I'm using ferrous chloride to etch my head tube badge out of copper or bronze. My initial tests have been with copper.
Ferrous chloride is pretty mild, as a far as stuff that can eat metal goes, but that's still a scale you have to respect overall. As long as you don't get it in your eyes or let it linger on your skin, it's not going to mess you up (use ventilation too). It will eat metal, obviously, including your sink. The copper ions it absorbs when you use it are also environmentally harmful. You have to save your waste and dispose of it properly (local hazardous waste disposal should have some idea how to handle it).
It won't eat through plastic, glass, or a few other random things. I made sure that all of the dishes and bottles that I used were plastic.
I won't go through all of my trial and error, because there are a million sites up with different instructions for etching with ferrous chloride, for both art and making circuit boards. The short version is: we need to get something onto the metal that the etchant won't eat (a resist) in the shape of the image. Then we dip it, and everything else gets eaten down a few mm.
Eventually, I ended up printing the design (reversed and inverted) onto a page out of a National Geographic using a laser printer (toner makes a great resist, inkjet ink doesn't). Any shiny magazine paper should work as well, but National Geographic adds a bit of class. I ironed it in place, soaked it and sprayed it with water until most of the paper flaked off, leaving just a thin layer stuck to the toner. I found that the transfer worked best if I left the iron on the back of the plate for about five minutes to get the plate nice and toasty, then flipped it and heated the back of the image for another five. I touched up the finished product with a few dots of Sharpie marker, which is a passable resist, and then used a vertical band saw to trim the metal around the edges.
I did a quick test piece first, which failed miserably. I was pretty bummed until I put together that the 40 degrees out on my porch probably wasn't an optimal temperature for this reaction. I suspended the plate upside down in a piece of Tupperware using tape, with ferrous chloride just covering it. Leaving it upside down keeps the waste products of the etch falling off of the plate so that they don't leave uneven bits.
Next, I moved the whole deal into my apartment (inside a trash bag, to prevent leakage and contain fumes), and left it in a shallow plastic dish of hot water for an hour and a half.
When I removed the plate, I had this:
Good so far! To neutralize the residual etchant, I dipped it in a mixture of baking soda and water. Then I scrubbed it with acetone and, as a quick final touch, repeated the entire process with a slightly different design.
Now I just have to hammer it until it'll curve around the head tube and then paint it. I plan to add some black paint to the lower sections and then clear coat it.