Not sure yet. I'm still experimenting as we speak. Sorry I can't
give you anything definite yet.
The two recipes I can compare are from
http://users.rcn.com/rexa/Projects/CuCl_ech.html and
http://www.pcbfab.com/iepart3.html1st2nd
dissolved copper 75 180grams/liter
hydrochloric acid 1.0 0.2moles/liter
specific gravity1.171.118g/cm^3
temperature 2149°C
I've made up some cupric chloride solutions by dissolving copper
in HCl over a period of about a week. I'm using this as a base
for experimenting with. I'll make up the above two solutions and
compare the etch rates (only at room temperature). I know the
more copper concentration the faster the etch, but I thinking
there must be a level where the etch rate drops off.
The hydrometer is a good way of getting an indication of the
copper level. I made a small colorimeter device with a red LED
and PIN photodiodes to measure absorption. This gives me a direct
reading of copper concentration. Of course you don't need a
colorimeter to maintain the etching tank, but I'm using it for
analysis while experimenting.