The very start of Chapter 3, Pigments, in The Materials and Techniques of Medieval Painting, describes methods of cleaning impurities and sizing pigment particles. Firstly, floating organic material such as peat, humus, and roots is skimmed off.
After a skimming, I dug around, grinding up little bits of minerals as best I could.
I then went after the larger particles that wouldn't break down. There wasn't much of that material so I think I will simply toss it.
And here's this evening's final product. It is still a wide-ranging mix of particle size. Perhaps it's too soon to be sure but it seems like the smaller particle sizes present the richest color. I'm thinking that the next step is to test whether this material can be reduced by mulling or if mortar and pestle is needed.
I hold such a feeling of history and tradition while playing in my red mud. I'm now heading over to this inspiring virtual tour for another visit.