SPECIAL PRINTING PROCESSES 475 



sufficiently to let the gelatin of the carbon tissue adhere to it. However, any paper 

 which is satisfactory for bromoil will work well with nontransfer carV)ro, and there are 

 several English papers of this sort. Kodak Royal Bromide is excellent for the purpose, 

 and in this country, the Defender Photo Supply Company will furnish, on order, 

 several grades of Velour Black which are specially manufactured for bromoil use. 

 Doubtless there are many other papers which will give good results with carbro. An 

 experienced worker can tell whether or not any particular paper will be satisfactory 

 by soaking it for 3^2 hr. or so in cold water, then judging the degree of swelling of the 

 gelatin by feeling it between finger and thumb. 



The bromide print should be completely developed as is done with one which is to 

 be toned by the bleaching and sulphiding process, i.e., development should be con- 

 tinued for at least 2 min. after the print has ceased to gain strength. It should be 

 printed a trifle darker than would be the case with an ordinary bromide print since it is 

 easier to retain gradation in the high lights of the carbro print if this is done. It 

 should be fixed in plain, not acid, hypo, and it should not be hardened in any way 

 whatever. If the bromide print is slightly blocked up in the shadows, this does no 

 harm, since the carbro print will bring out detail which is not visible in the darker 

 parts of the bromide. Any ordinary developer may be used. 



Care must be taken that the bromide print is entirely free from hypo since, if any 

 remains, it will combine with the potassium ferricyanide in the sensitizer to form 

 Farmer's reducer, which will dissolve the silver image, causing pale, or even completely 

 blank, spots in the carbro print. Potassium permanganate must not be used as a 

 hypo eliminator; the only safe procedure is thorough washing with plain water. 



Solutions Required. — Two solutions are necessary, the sensitizer and the control 

 bath. The sensitizer is made up as follows: 



Water, distilled 1000.0 cc. 20 oz. 



Potassium bichromate 12 . g. 110 gr. 



Potassium ferricyanide 12.0 g. 110 gr. 



Potassium bromide 12.0 g. 110 gr. 



Note that potassium ferricyanide, not ferrocyanide, is used. This sensitizer will 

 keep indefinitely if protected from strong light and may be used repeatedly, merely 

 adding to the bulk from time to time, as required. 



The control bath is made up as follows: 



Formaldehyde (40 per cent) 650.0 cc. 22 oz. 



Acetic acid (glacial) 30.0 g. 1 oz. 



Hydrochloric acid (c.p.) 30.0 g. 1 oz. 



Water, distilled 45.0 g. li-2 oz. 



This forms a stock solution. Probably, after a few days, and especially if the 

 solution is exposed to light, a white precipitate will settle in the bottle. This may be 

 filtered off, or it may simply be ignored, provided care is taken to decant the clear 

 solution and to avoid getting any of the precipitate into the working bath. This 

 stock solution keeps indefinitely. 



For use, take 



Stock solution 30 . oc. 1 oz. 



Water 1000 . cc. 32 oz. 



This dilute control bath changes in strength as successive prints are treated in it, 

 therefore it should not be used for more than half a dozen or so and should not be kept 

 but should be thrown away when used. 



Sensitizing. — The carbon tissue is sensitized exactly as described in the section on 

 Carbon Printing, except that the time of immersion should be 3 instead of 2^4 min. 



