PHOTOGRAPHIC ENGRAVING 563 



The process of M. Charles Negre was more complicated than the preceding, but 

 yielded superior results. His process was not unlike that of M. Fizeau. 



Mr. Fox Talbot thus describes his process of Photoglyphic Engraving : 



' I employ plates of steel, copper, or zinc, such as are commonly used by engravers. 

 Before using a plate its surface should be well cleaned ; it should then be rubbed with 

 a linen cloth dipped in a mixture of caustic soda and whiting, in order to remove any 

 remaining trace of greasiness. The plate is then to be rubbed dry with another linen 

 cloth. This process is then to be repeated ; after which, the plate is in general suffi- 

 ciently clean. 



' In order to engrave a plate, I first cover it with a substance which is sensitive to 

 light. This is prepared as follows : 



' About a quarter of an ounce of gelatine is dissolved in eight or ten ounces of water, 

 by the aid of heat. To this solution is added about one ounce, by measure, of a satu- 

 rated solution of bichromate of potash in water, and the mixture is strained through 

 a linen cloth. The best sort of gelatine for the purpose is that used by cooks and 

 confectioners ; in default of this, isinglass may be used, but it does not answer so well. 

 This mixture of gelatine and bichromate of potash keeps good for several months, 

 owing to the antiseptic and preserving power of the bichromate. It remains liquid 

 and ready for use at any time during the summer months ; but in cold weather it 

 becomes a jelly, and has to be warmed before using it : it should be kept in a cup- 

 board or dark place. The proportions given above are convenient, but they may be 

 considerably varied without injuring the result. 



' The engraving process should be carried on in a partially-darkened room, and is 

 performed as follows : 



' A little of this prepared gelatine is poured on the plate to be engraved, which is 

 then held vertical, and the superfluous liquid allowed to drain off at one of the corners 

 of the plate. It is held in a horizontal position over a spirit-lamp, which soon dries 

 the gelatine, which is left as a thin film, of a pale yellow colour, covering the 

 metallic surface, and generally bordered with several narrow bands of prismatic 

 colours. These colours are of use to the operator, by enabling him to judge of 

 the thinness of the film : when it is very thin, the prismatic colours are seen over 

 the whole surface of the plate. Such plates often make excellent engravings ; never- 

 theless, it is perhaps safer to use gelatine films, which are a little thicker. Experience 

 alone can guide the operator to the best result. The object to be engraved is then 

 laid on the metal plate, and screwed down upon it in a photographic copying-frame. 

 Such objects may be either material substances, as lace, the leaves of plants, &c., 

 or they may be engravings, or writings, or photographs, &c. &c. The plate bearing 

 the object upon it is then to be placed in the sunshine, for a space of time varying 

 from one to several minutes, according to circumstances ; or else it may be placed 

 in common daylight, but of course for a long time. As in other photographic pro- 

 cesses, the judgment of the operator is here called into play, and his experience 

 guides him as to the proper time of exposure to the light. When the frame is 

 withdrawn from the light, and the object removed from the plate, a faint image is 

 seen upon it the yellow colour of the gelatine having turned brown wherever the 

 light has acted. 



'When the plate bearing the photographic image is removed from the copying 

 frame, I spread over its surface, carefully and very evenly, a little finely-powdered 

 gum copal. It is much easier to spread this resinous powder evenly upon the surface 

 of the gelatine, than it is to do so vipon the naked surface of a metal plate. The chief 

 error the operator has to guard against is that of putting on too much of the powder : 

 the best results are obtained by using a very thin layer of it, provided it is uniformly 

 distributed. If too much of the powder is laid on it, it impedes the action of the 

 etching liquid. When the plate has been thus very thinly powdered with copal, it is 

 held horizontally over a spirit-lamp in order to melt the copal ; this requires a con- 

 siderable heat. It might bo supposed that this heating of the plate, after the forma- 

 tion of a delicate photographic image upon it, would disturb and injure that image ; 

 but it has no such effect. The melting of the copal is known by the change of colour, 

 The plate should then be withdrawn from the lamp, and suffered to cool. This pro- 

 cess may be called the laying an aquatint ground upon the gelatine, and I believe it 

 to be a new process. In the common mode of laying an aquatint ground, the resinous 

 particles are laid upon the naked surface of the metal, before the engraving is com 

 menced. The gelatine being thus covered with a layer of copal, disseminated uni- 

 formly and in minute particles, the etching liquid is to be poured on. This is prepared 

 as follows : Muriatic acid, otherwise called hydrochloric acid, is saturated with -per 

 oxide of iron as much as it will dissolve with the aid of heat. After straining the 

 solution, to remove impurities, it is evaporated till it is considerably reduced in 

 volume, and is then poured off into bottles of a convenient capacity; as it cools, it 



oo2 



