The effect of the unequal expansion of paper, when wetted, in causing
distortion of the photographic image impressed upon it, has, in the case
of ordinary photographs upon albumenized paper, been well recognized; but
the extent to which such distortion may exist under different treatment
is worthy of some special consideration, particularly with reference to
the method of printing upon gelatinized paper, which has been thought by
some likely to supersede the method now usually employed with albumenized
paper.
When a print upon the ordinary photographic (albumen) paper is wetted,
the fiber expands more in one direction than in the other, so that the
print becomes unequally enlarged, very slightly in one and much more so
in the other way of the paper. When the paper is dried without any strain
being put upon it, the fibers regain very nearly their original
dimensions and position, so that the distortion which has existed in the
wet condition nearly disappears.
If the photograph is cemented, while in the expanded condition, upon a
rigid surface, the distortion then existing is fixed, and rendered
permanent. Such a cementation or method of mounting is that which has
been generally adopted, and the consequence has been that every now and
then complaints have justly been made of the untruthfulness--owing to
this particular distortion--of photographs; productions whose chief merit
has often been asserted to consist in their absolute truthfulness. This
distortion is very manifest when, in a set of portraits, some of the
prints happen to have been made in one direction of the paper, and others
with the long grain the other way. I have known a case where a proof
happened to increase the face in width, and all the other prints
increased it in length. Of course, neither was correct, but the proof had
been accepted and liked, and the remainder of the set had to be reprinted
with the grain of the paper running in the same direction as that in the
first one which had been supplied.
Another evil arising from mounting prints while expanded with moisture
is, that in drying the contraction of the paper pulls round the card into
a curved form and although by rolling this curvature may be temporarily
got rid of, the fiber of the paper is in a strained condition, and the
bent state of the mount is, sooner or later, renewed thereby.
To remedy these evils it has been proposed to mount the print when dry,
by forcible pressure against a slightly damped card, the back of the
print having been previously coated with a cement and dried. This plan
is, to a great extent, successful; but that it does not give absolute
immunity from distortion is, I think, evident from the following
consideration. The prints, after being mounted a few days, will show a
certain tendency to curl inward. This curling, I take it, is a measure of
the strain upon the print, produced by the more complete return to its
original dimensions of the paper photograph. Probably it would be well to
keep the prints a few days after drying, or to subject them to
alternations of damp and dryness, in order to facilitate this complete
return before being placed upon the card. The evil of distortion is,
however, very slight--perhaps imperceptible--compared with that existing
when the prints are mounted wet. I may mention, _en passant_, that I have
found gum much more satisfactory as a mountant than starch paste in what
is known as the "dry mounting" system.
The paper which has recently been introduced for producing prints by
development upon a gelatine surface does not generally, when dried in the
usual way, give so good or so brilliant a surface as that of albumenized
paper; but on the other hand it is very easy with it to obtain what is
called an enamel surface, by simply allowing it to dry in contact with a
prepared surface of glass. This method of finishing has therefore been
much recommended and adopted, but without consideration of the effect of
distortion in connection with it. In an ordinary photograph the print is
mounted damp, but in the case of a print squeegeed on to the glass, the
paper is saturated and thoroughly swollen, and the use of the squeegee
strains it out to its fullest extent. By drying in the position in which
it has been held by contact with the glass, the distortion becomes fixed,
and if the print is mounted while in this state the distortion is made
permanent. How long the strain and distortion remain in an unmounted
print, and whether by time and alternations of moisture and dryness the
strain would be lost, and if so, whether the brilliant enamel surface
would go at the same time, are questions worthy of further investigation
and discussion.
For mounting prints upon developed gelatine paper, it has been
recommended to cement the edges only, so as to leave the greater part of
the print with its enamel surface. This plan is unsatisfactory, for two
reasons, besides the objection on the ground of distortion. There is a
rough-looking margin which spoils the continuity of appearance,
especially (as in the specimens I have seen) where the line of cement is
not kept at an exact width, but encroaches here and there.
Secondly, the print, from not being attached to the mount all over, is
apt, especially when in a large size, to be somewhat wavy and wanting in
flatness. Another plan recommended, as giving a surface resembling
albumen paper, is to paste the back of the print without moistening the
surface, and so mount. Some prints that have been shown thus treated had
so strongly curled the cards upon which they were mounted that it is
evident there was considerable strain and consequent distortion.
A third plan recommended is to paste the back of the print while in
contact with the glass upon which it has to dry; and, when dried, to
mount by passing through a rolling press with a damped card. This plan
looks, at first sight, like that recommended for albumen paper, and
called "dry" mounting. Consideration, however, will show that there is a
radical difference. In the case of the albumen paper the print has been
dried without strain, and therefore but little change is to be looked
for, while the print dried in contact with glass is strained to the
utmost, causing present distortion and future curling of the mount.
Perhaps the evil of distortion caused by enameling may be reduced to a
minimum by soaking the print in alcohol previous to laying it upon the
glass.
Since the distortion of the photograph arises from the unequal expansion
of the paper when wet, it becomes a question whether something may not be
done in the selection of the paper itself. It may be that some makes vary
much less than others in the "length against width" extension of the
surface by wetting. It must be remembered that for gelatine emulsion we
are not nearly so limited in the selection of paper as when it is
required to be albumenized. In the latter case the image is in the paper,
whereas with gelatine the image is contained in the surface coating. I
may mention that the best plain, i.e., not enameled, but resembling that
of ordinary albumen paper, surface that I have seen upon gelatine paper
was upon some foreign post that I had obtained for another purpose. The
emulsion employed was that described by Mr. J.B.B. Wellington, and this
gentleman agreed with me in attributing the superiority of the surface
obtained to the fine quality of the paper upon which the emulsion had
been coated. Some commercial samples appear to be coated upon paper of
somewhat coarse texture. This does not show when the print is enameled.
The unequal expansion of paper is a subject of interest, not only in
connection with gelatine paper for development, but with various
photographic processes. In making carbon transparencies for instance, the
gelatine film which is squeegeed against the glass necessarily takes its
dimensions from the paper to which it is attached, and if that be
expanded more in the one direction than another, the transparency is
similarly deformed; and so, of course, is any negative, enlarged or
otherwise, produced in the camera therefrom. A reproduced negative by
contact printing may either have the distortion due to expansion of the
paper bearing the gelatine film removed or doubled, according to the
direction in which the paper is used for the new negative.--_W.E.
Debenham, in Br. Jour. of Photography_.
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