Received from Harvey White at 3/5/2019 04:22 PM UTC:
The Ferric Chloride eventually becomes CuCl, but will run out of Chloride, hence adding HCL is needed. In the HCL/H2O2, you start out with enough oxygen from the H2O2, run out of that, then bubble. You really ought to bubble in the first place, since it keeps the etchant moving.
Exactly!
And using just plain CuCl + HCl, with no iron present, avoids the brown color the iron imparts to the solution so it's much easier to tell when the solution needs rejuvenating by bubbling. Emerald green is good, muddy brown is bad. Somewhere in between etches just fine, but most will prefer to maintain emerald green.
Jim H
