Hydrochloric acid, also called muriatic acid, is available at pool chemical supply houses or builders supply stores. Hydrochloric acid will etch most metals but should only be used by trained personnel. It's a very harsh chemical. It releases toxic vapors during use and heats up the metal considerably during the etching process. Safety equipment, such as sealed goggles and rubber gloves, should be used during the etching process. Disposal of unused acid requires taking it to a hazardous disposal facility, since it's a regulated hazardous material.
Saline sulfate is also used widely in etching. The chemical is made by combining copper sulfate with sodium chloride. According to etching artist Friedhard Kiekeben on the Nontoxic Paint and Print website, this chemical is less toxic than acids. Note, however, that "less toxic" does not mean that it is 100 percent nontoxic. It's still a harsh chemical and should be handled by those who have safety expertise training.
Etching artist Alfonso Crujera, also on Nontoxic Paint and Print, explains an etching process that uses electricity combined with chemicals. The chemicals used are copper sulfate, zinc sulfate or ferrous sulfate. An electrolytic bath is made, and the plate to be etched is immersed in the liquid solution. The plate is connected to the positive side of a power supply. The etching occurs by an electrical process that removes metal from the unmasked areas, transferring it to another plate connected to the negative side of the power supply. Again, this process is less toxic, but not nontoxic. The chemicals involved are still very harsh chemicals, requiring proper training and safety gear.
Hydrogen peroxide tarnishing is a relatively nontoxic method of metal etching. To tarnish, polish your plate to a high gloss and remove all residue. Glue on your mask. The mask can be made out of adhesive plain white shelf paper. Draw your design on the paper, and cut it out with a very sharp razor knife. Pour consumer-grade hydrogen peroxide, available at any pharmacy, onto the metal. The peroxide will tarnish the exposed areas. After tarnishing, remove the mask and spray on a clear coating, such as polyurethane. Consumer-grade hydrogen peroxide is not considered hazardous and can be washed down the drain. As with any chemical, safety equipment, such as goggles and gloves, should be used as well.