INTENSIFICATION AND REDUCTION 551 



is that given by curve A, and most intensifiers have, at least approximately, this 

 general characteristic of proportionality. 



As shown in Fig. 2, another method of showing the characteristics of intensifiers is 

 to plot the percentage increase in density due to intensification as ordinates against the 

 original density of the test strip as abscissas. The curves of Fig. 2 correspond to 

 similarly lettered curves of Fig. 1. The curves of Figs. 1 and 2 are idealized for the 

 purpose of illustration, and do not represent the measured characteristics of any 

 particular intensifying agent. 



According to whether the slope of the curve of Fig. ? is positive, zero, or negative, 

 intensifiers have been classified as positive-gradient, proportional, and negative- 

 gradient intensifiers, respectively. According to the published work of Nietz and 

 Huse,i and Crabtree and Muehler,^ the following classification of intensifiers with 

 examples of each type, is permissible: 



1. Proportional or zero-gradient intensifiers 



Chromium intensifier 



Mercury intensifier (Monckhoven's intensifier) 



Silver intensifier 



2. Positive-gradient intensifiers 



Copper-tin 



3. Negative-gradient intensifiers 



Mercuric iodide and Schlippe's salts 

 Mercuric iodide and paraminophenol 

 Bichromate and hydrochloric acid and Amidol 



Because all densities are increased by the same per cent in proportional intensifiers, 

 such intensifiers have the eff'ect of increasing the y of the developed film. The effect is 

 much the same as if the negative had been developed for a longer time or at a higher 

 temperature since the contrast is increased. Because of their desirable characteristics 

 and their extensive use, only proportional intensifiers will be discussed in this chapter. 

 For details of the other types of intensifiers, reference should be made to the paper by 

 Nietz and Huse. 



Intensifiers. Chromium Intensifier — Intensification with chromium is a favorite 

 method for moderate degree of intensification and lends itself to various control 

 methods, depending upon the composition of the bleaching bath. 



In chromium intensification, the original silver image is bleached in a solution of 

 potassium bichromate and hydrochloric acid or hydrobromic acid or, alternatively, 

 a solution of chromic acid and sodium chloride. The bleaching process is then 

 followed by redevelopment in any ordinary developer and by washing. 



Although the exact proportions of the various constituents may be varied over 

 rather wide limits, a suitable bleaching bath may be made as follows: 



Potassium bichromate 8 g. 1 oz. 



Hydrochloric acid 6 cc. ^ oz. 



Water 11. 1 gal. 



Research by Carnegie and Piper^ has indicated that the degree of intensification 

 depends upon the concentration of the constituents, especially that of the acid. By 



1 Nietz and Huse, Phot. J., 58, 81 (1918); /. Franklin Inst., March, 1918; Brit. J. Phot.. 65, 179, 191 

 (1918). 



" Crabtree, J. I., and L. E. Muehler, Reducing and Intensifying Solutions for Motion Picture 

 Film, /. Soc. Motion. Picture Engrs., 17 (No. 6), 1001-1053 (1931). 



3 Carnegie, D. J., and C. W. Piper, Intensification and Redevelopment, Am. Phot, 40, 336, 1904. 



