Waiting by a row of variously muddy jugs for three days, it occurs to me that my efforts may be otiose. I picked the most evenly-textured bit of landslip for the terra sigillata experiment, and it’s possible that, hundreds of aeons gone, the coastal marshes did all this levigation stuff already. Certainly the clay particles are so fine that they seem capable of remaining in suspension almost indefinitely.
A clear inch of water did eventually form on top, which I decanted; and then decanted the next few inches of the jug, which poured smooth as milk (never mind the smell). In case I want to use it, the thicker layer from the bottom of the first sedimentation is drying out on newspaper, and as I continue to wait there is plenty of time to appreciate its subtle perfections.