The process was first introduced into this country by a firm of printers in Buffalo, New York, and was used by them for several years for illustrating the United States patent office reports until it was superseded upon the introduction of photo-lithography and the subsequent adoption by the government of a uniform standard for patent drawings.
This process may be described in a general way as follows: A copper plate having a highly polished surface is first blackened by the application of a weak solution of sulphuret of potassium, or other chemical which will oxidize the copper. Then a composition, made by melting together in proper proportions, beeswax, zinc-white, and paraffin, is "flowed" over the blackened surface, producing an opaque whitish engraving ground. The thickness of the wax is varied according to the subject to be engraved, but in general should not exceed that of heavy writing paper. After it has been allowed to cool with the plate lying perfectly horizontal, the wax is smoothed down to an even thickness by a steel scraper, and the plate is then ready to receive the engraving.
Taking for an example the engraving of a map, the original copy is either photographed on the wax surface, or is transferred to it by covering the back of the copy with red chalk and tracing over every line with a steel point. The photograph, or the tracing, on the wax must not be a reversed one, as might be supposed, but should "read right." The outlines of the map are then gone over, with an engraving tool which cuts out a small channel in the wax, down to, but not into, the surface of the copper plate. The bottoms of these channels will eventually form the surface of the relief lines in the resultant electrotype plate, but now appear as dark lines against the whitish groundwork of the wax.
The engraving tools are made in different sizes, and therefore channels of varying widths at the bottoms may be cut in order to produce lines of different sizes. In cutting lines to indicate rivers,--which must be thin at the source and increase in thickness as they approach the mouth,--tools are used in graduated sizes. The first one cuts its own line of equal width for a very short distance, then another and slightly wider tool is used, the next still wider, and so on until the river line is completed. In reality a series of steps, the work is so done that the line appears to the eye to increase in width evenly and gradually from a very fine beginning to a heavy ending. The wavy lines indicating hills and mountains are made in substantially the same way. Special steel punches are pressed through the wax to the copper to show town and capital marks, and after all the lines and marks are completed, the plate is ready to receive the lettering. The name of each individual town, city, state, or river is set up in printer's type and stamped one name at a time into the wax. The type is placed in a small tool resembling a vise, which holds it in perfect alignment and on a perfect level. Tools of various shapes are used for stamping the names in straight and curved lines. It is necessary to wet the type to prevent its adhering to the wax.