February 14, 1901] 



NATURE 



173 



THE PRESERVATION OF PHOTOGRAPHIC 

 RECORDS. 



A FTER the article by Dr. W. J. S. Lockyer on the 

 ■^"^ disappearance of photographic images (p. 278) and 

 other references that have been made to this subject, 

 there is no need to dilate upon the importance of the 

 preservation of the vast number of photographic records 

 now being produced in our observatories and labor- 

 atories. I would, however, venture to express surprise 

 that the fact established by Dr. Isaac Roberts as to the 

 want of permanence of a silver photographic image 

 appears to have been unexpected, bearing in mind the 

 general procedure that appears to be the rule in the 

 production of such photographs. 



Photographic procedure is based chiefly, practically 

 speaking, indeed, entirely, upon " rule of thumb," and if 

 each modification requires a ten years' trial of it 'to 

 establish its advantage or otherwise, it is at least desir- 

 able that more attention should be paid to the principles 

 which govern the production and the permanency of 

 photographs. It is well known that metallic silver is 

 liable to change, yet we cannot get away from the use of 

 silver salts, and these, of course, give silver images. 

 But silver ought never under any circumstances to be relied 

 on. The questions that present themselves, therefore, 

 are : (i) how best to preserve the original silver image, 

 and (2) how best to copy it. 



It has been suggested to give up gelatine and go back 

 to collodion. But this suggestion appears to be founded 

 on nothing better than the quite insufficient evidence of 

 general experience, and partly, also, on false inference. 

 Metallic silver, we know, is affected by the air, and in a 

 wet collodion plate the silver lies chiefly on the surface 

 of the film, while in the ordinary dry plates it is buried in 

 the gelatine. Moreover, experienced photographers tell 

 us that the image on wet collodion plates gets denser 

 with age. If it does so the image is unstable, and 

 change, however it manifests itself, must be avoided if 

 the photograph is to be accepted as trustworthy. The 

 coarser grains and crystalline character of the image 

 produced on wet collodion would account for its superior 

 resisting power to outside influences, but this alleged 

 superiority has not yet been proved. We cannot go 

 back to. collodion, but under any circumstances there 

 appears to be no good reason for expecting any gain in 

 that direction. Gelatine is the medium of to-day, and 

 no evidence of its unfitness has been adduced. 



Metallic silver is soluble in such reagents as dissolve 

 silver oxide, if air or an oxidiser is present. Ammonia, 

 potassium cyanide and sodium thiosulphate readily dis- 

 solve silver in the presence of air. The oxidation pro- 

 ducts of developers act as oxidising agents. Ferric 

 oxalate, the product t)f development with ferrous oxalate, 

 is particularly active in attacking silver, and is practically 

 used for the purpose of thinning silver images. And the 

 coloured products of oxidation of alkaline developers 

 are well known to retard the reduction of silver salts, 

 though apparently they have not been shown to be able 

 to directly attack the metal itself But it must be re- 

 membered that that which produces no appreciable effect 

 in a few months, or even years, may have a disastrous 

 action in a generation, so that the only safe course is to 

 eliminate or exclude every suspicious substance. In 

 short, the photographic film should consist of pure silver 

 in clean gelatine, for anything more than this that is 

 likely to be present will, in all probability, prove dele- 

 terious. 



There is no difficulty in banishing at once ammonia, 

 ferrous oxalate and potassium cyanide. The ammonia 

 in the developer is replaced by sodium carbonate, an 

 exchange in every way advantageous, the ferrous oxalate 

 by alkaline developers, a change which has already met 

 with general approval, and potassium cyanide as a fixing 



NO. 1633, VOL. 63] 



reagent is practically obsolete. The two great dangers- 

 that are not sufficiently appreciated by many photo- 

 graphers are the presence of thiosulphate from the fixing 

 bath and oxidation products from • the developer. To- 

 remove the first, the usual half-hour of washing is not 

 sufficient, however the water may be applied. There is 

 no particular virtue in running water or in washing con- 

 trivances ; it is the prolonged soaking in clean water 

 that is wanted. However the washing is done, it is easy 

 to remove the greater part of the thiosulphate ; it is the 

 last traces that are hardly, if at all, susceptible to detec- 

 tion by any of the ordinary methods that need attention. 

 If after half an hour it is not possible to detect any thio- 

 sulphate in the wash water, a further soakmg for ar» 

 hour and a half, with suitable changing of the water, 

 would not be excessive treatment. 



The same washing that gets rid of the fixing reagent 

 washes out the developer and its oxidation products, if 

 the work has been carefully and successfully done. But. 

 to ensure this the developer must have sufficient sulphite 

 (sodium sulphite) in it to prevent its discoloration or the 

 staining of the film. The deposition of staining matter 

 should be prevented, as removal is tedious and trouble- 

 some. Many published formulae for developers prescribe 

 an insufficiency of sulphite, and it is not possible to state 

 definitely how much is required, because that will depend 

 on the time taken for development and the amount of 

 sodium carbonate present. But generally, if not always^ 

 and certainly when using pyrogallol, the sulphite should 

 be proportioned to the bulk of developer — that is, to the 

 water ; it is unsafe to dilute the prepared developer with- 

 out adding the further quantity of sulphite to maintain 

 its due proportion. 



The method generally adopted at the present time to 

 get nd of stains is founded, like so many other photo- 

 graphic methods, on a false basis. The idea is that if a 

 stain disappears, it has gone. The appearance, truly, has 

 gone, but the matter that constituted the stain may re- 

 main, and perhaps in a more dangerous form than it was 

 at first. In almost all cases the effect of acids on stains 

 due to oxidised developers is to lighten the colour of the 

 staining matter and to render it insoluble. The action 

 of alkalies is to darken its colour and to render it soluble. 

 Alkaline solutions are, therefore, true clearing reagents, 

 while acids are actually prejudicial, although they appear 

 to effect improvement. It is desirable to carefully avoid 

 the use of any acid solution whatever, and by doing so it 

 will be found that cleaner and chemically purer plates are 

 produced. After developing in an alkaline solution and 

 rinsing the plate, it should be fixed in a solution of 

 sodium thiosulphate made alkaline with sodium car- 

 bonate, and then well washed. The washing may be 

 done with plain water, but if there is the slightest trace 

 of colour due to stain, it will be found of advantage to add 

 a little carbonate of soda or a very little caustic soda to 

 the first wash waters. Acid fixing baths should be abso- 

 lutely eschewed. The very grave risk that accompanies 

 their use is not appreciated, or they would never be 

 recommended. 



Having thus obtained a really clean (that is, chemically 

 pure) plate, the exposed surface of the film must be pro- 

 tected in some way to keep the image as much as possible 

 from the air, and also to prevent contamination by the 

 acid perspiration from fingers when handling it. 



For this purpose a celluloid varnish will be found a 

 better protection than the ordinary lac varnishes, but 

 whatever is used it is desirable that the gelatine be dried 

 before it is applied. By warming the plate until it is as 

 hot as the hand can bear, and then allowing it to cool 

 to the desirable temperature for varnishing, even though 

 the varnish may have to be applied to the cold plate, the 

 film is probably effectively dried. But what seems to be 

 a much more effective method of preserving the film from- 

 outside influences is to cement on to it a cover-glass by 



