Jim, thanks for the clarifications and additional info.
THe Fe2Cl3 concentration reduction resulted from a discussion with Oscar, to address issues I was having. (As you said, reaction rates double for every 10 deg C increase, and my conditions generally ran in the mid 80s.
My problems were slow/poor contrast development and unevenness, particularly "slower results" toward the breech, and also difficulty controlling the etching.
I was dipping into a PVC tube tank of Fe2Cl3 (stoppered bbls of course), holding the bbls by a coat hanger wire thru the hole in the rib extension. It was a quick dip to full immersion, nearly immediate removal and immediate hosing down (literally).
The muzzles had a longer residence time in the bath because they were "first in -- last out," and therefore results weakened going back toward the breech. With a short etch time this became siginficant.
I've been told (but not by Oscar) that he modified his process to use ~ 10% Fe2Cl3, in his later years. I have the impression that this improved contrast somewhat, as well as controlability. In any case it certainly did for me.
I'm about to start a set of Lefever G bbls, here in the moderate clime of So CA, and plan to start at 10%. If anyone has confirming (or conflicting) experience in this I'd appreciate hearing.
Also, I got good results (but needed over a dozen passes) by Fe2Cl3 etching every pass (rusting cycle) -- in the pre-logwood era. The Parker Factory Process states just one, fairly heavy etch at the outset, and Oscar's "Parker Factory - adapted" process implies just one. Does anyone have experience in using just one, pre-etch?