HAVING read with much interest the various remarks which have appeared
in your columns of late on the blistering of albumenised surfaces, I
will, with your approbation, state my conviction of the cause.
First, let us examine the blisters and get at a clear notion of their
physical aspect.
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.
A word on filters in conclusion. The public purchase filters, and
imagine that they are to have pure water, or clarified solution, for
evermore without trouble; let them be undeceived, because when charcoal
becomes saturated it must be re-burned to restore its purifying
properties.