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- Bandzählung
- No. 1124, March 19, 1880
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The photographic news
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be 9hotographit Eths Vol. XXIV. No. 1124.—MARCH 19, 1880. is not easily obtainable otherwise, except by means of a ing away the film by spirit. Indeed, gelatine only becomes the harder on the application of alcohol. want to absorb moisture from a piece of gelatine and to harden it, put it into alcohol, and the thing is done. Pro frilling is, to say the least, as uncommon as central frilling, i This seems to point to the fact that too rapid drying of the I films is a source of danger. If we consider why this is so, I the cause may be apparent; the fact is, that too rapid ; drying puts the layers of the gelatine at different depths in 1 unequal tension. We all know that in order to prevent are very apt to think that some method which is suitable to Thus French poses. By T. J. Pearsall, F.C.S... Correspondence Proceedings of Societies To Correspondents Patents. Compiled by Mr. Des Voeux THE UNVARNISHING OF GELATINE NEGATIVES. these columns—to varnish their gelatine negatives before printing at all. It is rarely indeed that one miscalculates the intensity of a negative after a little practice, so long, that is, as you employ a sufficiency of developing solution, and thus remove all tendency to brownness in the trans parent parts of the image ; otherwise, as everybody knows, the printing qualities of a gelatine negative are considera bly lowered. But if you do want to modify the character of your film subsequently, you run less risk, we think, by having to unvarnish once in a while, than in printing persistently from the bare film. In the case, however, of silver stains getting upon an unvarnished gelatine film through careless printing, these, our readers should bear in mind, are usually to be removed by the application of a weak solution of cyanide of potassium. At the same time, photographers should recollect that it is a very easy matter to remove varnish from gelatine plates. Unlike collodion, there is no risk here of dissolv trous evil. There is no doubt whatever that long cooking of the gelatine emulsion, when it contains the full amount of gelatine as was formerly the case, is conducive to it, and if this plan is resorted to, it can only be guarded against—or, we may say, be mitigated—by never raising the temperature above 90°, as first indicated by Mr. Bennett. Gelatine under goes chemical changes by raising a solution of it towards the boiling point, particularly if the operation be prolonged. Putting aside, however, the gelatine itself as a cause of frilling, it should be asked if there is nothing else which will encourage its appearance, and to this we say that there are other points to which attention should be paid. The cleaning of the plates is an important item. In some recent experiments it has been demonstrated that when plates have been French chalked, frilling is of a most determined character, and that when cleaned with potash, rinsed, and nearly dried, the same emulsion gives perfect immunity from the evil. Again, if a plate be cleaned with Tripoli powder, and then polished, as usually is done for the wet process, it frequently happens that the film rises into small blisters during fixing, and the direction of the “rub” of the polishing handkerchief or cloth can be readily traced in the curves in which these blisters lie. We wish to lay stress on these facts, as photographers I substratum. Again, those who prepare gelatine plates I are aware that under some circumstances frilling at the films something of the same series of strains exist, if the drying be not regular and slow. When water is applied to a rapidly dried film, the expansion of the top surface appears to be less rapid than that of the bottom surface ; there is a consequent strain exerted to lift the film away from the plate, and the result is frilling in its most uncon querable form. Too great a thickness of gelatine is also an aid to the evil; but in this case it is probable that the ad hesion to the glass is overcome by the tendency for the gelatine to expand ; it is, therefore, needful in producing plates to guard against a too liberal application of the emulsion, unless chemical means be employed to prevent it. These mechanical inducements to frilling, we believe, cannot be got over by any mode of development, or appli cation of alum or spirit, but must be met by some mecha nical means. Frilling induced by chemical condition of the gelatine, or by very modified mechanical tension, can be battled with by the means which have been indicatedin our columns. We propose to revert to this matter at an early date. margins of the plate is much more common than in any other part, and this is particularly observable when the ! emulsion is made up without the addition of any alcohol. In this case there is a repulsion from the edges of the - plate, and a narrow band of gelatine dries first and rapidly, If you and it is just where this rapid drying takes place that frill- mnd + ing is most to be dreaded. In an emulsion made up with alcohol, this marginal fessor Hermann Vogel, of Berlin, has advocated the em ployment of alcohol for hastening the drying of gelatine plates during their preparation. While they are still damp, he says, flow them with spirit, and this will soon take the moisture out of the plates. Therefore, if a gelatine nega tive has been varnished, nothing is more simple thantouuequarvno. n. al nivwuuau 111 U1UU1 IV pieveuu take the varnish off again, if you want to manipulate the I glass from cracking with the slightest blow, it is annealed plate once more. We have scrubbed away at a negative | by a very gradual withdrawal of temperature, thus putting with a tuft of cotton-wool moistened in spirit, without ’ all parts into equal tension. If the glass be cooled suddenly, failing to work the least damage to the film, and washing ! as by dropping a small ball of molten glass into with spirit may certainly be resorted to in any case. | water, we have what are known as Prince Rupert’s drops, Under these circumstances, we are rather inclined to I which on the smallest provocation fly into dust, owing to press upon our readers the advice given by Mr. Swan in ' the different strains existing in the interior. So, again, if । a wine glass be unannealed, the gentlest tap will shatter it to atoms from the internal forces. With our gelatine THE FRILLING OF GELATINE NEGATIVES. In our columns last week a well-known manufacturer of gelatine plates pointed out a plan by which frilling of gela tine negatives could be corrected, and the specific given is simple and easily applied. It may, however, be worth while briefly to dwell on some of the causes of this disas . 142 . 142 . 143 . 144 . 144 “ It is all very well to tell us not to print our gelatine negatives before varnishing ; it must be done,” said a warm photographer of considerable decision, to us, the other day. We therefore suppose it must. But photographers j . g should remember that it is a far more risky thing to print one process must be equally so for another, an unvarnished gelatine film, than one of collodion, chalk is a very useful adjunct when preparing collodion Albumenized paper, even if damp, rarely stains a collodion emulsion plates, since it gives a bite to the collodion, which film ; the danger with the latter is more mechanical than ’ ” ’ ’ anything else. But in the case of an unprotected gelatine film, if printing-frame, pad, and paper are not “ dry as a bone,” then stains must be the inevitable result. CONTENTS. PAGB Photography In and Out of the Studio 133 French Correspondence. By Leon Vidal. 134 What is an Artistic Photograph ? By E. Dunmore 135 Remarks on Washing Prints. By Marshall Wane 135 At Home.- Mr. Valentine Blanchard in Regent Street 137 The Unvarnishing of Gelatine Negatives. 138 The Frilling of Gelatine Negatives 138 Notes 139 Topics of the Day.—Modern Photo-lithography. By Henry Butter 140 Photography and Colour, for Educational and Technical Pur-
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