The British Journal of Photography, No. 613, Vol. XIX, February 2, 1872
JOURNAL OF PHOTOGRAPHY.
A CHARACTER OF CERTAIN PRESERVATIVE FILMS.
Herr Schultz-Sellack has recently called attention to a remarkableagreement in certain properties between chemically-clean glass surfacesand photographic films coated with tannin or albumen. Theparticular property in which they appear to agree is this—that analbumenised or tannined surface, when breathed upon, takes themoisture evenly and loses it gradually by evaporation, just like aclean glass plate.
This wonderful discovery (!) is announced in Poggendorf’s Annalen,and a notice of it appears in the last Journal of the ChemicalSociety. If Herr Schultz-Sellack had endeavoured to make himselfacquainted with the literature of photography he would scarcelyhave taken the trouble to publish the statement of a fact well knownto all who have much experience in dry-plate work. A film protectedby albumen, tannin, or any other substance capable of absorbingmoisture, resembles a perfectly-clean glass plate in the mode ofcondensation of moisture and the manner in which an aqueous filmdisappears from the surface: but this property of a somewhat hygroscopicsurface is one which might be easily anticipated and has longbeen observed.
“Whether this hygroscopic property is beneficial or otherwise weare scarcely in a position to decide, as wide differences of opinionexist amongst practical men on this subject—on the one hand, Mr.M. Carey Lea considering nearly complete desiccation of a dry plateconducive to greater sensitiveness, and on the other, a large numberof operators declaring that a preservative capable of keeping theplate in a semi-moist condition is most advantageous. Our own experienceis in favour of Mr. Lea’s position; but it is by no meansimprobable that some of the so-called preservatives act best whenmoist, and others when the film is fully dried. We shall contentourselves with citing a single case in point.”
When a film of iodide of silver is washed free from extraneousmatter, and then covered with a solution of ferrocyanide of potassium,a very sensitive layer is obtained while the film is moist, but if driedfully the action of light upon the surface is very slow. Suppose,however, that we add to the ferrocyanide solution, previous to its applicationto the iodide film, a quantity of honey, a little glycerine, ora very minute amount of nitrate of magnesia, a comparatively sensitivefilm is obtained, which, though apparently dry, is still not completelyso. Here, then, is a remarkable case, parallel with theanalogous action of nitrate of silver moist and dry upon sensitivelayers.
A SHELLAC VARNISH.
In our last volume there appeared two very interesting notes—oneby Mr. G. Watmough Webster, F.C.S., and the other by Mr.A. R. Brown—on the preparation of a shellac varnish of a peculiarkind, and easily miscible with water. Though we have been longfamiliar with the mode of preparing such a varnish—thanks to thekindness of a friend—we have lately gone over the whole matter,and have been so interested that we now venture to recall thematter to the consideration of our readers—this week mentioningthe most convenient mode of preparing the so-called varnish, andreserving to another time an account of several applications of itwhich appear to possess some interest.
The most rapid mode of obtaining the shellac varnish convenientlyis certainly that of Mr. Webster. This gentleman dissolves theresin in spirit. We may remark that strong spirit gives the mostsatisfactory results. The solution when now treated with a very smallamount of the ordinary liquid ammonia becomes immediately misciblewith water. Care must be taken, however, only to add sufficientammonia to accomplish the desired end, and this can be easilyaccomplished by testing a little of the mixture by addition of waterafter every fresh dose of alkali. In this way any unnecessarydeepening of the colour is avoided, and a useful liquid obtained.
We now turn to the second plan—that mentioned by Mr. Brown.It has been long well known that fused shellac dissolves much moreeasily in alcohol than the resin which has not been melted. Mr.Brown adopts the same plan in obtaining solution in ammonia.The resin is melted under a layer of water, and, when perfectlyfused, strong liquor of ammonia is slowly poured in, the whole beingcarefully mixed. Gradually the resin is taken up by the liquid, anda good, brown mixture obtained. If too much ammonia be usedhere the colour is greatly deepened, owing to the action of the alkalion a peculiar impurity in the resin.
Both the above processes work well; but, when small quantitieshave to be prepared, Mr. Webster’s plan yields the best product, asa solution of shellac in spirit can be obtained having a comparativelyslight colour, whereas the direct use of ammonia deepens the tint.Whichever plan be used a good result can be arrived at, and a liquidobtained capable of numerous and most useful applications, whetheras a species of varnish, a cement, or material for, in a sense, waterproofingpaper.
Chromate of potash is used in dyeing in conjunction with dextrine,&c., for discharging the colour from various fabrics; it seems, however,that a difficulty has been noted in using such a mixture, forit is found that certain of the goods become discoloured by thechromate, and that this undesired colour is not easily removed.Will our readers be “surprised to hear” that the cause of this circumstanceappears to have been only just found out, if we mayjudge from a quotation in the Journal of the Chemical Society?It has been discovered that it is exposure to light during treatmentwhich prevents the removal of the offending material. This circumstanceindicates how little attention is in general paid to thephenomena attending the chemical action of light; and, notwithstandingthe fact that the remarkable property possessed by thechromates of rendering insoluble gelatine, &c., has been utilised toan extraordinary extent in the various pigment printing processes,operative chemists seem to have only now become alive to the fact,and able to apply the obvious remedy in using yellow glass for therooms in which the treatment of goods with the chromated dextrineis carried on.
BLISTERING OF ALBUMEN SURFACES.
Having read with much interest the various remarks which haveappeared in your columns of late on the blistering of albumenisedsurfaces, I will, with your approbation, state my conviction of thecause.
First, let us examine the blisters and get at a clear notion of theirphysical aspect.
All who have spoken on the subject appear to agree that the morehighly a paper is albumenised the more do we suffer from the annoyance.Well, such a strong film gives us the opportunity to detachsome of those large blisters which have been formed by thecoalescence of a number of small blisters.
Proceeding after the manner of a microscopist in his dissection ofanimal structure, we depress the sheet or photograph to the bottomof a dish with water, and there introduce a sharp-pointed knife,whereupon air escapes. Here we simply note this fact, and continuethe dissection. Having detached a vesicular film, and caught it ona small plate of glass with the innermost side upwards, it will befound in almost every experiment to have the surface fibre of thepaper attached, thus showing that it does not part from the surfaceof the paper as a film pure and simple. We therefore dismiss fromour minds the idea that the outer surface only had been renderedinsoluble, while the inner portion had been dissolved; were this thecase the blister would be filled with the albumen solution.
In accordance with the law discovered by Graham—that a colloidsolution, such as albumen, cannot pass through a colloidal septum—theblisters cannot be the result of expansion. Further: where acolloid septum is wetted on both sides, liquids of the nature of watercan diffuse through, and therefore the blisters would be filled withwater or solution of crystalloid salt, such as hyposulphite of soda.
Now we must search deeper and wider for the cause. If I amnot mistaken, the blisters show themselves in greater force, in thiscountry, from the middle of May to the end of July. During thisperiod the natural waters are of the most favourable temperature forthe absorption of air or gases on which the aquatic plants feed;gases coming in contact with or passing through the cells of theliving plant are decomposed, and the residual gas discharged intothe water. As the water becomes lowered in temperature it isunable to hold these absorbed gases, the minimum temperature beingfrom 32° to 40° Fah.
It is from this cause, in my opinion, that ice floats on the surface.As water becomes lower in temperature minute bubbles of air areseen to grow, as it were, on submerged objects; eventually the buoyancyovercomes the force of adhesion and gravity, whereupon theair-beads rise and become entangled amongst the water crystals—ice—andthus render the ice lighter, bulk for bulk, than the water fromwhich it has been formed.
Some of my readers will, most likely, say—“What has this to dowith the blistering of albumen surfaces?” Well, let us see howthese ideas, derived from anterior observations, assist us in thematter.
There are many substances beside the growing tissue and cellshaving the property of condensing gases in their pores, and oftenentering into combination with such avidity that sufficient heat isliberated to render the substance incandescent. Such is the casewhen hydrogen gas is impinged on platinum in a spongy form.Fresh-burned charcoal is another substance having the property ofcondensing many times its own volume of gas, and by mutual attractionto the occluded air the external air sticks as a film on the substancewith considerable force—wets the surface, if I may use thesimile. Most substances possessing extremely minute pores or intersticeshave this property, in a more or less degree, for different kindsof gases; therefore we may fairly class the fine texture of photographicpaper amongst such bodies.
Grant that there is an analogy between the above-mentioned cases,then the cause and effect become intelligible. First we have a hard-sizedpaper with gases or air condensed in the interstices, and withthe external air adhering to it; in this condition it is coated withan amorphous film of albumen. All appears to go on favourablyuntil the fixing process; here the temperature is reduced by theaddition of water contained in the prints. Now the absorbed airwhich the water contains finding a nucleus on its permeating thebody of the paper grows immediately (as I pointed out when directingyour attention to the phenomena proceeding from the freezing ofthe water) in the surface-fibre, thus it is likely that the blisters areformed is all cases where air is enclosed. Hyposulphite of sodawhen entering into solution reduces the temperature considerably—theair being then held with so little force readily unites with anucleus.
To counteract this natural effect it is obvious that means shouldbe taken to prevent the lowering of the temperature on the additionof water to the hypo, solution, or on the transference of prints fromthe hypo, into water containing absorbed air.
If this be unmanageable, the easiest thing to do is to dip the printsinto methylated alcohol, after leaving the toning bath. This treatmentremoves the sizing from the paper, and the prints appeartranslucent while wet. They should now be well washed; any disengagedair will then be able to escape through the porous body ofthe paper. It has been recommended to employ a large charcoalfilter to absorb the air or gases, but, like all filters, it will requirecleaning out, and the charcoal needs re-burning.
A word on filters in conclusion. The public purchase filters, andimagine that they are to have pure water, or clarified solution, forevermore without trouble; let them be undeceived, because whencharcoal becomes saturated it must be re-burned to restore itspurifying properties.
In last week’s issue your idea of combining silver with a new basein the collodio-bromide is so good that I think it just as well to mentionthat the very notion of acetate of silver has been carried out byme a dozen years ago, although in another form. I made a collodionfor wet-plate work in which an iodide was used with about