Thanks for your answer but all you said was done! We solved the problem increasing the temperature from 60ºC to 80ºC and develop for 4 minutes instead 10 minutes and those very circular spots disappear!
We only had those spots with Mdag not with Bdag.
Unless you also cleaned the walls of your mercury box, it is puzzling why the temp/time change worked so well.
Perhaps the higher temp allowed refluxing mercury to rinse accumulated polish particles from the walls of the box.
I develop under vacuum for 2 hours at 20C and get no spots if the plates and box are clean.
Here is a little “collection” of our various black spots, taken from a bad miscroscope.
We continue doing tests and reading, changing lots of variables, even plate provider…still we do have some.
We’ve started the developpement from very cold, for a test, and it did increase huggely the amount of spots.
We went back to a 85/90 degre developpement…
from the same plates, a becquerel comes casi perfect.