1854.] 



METHOD OF PRESERVING THE SENSITIVENESS OF COLLODION PLATES. 



up to tlie present time its use has been greatly resti-icted by 

 the necessity for preparing the plate and completing the whole 

 of the manipulatory details within a comparatively short space 

 of time, thus rendering this beautiful process practically inap- 

 plicable in all cases where the conveniences of a photographic 

 laboratory are not at hand. 



For some time past we have been investigating the causes 

 which operate to prevent the excited plate retaining its efficiency 

 for more than a few hours. It seemed highly probable that the 

 permanent sensitiveness of the film was principally dependent on 

 the retention of a moist surface ; and if by any artificial means 

 this end could be secured, the original sensitiveness of the 

 film would be, for at least- a reasonable time, preseiTcd unim- 

 paired. 



The only attempts up to the present time to effect this object 

 are, we believe, that of M. Girod*, who proposes to enclose the 

 sensitive collodion film between two plates of glass, with only .so 

 much of the exciting silver solution as might be retained bj' 

 capillary attraction ; and thus by retarding the evaporation of 

 the water, to keep the surface moist, and consequently .sensitive 

 for a longer period ; and secondly that of M. Gaudinf , who 

 suggests the use of perfectly air-tight dark frames or boxes, 

 in which a number of the wet plates could be arranged in a 

 horizontal position, and there kept until required. Besides 

 these two methods, it is well known that the plate will remain 

 excited for a considerable time if kept immersed in a solution 

 of nitrate of silver; in fact, a glass bath in the camera has 

 been often used in cases where the length of exposure was 

 likely to be too prolonged to admit of the plate being placed in 

 the ordinary slide. 



Instead, however of having recourse to a mechanical means 

 for preventing the evaporation from the surface, we have endea- 

 voured to avail ourselves of a chemical process, by the employ- 

 ment in the bath of substances having a powerful affinity for 

 water; in the choice of those, however, we are necessarily limited 

 to such as are neutral in constitution, and do not form insoluble 

 compounds with silver. The nitrates and acetates, especially 

 the firmer, seemed most convenient for our purpose on account 

 of their general deli(jueseent nature, and for our first experi- 

 ments we selected the nitrates of lime, magnesia, and zinc, as 

 most promising of success. These agents were at first tried in 

 the above-mentioned order; but from a few preliminary trials 

 we were inclined to give the preference to the zinc salt, and 

 having obtained such .satisfactory results with its use, are in- 

 duced to communicate them at once rather than withhold them 

 until our investigation of the other compounds shall have been 

 completed. At first wo endeavourcil to add the nitrate of zinc 

 direct to the exciting bath, but the quantitj' required to prevent 

 so large an amount of nitrate of silver from crystalizing out on 

 the plate rendered the solution too dense to work with. 



The followinfr process can lie recommended as having proved 

 perfectly successful in our hands; we do not doubt that with 

 more general use it may bo considerably modified and improved, 

 but at present we have rather contented ourselves with esta- 

 blishing the broad principle with such details only as will suf- 

 fice to ensure good rosultsS, and to leave to a future period the 

 consideration of those minor points which only a long experieuee 

 can devclopc. 



The plate coated with collodion (that which we employ eon- 

 tains iodide, bromide, and chloride of ammonium, in about equal 

 proportions,) is made .sensitive by immersion in* the ordinary 



*Jouruul Pliii. Soc. No. 9. 



tibiii. No. 10. 



solution of nitrate of silver (30 grains to the ounce), and after 

 remaining there for the usual time is trausfen-ed to a second 

 sojution of the following compo.sition : — 



Nitrate of zinc (fused) . . . 2 ounces. 



Nitrate of silver 3.5 grains. 



Water -. . . . . . . G ounces. 



The plate must be left in this bath until the zinc solution has 

 thoroughly penetrated the film (we have found five minutes 

 amply sufficient for this purpose, alth;,ugh a much longer tinie 

 is of no consequence) ; it should then be taken out, allowed to 

 drain upright on blottiijg-paper until all the surface moisture 

 has been absorbed (about half an hour), and then put by until 

 required. The nitrate of zinc, which is still retained on the 

 plate is .sufficient to keep it moist for any length of time, and 

 we see no theoretical or practical rea.son why its sensitivone.«s 

 should not be retained as long; experiments on this point are 

 in progress; at present, however, we have only .subjected them 

 to the trial of about a week ; although at the end of that period 

 they were hardly deteriorated in any appreciable degree. It is 

 not necessary that the exposure in the camera should be imme- 

 diately followed by the development, as this latter process can 

 be deferred to any convenient opportunity provided it be within 

 the week. Previous to development, the plate should be al- 

 lowed to remain for a few seconds in the original 30 grain 

 silver-bath, then removed and developed with either pvrogallic 

 acid or a protosalt of iron, and afterwards fixed, &c. in the usual 

 manner. 



The advantages of this process can scarcely be overrated. — 

 Besides the facility it afibrds of working iji the open air with- 

 out any cumbrous apparatus, photography may now be applied 

 in ca.ses where it would have been hitherto impiacticable, owing 

 to the feebleness of the light e.rj. badly illuminated interiors, 

 natural caverns, &c. ; if necessary, the exposure could be pro- 

 tracted for a week, or possibly much longer, and the deficiency 

 of daylight compensated for by the employment of the electric 

 or other artificial light. It will also be found useful where the 

 plate must be kept ready excited, but the exact moment of ex- 

 pjsure may depend upon possible contingencies rather than on 

 the will of the operator, or in cases where it would be imprac- 

 ticable to prepare the plate just before exposure ; for these 

 reasons it might prove a valuable adjtmct on the eve of a naval 

 or military engagement, for accurately recording the positions 

 of the forces. 



A small proportion of nitrate of zinc added to the ordinary 

 nitrate of silver bath in no way interferes with it.s action, and 

 might obviate the inconvenience sometimes felt during hot 

 weather in photographic rooms, of the film becoming partially 

 dry before exposure. If added in a still .smaller proportion t« 

 the silver solution used for exciting the ordinary Talbotype 

 paper (without the employment of gallic acid), it is verj- pro- 

 bable that its sensitiveness may be preserved during a much 

 longer period than is gcncndly possible. As fiir as our experi- 

 ments have gone, they tend to confirm (his supposition ; but at 

 ])rc.scnt wc can hardly speak more confidently on this point, an 

 it is still under investigation. 



There are, no doubt, many other sub.stanco.s which might 

 equally well answer the purpose of nitr.ilo of zinc ; beside.'* 

 those alrcadv mentioned, the nitrates of cadmium, niuntrnncio, 

 and perhaps also'thoso of copper, nickel, and cobalt mi>:Iit ho 

 founil serviceable. Glycerine at first seemed lo promise vor*- 

 good results, but the princijKil diflioulty wa.s the neces.siiry im- 

 purity of the comnierciid |)roduct, in con.sei|Uence of its being 

 obtjuned from the cxhaustt'd leys of the soap boilers. 



