So, an evening’s research on the portable difference engine later 🙂 It seems Copper Sulphate is the best option, as it is reasonably non-toxic, easily obtainable, and doesn’t generate toxic vapour during the etching process.
Another option is Ferric Chloride, which I’ve looked at before for etching circuit boards, but I was always put off by the toxicity/storage issues.
Using the Bordeaux Etch solution, Copper Sulphate can etch Zinc and Steel. According to Steampunkworkshop, it can also etch Brass and Copper.
Sounds ideal for a small run of experiments.
adq 