( Originally Published Early 1900's )
Photogravure.— The earliest attempt at photographic engraving dates back to 1827, which was six years previous to the introduction of the daguerreotype process, and was the invention of M. Nicephore Niepce of Paris, who first discovered that thin plates of bitumen were curiously affected by light. He therefore coated metal plates with a thin layer of bitumen of the kind called Jew's pitch, and placed them in a camera obscura, so arranged that he could insure their exposure to the same image for several hours. The plate was then submitted to the action of oil of spike, which readily dissolved those portions not acted upon by the light, but exerted little action upon the remainder. The metal exposed by the solution of the bitumen was then acted upon by acid, which produced a completeetching plate, the picture part being protected by its bituminous varnish from the action of the acid. The art, which can now be performed by several different methods, is also known by the names of photo-zincography and process-engraving. In ordinary zincography the picture is laid, by the help of transfer paper, on a zinc plate ; the parts to be protected are then covered with a varnish that will resist acid, and the whole is then dipped in a bath of dilute nitrous acid. This is repeated until the biting-in is sufficient, when the plate is dried and the ink taken off with benzine. In another process brass plates are used, which are covered with white wax, the design being drawn with an etching point upon the wax. The plate is then submitted to a powerful acid, which acts upon the parts of the metal exposed by the lines, but does not affect the wax. In photo-zincography the drawing is photographed to the right size, and an ordinary negative on glass is taken. This is then laid on a sensitized zinc plate, on which the picture is printed by the action of light. The zinc is coated with bitumen, and after the picture is printed, so mach of the bitumen as has not become insoluble by the action of light is removed by a wash of turpentine. In another process — the photo-graphic etching process —the negative is printed on a sensitized carbon paper, which is then laid on a polished zinc plate, and, being wet, all the carbon paper that does not hold the lines of the drawing is readily removed. The plate is then bitten-in in an acid bath. In what is called the Ives process a negative is applied to a gelatine plate, sensitized with bichromate of potash. This plate is then put into water, and all the parts not touched by the negative will swell. A cast is then taken of this in plaster of paris, which serves to form a base for electrotypes. The lines of engraving can also be reproduced by photography, and a late process produces successfully intaglio plates. Photo-engraving has enormously cheapened the reproduction of pictures, but it does not give plates that print with the clearness and distinctness, of those taken from wood engravings.