564 PHOTOGRAPHIC PRINTING 



Eolidifies into a brown semi-crystalline mass. The bottles are then well corked-tip, 

 and kept for use. I shall call this preparation of iron by the name of " perchloride of 

 iron." It is a substance very attractive of moisture. 



' Water dissolves an extraordinary quantity of it, sometimes evolving much heat 

 during the solution. I find that the following is a convenient way of proceeding : 



'A bottle (No. 1) is filled with a saturated solution of perchloride of iron in 

 water. A bottle (No. 2) with a mixture consisting of five or six parts of the 

 saturated solution and one part of water. And a bottle (No. 3) with a weaker liquid, 

 consisting of equal parts of water and the saturated solution. Before attempting 

 an engraving of importance, it is almost essential to make preliminary trials, in 

 order to ascertain that these liquids are of the proper strength. I have already ex- 

 plained how the photographic image is made on the surface of the gelatine, and 

 covered with a thin layer of powdered copal or resin, which is then melted by hold- 

 ing the plate over a lamp. When the plate has become perfectly cold, it is ready 

 for the etching process, which is performed as follows : A small quantity of the 

 solution in bottle No. 2, viz. that consisting of five or six parts of saturated solution 

 to one of water, is poured upon the plate, and spread with a camel's-hair brush 

 evenly all over it. It is not necessary to make a wall of wax round the plate, be- 

 cause the quantity of liquid employed is so small that it has no tendency to run 

 off the plate. The liquid penetrates the gelatine wherever the light has not acted on 

 it, but it refuses to penetrate those parts upon which the light has sufficiently acted. 

 It is upon this remarkable fact that the art of photoglyphic engraving is mainly 

 founded.' 



Photographic engraving has not, up to the present time (1874), been successfully 

 introduced into the arts. Many especially interesting processes have been devised; 

 and on the small scale, with the proper amount of care, they have been all that 

 is desirable ; but, when it has been attempted to apply the process on the larger scale, 

 it has either failed entirely, been uncertain in its results, or too costly for the general 

 public. 



PHOTOGRAPHIC PRINTING. Numerous experiments have been made 

 for the purpose of preparing surfaces upon which pictures could be obtained by the 

 action of sunshine, or on to which photographic pictures could be transferred. 



Most of the processes are founded on the peculiar action of the solar rays upon the 

 bichromate of potash dissolved in a solution of gelatine. It is not possible in a work 

 of this class to do more than deal with the general principles involved. In the last 

 edition, Woodbury's relief-printing was alone described. A portion of that description 

 is retained. 



The process is thus described : A sheet of talc (mica) of the size required is affixed 

 with gum or water to a plate of glass, and placed on a levelling stand ; some bichroma- 

 tised gelatine is then poured on its surface to form an even coating. When quite dry, the 

 tele by means of a knife is removed, the exposed surface carefully cleaned, and placed 

 in contact with the negative that is to be reproduced. The gelatine is protected by a 

 piece of blotting-paper, then covered with a glass to ensure uniform pressure and close 

 contact between the talc and the negative. After exposure to the sun for an hour, 

 the film must be placed face upwards in a dish of hot water ; this will dissolve all the 

 gelatine unacted upon by the sun, leaving a picture in relief, the parts most acted on 

 standing in highest relief. When no more gelatine will dissolve, the film is dried by 

 heat to a certain stage, then naturally. 



The metallic intaglio is produced in the following manner : The gelatine relief, 

 with a clean sheet of metal composed of type-metal and lead placed on it, is sub- 

 mitted to hydraulic pressure. As it is of the greatest importance that it should be 

 kept perfectly flat, a sheet of steel of sufficient thickness to prevent its bending or 

 yielding when in the press, is placed under the talc, and a similar one on top of the 

 metal. The amount of pressure varies with the softness of the metal employed, but 

 the approximate amount may be stated as 4 tons to the square inch. This process 

 does not in the least injure the gelatine mould, which will servo many times. 



The process of printing from the metal mould is conducted in the following 

 manner : The press is made in the form of a very shallow box with a winged lid. In 

 the bottom of the box is placed a plate of thick glass (resting on four screws), and in 

 the lid is a similar plate. The mould is placed face upwards on the glass in the lx>x, 

 and raised by means of the screws to come in contact with the glass lid when closed. 

 A small quantity of ink is then placed on the middle of the mould, the sheet of paper 

 is laid on the top of the ink, and the lid being closed, the ink spreads out between the 

 paper and the mould, filling up the cavities in the latter, the superfluous portion 

 escaping over the edges. The lid should remain closed until the ink is sufficiently set 

 to allow of its being removed with the paper. The conditions required in the ink are 

 fluidity with rapid setting, transparency, and facility for the removal from the mould 



