I was unhappy with the depth of my brass etching. Calculations
suggested that I should be getting about 0.020" depth for the given
current and time but I was getting about 0.006". I thought that the
explanation was that the current was wasted on gas production at the
anode (whatever the gas may be).

I was not sure what the problem was. I found a pot of my old
electrolyte (bought from a commercial vendor) and tried it. I quickly
found two things;

1) At current density over 0.1A/cm2 there was gas production on anode
and if one went even higher there was gas production on the cathode as
well.

2) At the beginning of the process there are some weird things
happening. My power source delivers constant current and to do so the
voltage fluctuates wildly (up to 20 V) and at one point there was an
oscillatory type of effect where the voltage dropped, gradually
increased and dropped again at about 1 of these events every two
seconds.

Anyway I managed to etch a piece of brass and separately a piece of
copper with an area of 100mm2 to the depth of 0.009" which was
consistent with calculated prediction.

Today I tried my new electrolyte (CuSO4, H2SO4 in H2O) on a piece of
brass with an area of about 450 mm2 (I had an accident with the
resist). At the start of the process I set the current at 0.45A and
got it with hardly any voltage at all. At 8 minutes the instability
started as above, then it settled with the current steady and voltage
of 10V. After 90 minutes the instability reappeared but was less
dramatic, the voltage oscillating between 2 and 4 volts keeping the
current steady.

There was no detectable gas production throughout.

I went for total 2 hours. The result is he

http://www.flickr.com/photos/2768312...in/photostream

I cannot even begin to guess why the current blew a hole where it did
while only removing between 0.0065" and 0.0075" elsewhere.

The cathode, BTW, showed a pretty even deposit of copper throughout.

Michael Koblic,
Campbell River, BC