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Exploring Ferric Chloride Etching

The ethos of paper-works* is to use low tech, low impact techniques so we were immediately drawn to avoid the strong acids, organic solvents etc used in traditional etching processes . Reading around the subject the use of a Ferric Chloride solution to etch copper plates seems like the way to go.

Advantages of ferric chloride

Ferric Chloride has immediate advantages. It is a salt rather than an acid. It is a less noxious chemical to have in the studio than an acid. However it still needs to be treated with care!. Also when etching copper it produces no gaseous by products so fume extraction is not an issue. It can also produce a good quality clean etch. However it has suffered a couple of drawbacks which has limited its use historically as an etching medium. Namely the slow speed of etch and the way that the sludge that develops as a by product can inhibit the etching process.

At its simplest a copper plate can be etched in a shallow tray of a ferric chloride solution. Occasional agitation of the tray will help move the sludge out of the way of the etching process. The etch can be achieved but it is slow. The slowness of the etch increases the chances of breakdown of the resists used in the etching process resulting in etching of the copper plate in areas that it is not wanted.

A better etching tank

Electronics enthusiasts have used ferric chloride as a means of etching copper circuit boards for many decades. Their approach to improving the etching process has been to develop vertical tanks rather than horizontal trays and to equip them with aquarium style air bubblers to provide constant agitation and aquarium heaters to speed up the etching reaction. We have looked at a number of such tanks and opted for the “Velleman ET20 etching tank” which complete with air bubbler and heater can be purchased for as little as £90. It is capable of accepting plates up to 300mm x 200mm.

Ferri chloride etch tank

A better etching solution

Printmakers have meanwhile adopted a differing approach to improving ferric chloride etching by experimenting with the solution itself. The result has been the “Edinburgh etch” a combination of controlling the strength of the ferric chloride and adding citric acid to obtain an etching solution which both acts faster and minimises the production of sludge.

To progress our experiments we have decided to go for the combination of the improved tank system tank and the improved etching solution.

Acrylic resists

The other element in copper plate etching is of course the resists being used. Again our desire for a low tech, noxious free approach led us straight to acrylic based resists. Acrylic resists can be quite simple from the use of acrylic based floor varnishes to a variety of proprietary acrylic resist systems. For our initial experiments we have opted to test Lascaux acrylic resists which come in a wide variety to provide hard ground, soft ground and aquatint style formulations. The full set are obtainable as a relatively modestly priced boxed set of nine 85ml bottles obtainable for around £60.

Chemical Disposal

Edinburgh etch solution is reported to remain potent for up to a year even with regular use but at some point it will need to be replaced. Unwanted Edinburgh etch should be disposed of appropriately. The following advice was obtained from MG Chemicals website

“The solution must not be put down the drain because of residual copper ions left in it. To make it safe for disposal, you can add sodium carbonate (washing soda) or sodium hydroxide to it to neutralize it, until the pH value goes up to between 7.0 and 8.0, testing it with indicator paper. Copper will be deposited as a sludge. Allow the sludge to settle, pour off the liquid, further dilute it with water and then it can be poured down the drain. Collect the sludge in plastic bags and dispose of it as required by your local waste authority”.