Electrotyping
Electrotyping (also galvanoplasty) is a chemical method for forming metal parts that exactly reproduce a model. The method was invented by Moritz von Jacobi in Russia in 1838, and was immediately adopted for applications in printing and several other fields. As described in an 1890 treatise, electrotyping produces "an exact facsimile of any object having an irregular surface, whether it be an engraved steel- or copper-plate, a wood-cut, or a form of set-up type, to be used for printing; or a medal, medallion, statue, bust, or even a natural object, for art purposes." In art, several important "bronze" sculptures created in the 19th century are actually electrotyped copper, and not bronze at all; sculptures were executed using electrotyping at least into the 1930s. In printing, electrotyping had become a standard method for producing plates for letterpress printing by the late 1800s. It complemented the older technology of stereotyping, which involved metal casting. By 1901, stereotypers and electrotypers in several countries had formed labor unions around these crafts. The unions persisted into the 1970s, but by the late 20th century, after more than a century in widespread use for preparing plates, the two technologies had been bypassed by the transitions to offset printing and to new techniques for the preparation of printing plates.
As with metal casting and stereotyping, a mold is first formed from the model. Since electrotyping involves wet chemical processes and is done near room temperature, the molding material can be soft. Materials such as wax, gutta-percha (natural latex), and ultimately ozokerite were used. The mold's surface is made electrically conducting by coating it very thinly with fine graphite powder or paint. A wire is attached to the conducting surface, and the mold is suspended in an electrolyte solution.
Electrotyping is activated by electric currents that flow between anode wires that are also immersed in the solution and the wire connected to the coated mold (the cathode). For copper electrotyping, a typical aqueous electrolyte contains copper sulfate (CuSO4) and sulfuric acid (H2SO4), and the anode is also copper; the arrangement is illustrated in the figure. The electric current causes copper atoms to dissolve from the anode's surface and to enter the electrolyte as copper ions (Cu++ in the figure). Copper ions are taken up by the mold's conducting surface at the same rate at which copper dissolves from the anode, thus completing the electrical circuit. When the copper layer on the mold grows to the desired thickness, the electric current is stopped. The mold and its attached electrotype are removed from the solution, and the electrotype and the mold are separated. An animation of the electrotyping process was produced in 2011 by the Metropolitan Museum of Art. Other metals besides copper can be electrotyped; similar procedures apply, but each different metal needs its own anode and electrolyte chemicals.
...
Wikipedia