Trial of Burton's Emulsion Process
Trial of Burton's Emulsion Process
As the above process, though introduced to the notice of the photographic world by my friend Mr. W. K. Burton as far back as the 14th of November, in an address delivered at the meeting of the Photographic Society of Great Britain, is still to a certain extent sub judice, a few remarks upon it by one who has given it a careful and unprejudiced trial may not be out of place. In every branch of science there is a danger that what claims to be an improvement may really be a retrograde movement, and this holds good particularly of photography. I do not think, however, that Burton's emulsion process can by any possibility be placed in this category of equivocal innovations.
Winter is, par excellence, the season for manufacture of emulsions. The cold is advantageous, while actual photography being at its minimum the photographer cannot do better than prepare for future campaigns. Though lack of time has caused me to fall back on commercial plates of late, it has been my custom, as far as possible to prepare a stock during the winter months which should suffice for the whole of the summer.
With this laudable object I was about to start with my old favourite, the boiling process, when I noticed the publication of Mr. W. K. Burton's modification. It recommended itself to me at once as being an improvement over everything which had gone before. The only objection which I could see against it was the great length of time from the beginning to the end of the manipulation, and this in my own case proved to be a positive advantage; for, having my time taken up with professional business, and being liable to interruption, I have under any circumstances to extend my operations over several evenings. Observe particularly that, in Mr. Burton's process, although the whole time needed is comparatively long, still that occupied in actual manipulation is shorter than is the case in any ordinary process. It commends itself to a busy man or to any amateur, situated like myself, with intermittent and irregular periods of leisure.
The details of the process have been already dwelt upon in THE BRITISH JOURNAL OF PHOTOGRAPHY, but still I may briefly recapitulate them for the benefit of anyone who may, notwithstanding, be hazy as to the details. Any formula may be retained during the early part of the process, as the special features are only introduced after the boiling is completed. Mr. Burton recommends, however, the employment of an unusually large amount of water, under the impression that it favours a fine state of division in the bromide of silver — a supposition in which I consider him to be perfectly correct. After cooling the emulsion down to 1200 Fahr., or thereabouts, strong ammonia and alcohol are added. This causes the bromide of silver to be precipitated, and the solution containing the gelatine and soluble salts may be poured off. This process is repeated, the former precipitate being stirred up in a fresh quantity of water, and allowed to settle once more. It is then washed by decantation, mixed with gelatine, and the emulsion is complete.
An excellent rough-and-ready method of improvising the necessary boiling apparatus is to use a jam pot covered by an inverted flower-pot saucer, and place standing in a good large saucepan with the lid on and half full of water. The whole is placed upon an ordinary cooking burner. This arrangement will be found to answer quite as well as a more pretentious one, and to give most satisfactory results. In this manner I have prepared plates more rapid than any I have got before, and of excellent quality. The emulsion needs some keeping, however, before it attains its maximum of sensitiveness. I find that, as a rule, the plates will not stand so strong a developer as pure, boiled emulsion plates; but, on the other hand, they will give density with a weaker one.
I have read with considerable interest a communication upon superficial fog in THE BRITISH JOURNAL OF PHOTOGRAPHY, by the originator of this process. In it he remarks that it only makes its appearance in the case of emulsions which are very rapid, and which are capable of giving dense images with clear shadows. My experience entirely corroborates Mr. Burton's researches in this matter. I have formed my own conclusions, however, as to the cause of the phenomenon. I believe it to be due to alkalinity of the emulsion, combined with the predisposing nature of the silver bromide. It is, I find, by no means uniformly amenable to the treatment recommended, namely, the addition of alcohol and preservative, though this has undoubtedly very frequently the desired effect. I find that the addition of any mineral acid (hydrochloric, for example) to neutralise the alkalinity of the emulsion, will invariably cure it. Care must be taken in adding it, however, as any considerable excess of acid has the effect of slowing the plates, and should, therefore, be avoided.
Another point worthy of notice in connection with the addition of acid is that ammonia has to be added to the emulsion to cause it to ripen by keeping. Hydrochloric acid is added to neutralise this ammonia, and, of course, chloride of ammonium in the film is the result. This, it must be remembered, is a powerful restrainer, so that the less there is of it present the better for the efficiency of the emulsion. The least possible quantity of ammonia, therefore, should be added in the first instance.
I have read the excellent editorial article, which holds out some prospect of doing away with the ammonia and of requiring a shorter time for precipitation. I shall certainly try it, and shall, with the Editor's permission, give the result to your readers in a future communication.