clay from your backyard or from anywhere(obtained naturally) is not a good material for sculpting or throwing as there are many impurities in it, once air dried it’ll most likely cracked, likewise if its fired in the kiln, commercially bought clay, had all (or most) of the impurities removed, you can however, removed the impurities yourself, it’s a long & troublesome process, many will gaveup once they heard of the process, theorically this is the process to obtain clay,
1)mine the clay, dissolved it water
2)seive it several times to remove floating & suspended impurities, each seiving, use smaller hole seive, to remove smaller suspended impurities
3)leave it for a couple of days ofr it to settle
4)once the water is clear, remove as much water as possible, & let evaporation does the rest,
5)once cracks starts to form on the surface you can remove the top layer & starts knealding, (the clay should not be too wet, where it just stuck to your hand & you can’t do anything with it, it’s still too wet, & needs more drying)
again, clay for throwing & sculpting needs different consistancy