486 HANDBOOK OF PHOTOGRAPHY 



If potassium bichromate is used in the developer, this chemical is used up and must 

 be renewed from time to time. This is not the case with the bichloride of mercury, 

 which never loses its effect. 



Trays and graduates should be kept scrupulously clean, or the prints may show 

 wliite spots. If enameled trays are used, care should be taken to avoid those which 

 are chipped or cracked, as the presence of iron in the developer may cause blue spots 

 in the prints. 



Gum Printing. — The printing process which is variously known as the gum 

 process, the gum-pigment process, and the gum-bichromate process is the most flexible 

 and at the same time one of the most beautiful of all photographic printing mediums. 

 It is commonly believed that it is a difficult process to use, but this is by no means the 

 case. The trouble lies in the facts that it is extremely flexible and that the average 

 amateur approaches it in a hit-or-miss fashion, so that he becomes confused among the 

 great number of variables inherent in the medium. But if anyone will systematize 

 his work, operating along definite lines and varying one element at a time, he will find 

 that the process, though somewhat laborious, offers no difficulties of any consequence 

 and that the beauty of its results well repays the necessary effort. 



Insoluble gum ^^ 



Soluble gum Surface of paper 



Fig. 2. — Diagram showing cross section of paper used in gum printing. 



Gum is analogous to carbon and Fresson in that it depends on the insolubilization 

 by light of a pigmented bichromated colloid film produced by light, but it differs from 

 these processes in that the colloid used is gum arable and that development is effected 

 simply by floating the printed paper face down on cold water. Also the paper is not a 

 commercial article but must be coated by the worker himself. There is still a further 

 difference, which has a profound influence on the results and which is illustrated in the 

 accompanying sketch. In this sketch it is seen that the outer portion, or layer, of the 

 gum film becomes insoluble first, when printed under a negative, and that this insoluble 

 layer adheres to the high points of the paper, allowing the soluble gum and pigment 

 to seep out through it. When the fully developed print is dried, this insoluble layer 

 retracts into the hollows of the paper, thus adhering to the latter wherever the gum 

 has been insolubilized. From this it follows that the gum-pigment emulsion must be 

 spread very thinly on the paper, or the soluble gum will not seep out through the 

 overlying insoluble layer but rather will tear it off, causing flaking of the print. From 

 this thinness of the emulsion it follows that single gum is a very short-scale process 

 and does not give rich blacks. If rich black are to be secured, it is necessary to resort 

 to multiple printing, from three to five coatings and printings being required for a 

 very deep black. If a long scale is desired, from five to eight printings will be 

 demanded. It is tiiis need for multiple printing which makes the process laborious, 

 but on the other hand it offers an exceedingly valuable means of control, as by this 

 method it is possible to render the gradations of a negative which is far too strong for 

 any other printing process and it is also possible to emphasize any desired set of tones, 

 either high, middle, or low, by varying the manner of coating and the time of printing. 



