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The photographic news
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- 24.1880
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- Bandzählung
- No. 1126, April 2, 1880
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The photographic news
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Band 24.1880
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- Register Index 631
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Ukge 8yotograpkir Stls, Vol. XXIV. No. 1126.—APRIL 2, 1880. CONTENTS. page Photography In and Out of the Studio 157 international Exhibition of Photography at Ghent 157 At Home 158 Notes on the Gelatine Emulsion Process. By T. Chilton ... 159 Principles or Rules of Pictorial Composition 160 The Effect of Temperature in Forming a Gelatine Emulsion ... 162 Retouching the Gelatine Film 162 Notes 163 Topics of the Hay 164 French Correspondence. By Leon Vidal 165 Northern Notes. By Jo. Vestris 1G6 Practical Experience with Gelatine Plates. ByG. Gregory... 166 Correspondence 167 Talk in the Studio 168 To Correspondents 168 THE EFFECT OF TEMPERATURE IN FORMING A GELATINE EMULSION. The best temperature and length of time to form an emul sion of bromide of silver in gelatine are points about which there is some dispute, and yet decidedly they ought not to be left in an unsettled state, since they are of the first im portance in the preparation of gelatine plates. It will be in the recollection of our readers that when Mr. Kennett first brought out his really practicable process the time of emul sification was comparatively short,and we can most honestly say that the plates which we prepared by his method, and also the plates which he prepared for us,were much quicker than wet plates, but certainly not so rapid as those pre pared by the later formulae. Amidst the various improve ments in the preparation of the emulsion, photographers are apt to lose sight of the debt that they owe to Mr. Kennett, though it is his process, as published, which has served as a basis for the later superstructures. Mr. Kennett’s plan of keeping the emulsion fluid for times varying from one to seven days, at a temperature not over 90°, produces excellent results and extreme rapidity ; but there is the great drawback to it, that the operation is long and, in some instances, tedious; in fact, so much so, that we know of several preparers of gelatine plates who tried it and abandoned it from this cause alone. Now, we cannot doubt that the use of this prolonged emulsification is to produce some physical change in the size of the par ticles of silver bromide emulsified, as a much shorter time would enable a combination to take place between the soluble bromide introduced into the gelatine and the nitrate of silver subsequently added ; besides which, if the conversion were incomplete, it would only be approxi mately so, and the subsequent washing of the gelatinous mass would eliminate any such uncombined salt from the emulsion. If, then, it is merely a physical change that is required, a higher temperature than 90° should affect it in a far less time. Capt. Abney, we believe, was the origi nator of the method of boiling the emulsion. In his de scription of his plan for the emulsification of the bromide of silver, precipitated from an aqueous solution, which was washed and then added to the fluid gelatine, he stated that sensitiveness was rapidly attained by boiling the sensitive salt in the gelatine solution. Colonel Wortley also stated he emulsified at a temperature of 150?, and obtained as great sensitiveness in a few hours as he did by a more prolonged emulsification at a lower heat. In our various experiments we have found that with certain gelatines, boiling the emul sion for an hour, or even half an hour, produced the maximum of sensitiveness,and that any extension of the time induced fog, which by some has been mistaken for increased sensibility to the light of the developing-room. The real fact is, that boiling decomposes gelatine to a certain extent, and produces organic compounds, which react on the silver bromide and induce this veil. The emulsifica tion may be continued for a much longer period without detriment if the gelatine, in the first instance, has a slightly acid reaction, which is the case in certain foreign gelatines. We also have reason to think that the intro duction of a suitable acid to the neutral gelatines is attended with advantage. There is one more point to which we wish to refer, and that is to the colour of the emulsified silver bromide. There is an idea prevalent at the present time that blue bromide is of necessity the most sensitive. This we demur to. If the silver compound be originally emulsified so as to produce a very fine emulsion perfectly ruby by transmitted light, the emulsion, after any amount of cooking or boiling, will retain a ruddy tint inclining to yellow. There are certain gelatines, however, as there are pyroxylines, which will invariably show a blue film by transmitted light, and a ruby coloured emulsion with them is almost an impossibility. In this case the blue colour is really dependent on the “ particular ” coarse ness, and not on what we may call the molecular coarse ness of the bromide. As a result of our experiments, we can unhesitatingly recommend that the emulsion be boiled for half an hour instead of being cooked for three or four days, and that, in the first instance, the silver nitrate and the soluble bromide should be each added alternately, little by little, to the gelatine solution, only one-fourth of the total amount of gelatine being used till the boiling opera tions are over. This, we believe, will give a plate which, with a tolerable light, and a lens fairly stopped-down, will enable views to be taken with a drop shutter. Beyond this point it seems unnecessary to get rapidity, unless for very exceptional purposes. RETOUCHING THE GELATINE FILM. It is strange that while some avow the gelatine film to be very difficult to pencil, others have made no complaint against the new plates on this score. Some photographers employ for retouching on gelatine very hard pencils, such as a four H, or even five H, while others again use points of very moderate hardness, like the H, or HBs but most agree that only pencils of the first quality and of certain makers are suitable for the purpose, since these alone give a suffi ciently fine point. The well-known firms of E. Wolff and Son, of Keswick, and A. W. Faber, of Nuremburg, appear to be the only manufacturers of pencils that photographers can use. There are two ways of rendering gelatine negatives proper to retouching; the first is to treat the surface with some gum or spirit, and the second to abrade the film and impart a tooth in this fashion. The first method is that in general practice, at any rate in this country. In the case of nega tives from which an ordinary number of impressions are to be taken, it is usual to varnish first and retouch afterwards, but when a photograph is wanted for publication, and con siderable pains and expense are devoted to retouching it, then the varnishing is done afterwards, so that the work upon the negative may be protected. Adam-Salomon advocates retouching on both sides of the negative. Only freckles and minor imperfections are treated with a fine- pointed pencil on the negative film itself; the broader effects of retouching take place on the reverse. The glass is covered with a matt varnish, and the artist does most of his work on this. Turpentine is employed by some to prepare the film for retouching; a little of the liquid is applied with the finger, and the film may be retouched as soon as the surface is dry again. In this case, however, hard pencils are necessary to do the work. One gentleman we met the other day told us he still employed gum-water to give a film for retouching, as he had done, he said, for twenty years; but a great many nowadays follow Mr. Maylend's example, and employ the Autotype medium, which renders the gelatine film as easy to work upon as one of collodion. “ You may pencil the film like paper,” Mr. Mayland says, " if you use this medium, and if you do not like your retouching afterwards, you may remove it without the least difficulty by means of the finger dipped into the liquid.”
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