Things have changed rather a lot in three years and a few months. The chickens now have a new coop which I bought and i'ts raised off the ground. In desperate need, the chicken will dust bathe as best they can under the coop.
More recently I knocked up a shade shelter for them. While the shade cloth isn't waterproof it does help to keep the ground below dryer than that in the run.
Plain fine earth is I've found best for dustbaths. Oddly, all the roosters I've known have prefered slightly damp soil.

I'm sure I've written about this in my thread somewhere. The only material I've added (long time ago) was hard wood ash. It looked the part but the chickens just walked past it and went and dug a hole under some plant.

Yes, they are very endearing most of them. I also find them very good looking hens, not too erm, heavy to be polite and good shaped heads with good vision.
We did have a couple of savages who may have been cage bosses at the battery. The extra freedom and reliable food melowed them out pretty quickly.