1878.] to tJic Reproduction of Mnps and Flans. T15 



solution of pyrogallic acid or of phosphorus in bisulphide of carbon. The 

 gelatine relief then receives a thin deposit of copper in the usual way. The 

 thin copper electrotype is backed up with type metal, planed and mounted 

 on a wooden block so as to be of the height of type. 



This method gives the finest results but takes time. 



The second method is to take a cast of the gelatine relief in type- 

 metal. A cast in plaster, wax, &c. must first be taken from the gelatine, a 

 second cast in plaster is made from this, and then stereotyped in the usual 

 manner. This method is quicker than the last, but the results are coarser. 



These processes are now largely used for illustrations in books and 

 newspapers, but, so far as I know, have not been regularly applied to the 

 reproduction of maps. 



JEtcJiing processes. — The processes in the second class, in which a metal 

 plate on which the image has been obtained on a sensitive coating of 

 asphaltum or gelatine is bitten in with an etching liquid, though capable of 

 giving very perfect results, are not, I believe, so much used as the other 

 methods which are quicker and more simple. 



A photographic image is impressed from a reversed negative on a co]3- 

 per or zinc plate prepared as in the Berlin engraving process described 

 at p. 103, and, after development with olive oil and turpentine, is bitten in 

 so as to yield an image in sufficiently high relief for surface printing, the 

 precaution being taken of protecting the finest parts of the work as soon 

 as they are sufficiently bitten, by covering them with stopping-out varnish. 



If the sensitive surface is chromated gelatine, the soluble gelatine may 

 be removed or not, but the etching fluid must be such that it will not 

 dissolve or remove the gelatine from the surface of the plate — solutions 

 of perchloride of iron, bichloride of platinum, nitrate of silver in alcohol, 

 bichromate of ammonia in dilute sulphuric acid are some of the most 

 suitable mordants for the purpose. In any case, the full amount of relief 

 cannot be obtained through the gelatine at one operation. After the first 

 biting-in the gelatine film must be removed and the lines protected from 

 the further action of the etching fluid. 



Gillotage. — The last class, in which a photographic transfer in resinous 

 ink is made on a metal plate, and then bitten in, comprises the simplest 

 and most largely used of these processes. 



The process generally employed is substantially the same as Gillot's 

 *' paniconography", now commonly called * Gillotage,' which is largely used 

 for illustrated papers and various other purposes. 



A polished zinc plate, which has been strongly varnished at the back to 

 protect it from the acid in the subsequent operations, receives a transfer in 

 greasy ink, either from an engraved copper plate, a lithographic drawing 

 on paper, or a photo-transfer print prepared as for photozincography. 



