That's very interesting, Tom -- I read the article you linked to. I eventually realized that what I was doing was lithography and not etching, which was a little disappointing at first. In etching you are making recesses that accept the ink, while in lithography you are making slightly raised areas that repel water, and so accept oil based ink. So the difference is in one it's a sunken image, and in the other it is a raised image.
The big difference is in how you ink them. In an etching you put ink all over the whole plate, and then remove it gradually from the high areas by wiping, which become the white space on the finished print. In lithography, you are rolling ink on the high areas (you hope) because the other (white) areas stay damp with water and repel the ink. Of course if you don't wipe the whole plate carefully and frequently with a water sponge before rolling each time, the ink sticks everywhere, and the image can be ruined. That's what happened yesterday.
I would still like to try etching, and I'm thinking the electrolysis method that Rob used to engrave would work as well as an acid bath. I think an etched plate is much more robust than what I've been doing -- even if you mess up a print, you can always clean back the plate because the image is incised. Doing the litho prints that I am, the plate can easily get ruined if you mis-ink it. I find that I have to do the artwork and print immediately, also -- the plate won't work after a day. That's an area that my be something I can improve upon, since commercially, litho plates can last longer, and be re-used, I think. I'm thinking the addition of gum arabic in the coke, might make the plate last longer.
Anyway, interesting stuff, and I would definitely like to try "real" etching with electrolysis, as well as improve the success rate of simple litho.
Josh, thanks. Yes I used to work on a newspaper in the '70s with our own press, and I remember the photo plates -- pretty thin, though <1 mm-- people used to patch their cars rust holes with used plates.
There are no print shops here, and the aluminum sheet I bought was only $17 and is infinitely re-usable for litho (not etching).
For etching, I think steel might be good -- it was used traditionally, and supposedly gave the longest lasting print runs, compared to copper and zinc, which were the other traditional types used for etching plates. Steel can definitely be engraved by electrolysis.