprocessed clay, to justify the cost. I do not mind using the clay in the box as long as I can make my own pottery. Painter uses the paint in tube. I do not think people ever asking the painter, where the paint or canvas comes from.