Homebrew PCBs

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.html

			1st	2nd
dissolved copper 	75 	180	grams/liter
hydrochloric acid 	1.0 	0.2	moles/liter
specific gravity	1.17	1.118	g/cm^3	
temperature 		21	49	\ufffdC


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.