I came across NontoxicPrint.com and they outlined a way to etch zinc plates (which I use) without nitric acid and the toxic fumes it emits. You can actually etch zinc, aluminum and steel in a mixture of copper sulfate, table salt and water.
So, I decided to do a test and etch a small zinc plate (6" x 3") with a 50/50 mix of copper sulfate and regular table salt (not iodized). See print above. The solution was a small amount -- 1L hot water mixed with 200ml of the powder mixture (100ml copper sulfate and 100ml salt). I used an old plate that had hard ground on it from a long time ago and didn't bother to reground the plate before I drew into it (hence the messiness of the lines and the bitten edges from old ground flaking off). I had no idea how much time would be needed so I sectioned the plate off into 15-minute increments except for the first 10 min section.
What's interesting is that copper sediment forms at the etched areas much the same as bubbles form when the plate's in nitric acid. So you need to brush off the sediment in the same way as the bubbles. I didn't brush anything off during the etch because I wanted to see what would happen. I did, however wipe off the sediment each time I took the plate out to cover a section for the next etch. And in doing so, I inadvertently removed some of the old hard ground so there are some foul bitten areas around the marks in some areas.
For the next test I'm going to use the acrylic, waterbased ground (Z-Acryl) and see how that holds up to this process. It looks like this process etches in about the same time as nitric acid and there are no harmful fumes in the air during etching. So an exhaust system isn't necessary and (yay) I can etch in my studio fume-free. The solution is considered a marine pollutant as it is toxic to water wildlife and needs to be disposed of properly (NOT down the drain), yet according to NontoxicPrint.com:
The left over copper residues from the etching process are a valuable resource. You may well be able to find a local recycling firm that actually pays for your etching residues - a substantial part of these is solid copper. Or, with the addition of sodium bisulfate to the etching residue you may actually be able to reclaim the solution a number of times.So, there you have it. No more need to handle deadly nitric acid when there's a much safer way to etch. And NontoxicPrint.com has recipes for solutions to etch copper and brass as well (amongst other non-toxic printmaking techniques).
UPDATE: I've updated the links for nontoxicprint.com since one wasn't working at all and the others didn't direct to the relevant page.
NOTE: There is a comment below from an anonymous commenter who explains a chemical reaction between copper-sulfate and salt. Since I have no chemistry background, I cannot verify whether or not this information is accurate. However, please note that there is always a risk involved in using and mixing chemicals that you are not familiar with. Please use caution and get as much information before-hand when using all etching methods whether they are considered traditional or "non-toxic." I find that nontoxicprint.com, Zea Mays, and others are good resources to look into for comprehensive information on non-toxic etching as well as other "greener" printmaking materials and techniques.