Eliminating the precipitate from Bordeaux Etch by generating electricity in the etching tray or tank. A "passive" process for zinc plates using Copper Sulphate. Application of the passive process to Galv-on semi-dry method. Extract from GREEN PRINTS by Cedric Green published by Ecotech Design, Sheffield, UK. - a handbook on new methods for non-toxic intaglio etching and metal plate printmaking, featuring the technique of Galv-Etch, a modern development of the 19th century electrolytic technique of Electro-Etching, and introducing Fractint and other new alternative methods avoiding the use of solvents and chemicals harmful to health and to the environment.
GALV-ETCHING WITHOUT ELECTRICITY
a 'passive' non-powered electrolytic process
One of the slightly irritating things about Bordeaux etch and its variants, is the precipitate produced that must be brushed away and can clog the lines and that requires regular filtering of the solution. Powered electrolytic processes have the advantage that no precipitate is produced, but it seems that a few etchers are inhibited or intimidated by the idea of having to buy electrical equipment and do calculations based on areas and amperages. But there is a way of using Bordeaux Etch electrolytically which dispenses with the need for an external source of power like a battery charger, or other power source - a "passive" electrolytic process which generates its own electricity. .(TOP)
generating electricity to eliminate precipitate
An obvious way of eliminating the precipitate in Bordeaux etch is electrolytic, by having a small direct current passing through the solution to carry away the copper ions and deposit them on a copper plate. The earliest batteries invented in the early 19th century consisted of a zinc plate and a copper plate in an acidic copper sulphate solution and the early experiments in electrotyping - depositing copper on the cathode - were done in the battery itself. Thomas Spencer and John Wilson in their patent in 1840 for "Engraving Metals by means of Voltaic Electricity" mention the possibility of two dissimilar metals in a suitable solution not needing to be connected to an external 'voltaic' battery (see Appendix 2 for their patent)
So, to create your self-etching "battery" lay a copper plate in the bottom of the etching tray with a copper strip taped to the back and sticking out of the concentrated Bordeaux etch solution (copper sulphate with or without sodium chloride added), then tape a copper strip to a bared area on the back of the zinc plate to be etched and connect it to the other copper strip, and laid the plate face down supported a centimetre away from the copper plate and leave it to etch. When you have given it about the same time as you would in a concentrated Bordeaux Etch solution take it out, and there will be little or no precipitate and the plate will be etched perfectly, showing all the characteristics of a plate etched electrolytically. If you put a multimeter across the terminals it will show about half a volt and 80 milliamps - not a lot, but a sufficient flow of electricity to carry the copper ions away. In addition to the zinc plate being etched, the copper is 'plated', only if it was very clean, or will have the precipitate collected on it. (TOP)
passive galv-on process
But there is another much more interesting reason to use this 'passive' galv-etching process, and that is that it can be used in the Galv-On semi-dry process that I have already described. The Galv-On process can only be used electrolytically, because any precipitate or bubbles that are produced cannot be brushed or agitated away, and they stay to clog the lines or texture and stop the etch after a while. But one of the problems of using a 'powered' electrolytic process is that the thin sandwich between the plates offers such a low electrical resistance that for large etched areas, a powerful and expensive direct current source is needed, or else the amperage has to be reduced so much that the time taken is far too long, with the practical consequence that it is a method suited only to small plates. But if you try the 'passive' galv-etch with a large zinc plate and layers soaked in copper sulphate backed with a copper plate just connected with a copper strip to the zinc plate, it works perfectly. The small distance between the plates is a positive advantage and a strong current is generated, carrying away all the precipitate and taking only about twice as long to etch as it would in Bordeaux etch. All the effects possible with the 'powered' galv-on process are possible and there is no limit to the size of plate that can be etched. The time taken does not increase with larger plates, as it does with the 'powered' process, as the current automatically increases with the size of the plates. (TOP)
how the passive process works
The electricity generated results from the difference in what is called the 'electrode potential' between zinc and copper, and closing the circuit between the plates allows a current to flow, 'pushed' by the flow of positively charged copper ions towards the copper or negative cathode, displaced from the solution by the zinc. The disadvantage is that the electrolyte becomes exhausted as its copper is gradually replaced by zinc, and it must be disposed of safely (see below). For those willing to tangle with a powered electrolytic process the great advantage is that the electrolyte can be used for years, and one is not limited to etching zinc.
For existing users of Bordeaux etch or its variants, I expect it will be good news to be able to eliminate annoying precipitates and to be able to use the galv-on semi-dry method, without having to invest in electrical equipment, special etching tanks, and grapple with the mysteries of voltage, amperage and calculations of the time required based on plate area. For many courses and studios that have to provide facilities for numbers of students at a time, the passive galv-etch and galv-on processes should be a welcome development.(TOP)