Precipitate forms DURING selenium toning.

I understand about precipitates forming in selenium toner during storage.
Just filter them out or decant carefully.

I am referring to black sand-like grains of precipitate forming on the surface of the print, in the tray, while it is being toned.

I am not worried about this, per se, because it is something I have noticed for a long time. I am just curious about it.

I feel pretty certain that these black grains are precipitate are forming as a consequence of the chemical reaction that takes place as the print is being toned.

I feel pretty certain that these black grains are not in the solution before the print is placed in the tray because I have filtered the solution before starting.

The grains are noticed, forming in darker and mid tone areas of the print. They just seem to materialize out of nowhere. They form little pools on the surface and roll around as the tray is agitated. Sometimes, they seem to disappear after a few a minute or two.

Again, I assume that this is all fairly normal. I am just curious about what is happening. I also wonder whether we can infer anything about the toning process by watching for the appearance of this precipitate. For instance, can I tell when toning is nearing completion when I see the grains?

Just curious...

Acid in the paper can cause elemental selenium to precipitate. Possibly from fixer. Neutral or alkaline fixer is supposed to prevent this, or a good soak in a mildly alkaline solution would help too (sodium sulphite would probably do). An alternative is a rather long wash before toning.

So, the thing that I might infer from this is that I need to rinse my prints better?
Good to know.

These are fiber based prints. Ilford Multigrade. I rinse them in Heico PermaWash for about four minutes.
I suppose I need to double check my process, then.