Etching of copper and zinc plates using diluted acids is a common intaglio technique. Acids can also be used for etching metals for jewelry and metalworking. Acids used include nitric acid and Dutch Mordant (prepared by mixing hydrochloric acid and potassium chlorate).
Skin Contact Concentrated acids are highly corrosive and can cause severe burns if splashed on the skin or in the eyes. The dilution of concentrated acids with water can be very hazardous because of the risk of splashes. When mixing, always add the acid to the water, never the reverse. If you add water to concentrated acid, large amounts of heat are produced which can result in boiling concentrated acid spewing all over.
Gloves, goggles, and protective clothing (e.g. plastic laboratory apron) should be worn when mixing concentrated acids. There should be an emergency shower and eyewash fountain in the area where concentrated acids are mixed. It is important to ensure that there are no electrical outlets or switches within splash range of the shower or fountain.
Dilute acids are less hazardous than concentrated acids. However, prolonged or repeated contact can cause skin irritation (irritant dermatitis). Even dilute acid solutions can cause eye irritation if splashed in the eyes. Gloves and goggles should be worn at all times even when working with dilute acids.
Inhalation Nitric acid etching produces brownish nitrogen dioxide gas. Unfortunately, nitrogen dioxide gas has poor odor warning properties and you cannot tell when you are being overexposed. Large acute overexposures (e.g. from using too concentrated a nitric acid solution) can result in chemical pneumonia. Low concentrations of nitrogen dioxide over a long period of time can cause emphysema. There have been a number of printmakers who have developed emphysema as a result of doing nitric acid etching without proper ventilation. I have inspected many printmaking studios and found that all the light fixtures and other metal surfaces in the acid room were corroded, a clear indication of inadequate ventilation of the nitrogen dioxide.
Dutch Mordant has a different problem. When the potassium chlorate and hydrochloric acid are mixed, highly toxic chlorine gas is released, which can cause chemical pneumonia. Over 15 years ago, five students and teachers in Alberta, Canada got chlorine poisoning from mixing Dutch Mordant without adequate ventilation.
Etching with both nitric acid and Dutch Mordant requires local exhaust ventilation. The acid trays and sink where acids are mixed should have either a slot exhaust hood or laboratory-type hood. Do not use an overhead canopy hood.
Ingestion Although unlikely to be a problem with adults, accidental ingestion of small amounts of concentrated acids by young children could be very serious and possibly fatal. For this reason, it is essential to keep acids and other adult art materials locked away from young children.
Substitution A possible substitute for acids in metal etching is ferric chloride (iron perchloride). Ferric chloride, when dissolved in water, produces hydrochloric acid in small amounts, and repeated contact could cause skin irritation. However, you do not have the hazards of mixing concentrated acids. During the etching process, the ferric chloride gets converted to iron that can accumulate in the etched area. Inverting the plate or using a stirrer can solve this problem. Storage and Disposal Concentrated acids should be stored separately from alkalis such as ammonia and other materials such as solvents because of the risk of a chemical reaction if there is a spill. Concentrated nitric acid can even react with other acids and should be stored separately from all other chemicals. Acids should be neutralized with baking soda (sodium bicarbonate) before being poured down the sink with lots of water. Schools and other large scale users of acids should have neutralization tanks and polyvinyl chloride plumbing.