WASHED SENSITIVE PAPER.
For some classes of work sensitized paper may be washed with advantage
previous to drying, and there is much economy in this plan, particularly
in hot weather, since it keeps of a purer white for a much longer
period than where the silver nitrate is allowed to dry on the surface.
It may not be out of place to call attention to the action of silver
nitrate on the paper. If a stick of lunar caustic be applied to the
skin when dried, there is a peculiar burning effect produced, and even
in the dark the cuticle becomes discoloured, though not black. In the
albumenized paper we have albumen and the gelatine sizing, and these
substances behave somewhat like the skin. The gelatine particularly will
become oxidized at the expense of the silver, a reddish organic oxide
being formed; and again, if the silver nitrate be alkaline or strictly
neutral, we have the same action occurring as when we precipitate
metallic silver by means of an alkali, and an organic body such as sugar
of milk. The gelatine takes the place of the latter. When the free
silver nitrate is removed, the tendency for the spontaneous darkening
of the paper is much diminished, since the chloride and albuminate of
silver are much less readily reduced than the nitrate. The following
plan is adopted for washing the paper:--The paper, after floating,
is drawn twice rapidly through a dish of rain or distilled water,
and, unless some other substance which can absorb chlorine be added
to the last wash water, care should be taken not to soak out all the
free nitrate, as then the paper would produce flat prints. It is then
hung up to dry as before. Immediately before use it must be fumed with
ammonia, in order that the prints may be "plucky," and free from that
peculiar speckiness of surface which is known to the silver printer as
"measles." We can readily trace the "measles" to their source. Suppose
all free silver nitrate is washed away, and the paper be then exposed to
light, the chloride is rapidly converted into subchloride, and chlorine
is given off (see page 5); if there be nothing to absorb it at once it
will attack the albuminate, which is blackened at the same time, and
fresh chloride will be formed in little minute spots. These discolour,
and are of different tint to the rest of the print, and give rise to
the appearance of measles. This, of course, is not so marked when a
little free silver nitrate is left in the paper; but as what is removed
is principally removed from the surface, it may still be unpleasantly
discernible. Fuming obviates it entirely if properly performed, for
chlorine and ammonia combine to form finally ammonium chloride, a
neutral and inactive salt.
Any other chlorine absorber may be substituted; thus citric acid,
potassium nitrite, and many others are effective, and cause vigorous
prints to be produced. Perhaps the easiest way of giving the paper the
necessary amount of ammonia is that recommended by Colonel Wortley. This
is to place overnight the pads of the printing-frame, if they be of
felt, into a closed box in which is placed a saucer containing a couple
of drachms of liquor ammoniæ, and to withdraw them as required for the
printing-frames. The pads will be thoroughly impregnated with the vapour
of ammonia, and a couple or more prints, in succession, may be made
before it is necessary to change them.
The ordinary method of fuming is that used in America. Hearn describes
a box, which is very convenient and simple in construction. He says:
"Take any common wooden box, large enough for the purpose, and make a
door of suitable size for it, which, when shut, will totally exclude
all light. Make a false bottom in this about six inches, or so, from
the real one, and perforate it with holes of about the same size that
a gimlet would make. These holes should be very numerous, and at the
centre there should be, if anything, a smaller number of them, because
the saucer containing the liquor ammonia is generally placed at the
centre of the real bottom of the box."
For our own part we dislike the false bottom as constructed, and
recommend one of fine gauze, and, instead of placing half-an-ounce of
ammonia in the saucer as Hearn directs, we prefer to soak half-a-dozen
sheets of blotting-paper in ammonium chloride solution, about 20 grains
to the ounce, and the same number of sheets soaked in lime water; one
sheet of each are placed together, and ammonia is liberated by double
decomposition; calcium chloride being also formed.
This method is excellent in hot, dry weather, since it imparts a certain
amount of moisture to the paper. In damp weather it is a good plan to
dry the vapour by sprinkling on the gauze calcium chloride, which will
rapidly absorb the aqueous vapour, and will allow the ammonia to pass
on unimpeded. The sheets of paper are held at the top of the box by
American clips, suspended from laths about three inches apart, and it
is not a bad plan to fasten a lath on to their bottom edge by the same
means, to do away with their curling. To fume a single piece of paper
it may be pinned up to the inside of the top of the lid of a box, and a
drachm of ammonia sprinkled on cotton wool distributed at the bottom.
The point to be attended to is that the fuming shall be even, and it is
evident that the ammonia should rise equally from any part of the bottom
of the box. In the plan of the box given above, the bottom of the sheet
is apt to get a little more ammonia than the top. The time of fuming
depends on so many things that a rule can scarcely be given for it;
twenty minutes may be considered about the extreme limit.
If this sensitizing bath be acid, the time must evidently be longer than
when it is strictly neutral or slightly alkaline; and if the negative be
hard, it will require to be less fumed than if it be of a weak nature,
since the action of ammonia is to cause rapid darkening in the deep
shadows. In hot weather the fuming should be shorter than in cold, since
the ammonia volatilizes much more rapidly when the temperature is high.
On the whole, we recommend Colonel Wortley's plan of fuming the pads in
preference to fuming the paper.
Another mode of preserving the paper from discolouration is to add
citric acid to the printing bath, which is effective owing to the fact
given at page 32. The following formula is a good one, and has answered
with the writer. It is--
Silver nitrate 50 grains
Citric acid 20 "
Water 1 ounce
The paper is floated for the ordinary length of time, when it is dried
thoroughly and placed between sheets of pure blotting-paper. It will
keep in its pristine state for months, if excluded from the air. It is
better to fume this paper strongly before use, or the toning becomes a
difficult matter.
Ordinary sensitized paper may be preserved for a considerable time if,
when dry, it is placed between sheets of blotting-paper saturated with a
solution of carbonate of soda, and dried.
Washed sensitized paper is also improved in sensitiveness by floating it
for a few seconds on--
Citric acid 20 grains
Potassium nitrite 10 "
Water 1 ounce
It can be fumed, when dried, in the usual manner.
In the YEAR-BOOK OF PHOTOGRAPHY for 1880 Mr. A. Borland
recommends the following modification:--
He floats the paper on nitrate of silver, as usual, and after it has
drained surface dry, blots off any drops that may remain at the edges,
and then floats the _back_ of the paper for about three minutes on the
following bath:--
Nitrate of soda 1 ounce
Distilled water 16 ounces
This is rendered slightly acid by a little solution of _freshly_
prepared citric acid in water. The degree of acidity is regulated by
litmus paper (the blue specimen), which should be slightly reddened
by it. After this solution has been mixed about ten minutes, it is
filtered, and the paper floated. Mr. Borland says the paper keeps well,
and prints the same as ordinary paper, and any tone may be produced.