Metal Salt Etching
1. Compared to Traditional Intaglio
Traditional Intaglio Etching Chemistry
Intaglio (from the Italian intagliare, to cut) is a category of printmaking techniques that creates depressions in a flat metal plate to hold ink. Intaglio includes engraving, drypoint, mezzotint and aquatint. Etching, the fastest and most preferred technique, requires exposure to toxic chemicals, including nitric acid or Dutch mordant to chemically etch exposed areas of the metal plate, asphaltum and rosin dust as grounds to resist etching and polycyclic aromatic hydrocarbons (PAH) as cleaning and thinning solvents.
Nitric acid etching solution
Nitric acid (HNO3) is highly corrosive and creates odorless toxic nitrogen dioxide fumes that are absorbed by the skin as well as by inhalation; the fumes can cause chemical pneumonia with acute exposure or emphysema with chronic exposure. The molecular structure is shown at the right (source: Wikipedia Online Dictionary). Contact of nitric acid with chlorine produces lethal mustard gas!
Material Safety Data Sheet (MSDS) information on nitric acid hazards states:
Poison! Danger! Strong oxidizer. Contact with other material may cause fire. Corrosive. Liquid and mist cause severe burns to all body tissue. May be fatal if swallowed or inhaled. Inhalation may cause lung and tooth damage.
Dutch mordant etching solution
Dutch mordant contains diluted hydrochloric acid (HCl) and potassium chlorate (KClO3); the molecular structure of hydrochloric acid is shown at the right above the molecular structure of potassium chlorate (source: Wikipedia Online Dictionary). Dutch mordant produces toxic chlorine gas fumes that cause acute respiratory damage. The potassium chlorate in Dutch mordant is a strong oxidizer that can cause fire or explosion. Graphic Chemical & Ink Company, a printmaking supplier, has refused to carry the ingredients for Dutch mordant since the 1980s.
Asphaltum and rosin dust resists
Asphaltum, a traditional resist, contains PAH which vaporizes above 200°F and is a probable carcinogen.
Material Safety Data Sheets says of asphaltum:
Vapors may be irritating to nose, throat and respiratory tract. High concentrations cause headaches, dizziness, nausea, drowsiness, anesthesia, and unconsciousness and can result in death. Vapors and liquid can cause eye irritation, redness, tearing and blurred vision...IARC monographs list refined bitumens as possibly carcinogenic to humans based on animal studies.
Rosin dust, the traditional aquatinting resist, is highly explosive according to MSDS.
Safer Etching Chemistry
Liquid ferric chloride (FeCl3) is a metal salt that produces no toxic fumes. Ferric chloride and The Edinburgh Etch (ferric chloride mordanted with citric acid, a chemical often found in foods) permit safer chemical etching of copper and brass plates. The Saline Sulfate Etching Solution provides a safer etch for zinc, aluminum and steel. Chemists at Rochester Institute of Technology who tested both solutions found that they give off no toxic fumes and permit safe disposal. Liquid acrylics (such as Future or screenfiller), wax, and fat are safer resists. Gum-based Akua inks are safer than oil-based inks. Vegetable oil and soap are safer cleaning agents than traditional hydrocarbons.
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