[pure-silver] Silvery spots on prints


Hi,
Now and then I've noticed odd "silvery spots" on my RC prints. The spots are of irregular shape, about the size of a fingerprint, and usually located on the unexposed area or near the edge of the paper. They are metallic silver or pale brown in colour.
I've taken them as a reminder to clean my trays, assuming them to be the product of dirty equipment, somehow. But the other day I began to wonder if the process could be controlled and the effect produced deliberately. 

So, a few questions:

What is the proper name for this silver spots phenomenon?
Do the spots form in the dev, stop, or fix bath?
How stable are them? I've noticed that at least the pale brown ones can be wiped away from the surface of wet emulsion.
I'm using mostly MGIV RC pearl, Calbe N113 and A300 chemicals. Plain water or weak household vinegar as stop. I'm not sure but usually the spots seem to form when processing the paper face down, and with chemicals that are near to exhaustion.
This is not a problem - it never happens with clean trays and fresh chemicals - but I'm curious about the chemical theory behind the phenomenon. 


Greetings from Finland,
Jukka Lehmus




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