Experiments in the electro etching of aluminium.

salt bridge and aluminium

Each of the cells is a 1.5 L flask. The electrolyte in the anode cell is copper sulphate 25g made up to 1L with tap water. The cathode is copper mesh in an electrolyte of sodium chloride 25g made up to 1L with tap water.

The salt bridge is made up of 50ml water 5g potassium nitrate and 1g-agar agar.

potassium nitrate salt bridgeThe aluminium anode measures 65 x 250mm divided into 8 sections each section was etched for fifteen minutes.

Etched at 7.5v. 0.04amps. Using two salt bridges of 50ml.

plate one

At the start of etching there were some static bubbles in the mesh. These were shaken free and a small drop of washing up liquid was added as a wetting agent, the bubbles did not re-appear.

Showing no bubbles at cathodeThe resulting etch is fine with a good tooth for holding ink. At the fifteen-minute intervals the newly exposed areas of the plate were degreased, it was also necessary to brush away copper deposits from the previously exposed areas of the plate. Etching was active for over two hours and the electrolyte was still active when etching was stopped. Very small fast moving bubbles were visible on the anode but there were none visible on the cathode.

There was no smell of bleach or chlorines coming from the set up. The Ph at the anode remained at 4 from start to finish. The Ph at the cathode started at 6 climbed to 7 at finish. Alumina gel appeared in the cathode cell after half an hour. This can be controlled by the use of sodium gluconate.


Prior to using the salt bridge a test plate was etched in each of the electrolytes, the plates were pitted rather than etched. Etching with the bridge produced a good tooth that holds ink.

Why is there no gas visible on the cathode?

Etching will take place with electrolytes made up of 50-50 sodium chloride, copper sulphate as low as 5g to a litre of water. Clearly this saline sulphate mix will etch aluminium without electrolysis but the resulting etch is pitting rather than etching. With electrolysis a nice tooth produced that holds ink really well. The staying power of this electrolyte has yet to be tested. The salt bridges also build a very high level of resistance into the system. It’ll be interesting to see what the reaction of it will be to an increase in the volume of electrolyte and increase in plate size. Without the resistance created by the salt bridges it will be possible to reduce the voltage and increase amperage and possibly further reduce the copper sulphate and sodium chloride used in the electrolyte. Given the scale this system is working at there is probably no danger from chlorine gas or other nasties like caustic soda.

But, because of the concerns about this Bob Perkin suggested that sodium carbonate might be used replace the sodium chloride solution. A quick and dirty experiment was run using sodium carbonate and the results are encouraging. The system was set up as above (fig. 1) with sodium carbonate in the cathode cell.  As with the sodium chloride there were no bubbles visible at the cathode.


There will certainly be no danger from chlorine in this set up but there could well be another genie in the bottle. Although this does not qualify as a non-toxic approach it is a little less toxic and slightly safer and easier to control than saline sulphate? Whether the mark making potential, length of working life or quality of etching is as good as or better than saline sulphate will take a little more experimentation.



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