Pho′to-gal-van′o-graph.
（Photography.) A process for producing printing-plates in relief by the consecutive actions of light and electro-deposition. As invented and practiced by Pretsch in 1854: Take a posi- [1680] tive photographic print on paper or glass; cover a sheet of glass with bichromated gelatine; when dry, lay the positive upon it and expose to light. Develop by immersing in water: where the light has not acted the gelatine swells and the parts where the light has acted do not absorb water, and are sunken relatively. Take a mold in gutta-percha, which gives an intaglio. Take a plate by treating the surface with graphite followed by electrodeposition, which gives a matrix like the gelatine. Take a cameo electroplate from the matrix, which gives a plate to be printed from by the ordinary printing-press. For metallic ornamentation. It has been suggested that the surface of a metallic object being polished or frosted, a film of iodide of silver be formed on it, and an image from a negative thrown upon the object in a camera. Being placed in an electrobath, a deposit of copper takes place on the parts exposed to light, while the unexposed parts, protected by the shadows of the negative, receive no deposit of copper. The object is now dried, etched with a solution of sulphuric acid, saturated with nitrate of potash. This attacks the silver and not the copper. When the silver is corroded the required depth, the copper is removed by aqua regia, which does not affect the silver. The surface previously protected by copper is in relief, and corresponds to the lights of the negative, the dark lines of the print, for instance, from which the negative was obtained.
