All who have spoken on the subject appear to agree that the more highly a paper is albumenised the more do we suffer from the annoyance. Well, such a strong film gives us the opportunity to detach some of those large blisters which have been formed by the coalescence of a number of small blisters.
Proceeding after the manner of a microscopist in his dissection of animal structure, we depress the sheet or photograph to the bottom of a dish with water, and there introduce a sharp-pointed knife, whereupon air escapes. Here we simply note this fact, and continue the dissection. Having detached a vesicular film, and caught it on a small plate of glass with the innermost side upwards, it will be found in almost every experiment to have the surface fibre of the paper attached, thus showing that it does not part from the surface of the paper as a film pure and simple. We therefore dismiss from our minds the idea that the outer surface only had been rendered insoluble, while the inner portion had been dissolved; were this the case the blister would be filled with the albumen solution.
In accordance with the law discovered by Graham—that a colloid solution, such as albumen, cannot pass through a colloidal septum—the blisters cannot be the result of expansion. Further: where a colloid septum is wetted on both sides, liquids of the nature of water can diffuse through, and therefore the blisters would be filled with water or solution of crystalloid salt, such as hyposulphite of soda.
Now we must search deeper and wider for the cause. If I am not mistaken, the blisters show themselves in greater force, in this country, from the middle of May to the end of July. During this period the natural waters are of the most favourable temperature for the absorption of air or gases on which the aquatic plants feed; gases coming in contact with or passing through the cells of the living plant are decomposed, and the residual gas discharged into the water. As the water becomes lowered in temperature it is unable to hold these absorbed gases, the minimum temperature being from 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 are seen to grow, as it were, on submerged objects; eventually the buoyancy overcomes the force of adhesion and gravity, whereupon the air-beads rise and become entangled amongst the water crystals—ice—and thus render the ice lighter, bulk for bulk, than the water from which it has been formed.
Some of my readers will, most likely, say—“What has this to do with the blistering of albumen surfaces?” Well, let us see how these ideas, derived from anterior observations, assist us in the matter.
There are many substances beside the growing tissue and cells having the property of condensing gases in their pores, and often entering into combination with such avidity that sufficient heat is liberated to render the substance incandescent. Such is the case when hydrogen gas is impinged on platinum in a spongy form. Fresh-burned charcoal is another substance having the property of condensing many times its own volume of gas, and by mutual attraction to the occluded air the external air sticks as a film on the substance with considerable force—wets the surface, if I may use the simile. Most substances possessing extremely minute pores or interstices have this property, in a more or less degree, for different kinds of gases; therefore we may fairly class the fine texture of photographic paper 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-sized paper with gases or air condensed in the interstices, and with the external air adhering to it; in this condition it is coated with an amorphous film of albumen. All appears to go on favourably until the fixing process; here the temperature is reduced by the addition of water contained in the prints. Now the absorbed air which the water contains finding a nucleus on its permeating the body of the paper grows immediately (as I pointed out when directing your attention to the phenomena proceeding from the freezing of the water) in the surface-fibre, thus it is likely that the blisters are formed is all cases where air is enclosed. Hyposulphite of soda when entering into solution reduces the temperature considerably—the air being then held with so little force readily unites with a nucleus.
To counteract this natural effect it is obvious that means should be taken to prevent the lowering of the temperature on the addition of water to the hypo, solution, or on the transference of prints from the hypo, into water containing absorbed air.
If this be unmanageable, the easiest thing to do is to dip the prints into methylated alcohol, after leaving the toning bath. This treatment removes the sizing from the paper, and the prints appear translucent while wet. They should now be well washed; any disengaged air will then be able to escape through the porous body of the paper. It has been recommended to employ a large charcoal filter to absorb the air or gases, but, like all filters, it will require cleaning out, and the charcoal needs re-burning.