THE SENSITIZING BATH FOR ALBUMENIZED PAPER.
To render albumenized paper sensitive to light it has to be treated with
a solution of silver nitrate, and the most convenient method of applying
it is to float it on a dish containing the silver salt in solution. The
first point to consider is the strength of the solution. If we float
albumenized paper (face downwards) on a solution of 10 grains of silver
nitrate to the ounce of water, we shall find, what at first sight may
seem to be remarkable, that the albumen will be dissolved away from the
paper, and that there will be a precipitate left in the silver solution.
Why is this?
It must be remembered that albumen is soluble in water: it is coagulated
or insoluble in water when combined with silver nitrate. The fact
is that the quantity of silver nitrate in the solution we have been
experimenting with is too small. The water dissolves the albumen first,
and then the silver has time to act upon it to form the insoluble
albuminate. If we soak paper in common salt, and treat it in the same
way with the same strength of solution, we shall find that this is not
the case: the silver chloride will remain on the paper. From this we
learn two facts.
1st. That the silver solution has a greater affinity for the chloride
than for the albuminate, and that in an equal mixture of the two more
chloride would be formed than albuminate; in other words, that the
ammonium chloride would be totally converted into silver chloride long
before the silver albuminate was formed.
2nd. That a certain strength of silver nitrate is necessary to prevent
the albumen dissolving from off the paper.
This last fact has fixed the lowest strength of any sensitizing solution
to be thirty grains to the ounce, and even if this be taken as a limit,
it is necessary that the water should be rendered less active by holding
some other soluble matter in its embraces. This is usually effected by
adding some other neutral and inactive nitrates. There does not seem to
be any theoretical limit to the amount of silver nitrate in solution,
but practically it rarely contains more than 80 grains to the ounce,
though occasionally we have heard of it being used of a strength of 100
grains to the ounce.
The important point now presents itself. How are we to fix the strength
of the bath? What principles must we follow?
To answer these questions we extract a passage from another work of this
series.[9]
"If a paper be coated with albumen (say) in which has been dissolved
a certain quantity of a soluble chloride, and floated on a silver
solution, both chloride and albuminate of silver are formed. It depends,
however, on the strength of the solution as to what proportions of each
are present, owing to the fact that the organic compound is much slower
in formation than the chloride, and has less affinity for the silver.
If the silver solution be not sufficiently strong, the chloride may rob
that portion of it with which it is in contact of all the silver before
any (or, at all events, sufficient) albuminate has been formed, the
molecule being composed almost entirely of silver chloride. The stronger
the silver solution the more 'organate' will it contain; whilst if it
be very weak, very little will be present. Hence it is with albumenized
paper which is weakly salted with a silver chloride a weak sensitizing
bath may be used, whilst if it be rich in the chloride it must be of
proportionate strength."
It will now be seen that the proportion of chloride to albumen has
to settle the point. We next have to consider the time during which
the silver should be in contact with the paper when the floating is
commenced. Let us take the case of a strong silver solution, and
consider the action that will follow. Immediately the paper is placed in
contact with the solution, silver chloride is formed, and the amount of
the silver nitrate in the layer of fluid in immediate contact with the
surface being scarcely diminished by the formation of silver chloride,
the albuminate is formed almost simultaneously, forming a film which is
to a great extent impermeable to the liquid. But even before this layer
is coagulated, the next layer of chloride will have been formed, so that
we may say we have one layer of albuminate and chloride of silver, and
one layer of chloride of silver alone.
The further penetration of the silver solution will be very slow; hence,
for fully saturating both the albumen and the salt with silver, the time
of flotation must be prolonged. For some purposes, however, this is not
necessary, as will be seen presently.
Next let us trace the action of a weak solution, not weak enough to
dissolve the albumen off the paper, but of the minimum strength. The
solution, as before, would immediately form the silver chloride, but
before the albumen had coagulated at the surface, the solution would
penetrate to the interior of the film, and then the formation of the
albuminate would proceed nearly equally throughout the whole of the
interior. Evidently, then, in this case, the contact of the silver
solution would be less prolonged than in the former case. If the
floating be prolonged the silver solution in the interior will become
weakened, and partially dissolve the albumen and be carried by the
water into the interior of the paper; it will also partially dissolve
off the surface, and a negative printed on such a paper would have all
the appearance of being dead in lustre, and existing in the paper itself
instead of on the surface.
We may thus summarize:--
1. A paper floated on a strong solution may require long floating.
2. A paper floated on a weak solution requires short floating.
3. And the strength of the solution may be between the 30 grains and
80 grains to the ounce of silver according to the amount of soluble
chloride dissolved in the albumen on the paper when the negative is
really good as regards opacity and delicacy.
The knowledge of the amount of chloride in the paper supplied by dealers
has to be arrived at somehow, and the following method will answer. Cut
up a quarter sheet of the paper into small pieces, and place them in a
couple of ounces of methylated spirit. This will dissolve out most of
the chloride, and should be decanted off. Another two ounces of spirit
should be added to the paper, and, after thoroughly soaking, should be
decanted off, and added to the other spirit. The spirit containing the
chloride may then be placed in a glass vessel standing in hot water,
when it will evaporate and leave the chloride behind. It may be weighed;
but since it is better to know how much silver chloride (AgCl) would be
formed, the residue should be dissolved in a few drops of water, and a
little silver nitrate added. The silver chloride will be precipitated,
and should be carefully washed with water, and then be filtered, the
paper being opened out and dried before the fire on filter paper. The
chloride is then detached and weighed; 3-1/2 grains of silver chloride
would show that a weak bath should be used, whilst 10 grains would show
that a strong bath was required.
With most brands of albumenized paper directions are issued as to the
best strength of silver nitrate solution for sensitizing, and a fair
estimate of the chloride present can be gained from such directions.
A weak solution loses much of its strength by each sheet of paper
floated, much more proportionally, in fact, than a strong solution,
since the same amount of fluid is absorbed by the paper in each case,
whilst the amount of silver abstracted from the _whole_ is also equal,
which reduces the strength per ounce more with the former than with
the latter. A weak sensitizing solution, therefore, requires much
more attention than a strong one: crystals of silver nitrate must be
constantly added to the former. In practice and for general work, then,
we recommend a moderately strong bath, the method of making up of which
we shall describe.
To make up 2 pints of solution with a strength of 50 grains to the
ounce, we shall require 2,000 grains of silver nitrate. This is
carefully weighed out in the scales, a piece of _filter paper being
placed in each pan_. By adopting this plan freedom from all impurities
that may cling to the pans will be avoided, and the silver nitrate
will be perfectly pure. Place the silver salt in a large clean bottle,
and add half-a-pint of water to it, and shake it to dissolve it. The
best water for the purpose is distilled water; but filtered rain, pure
spring, or river water answers well. If the water contain any chlorides,
it will be shown by a milkiness due to a formation of silver chloride.
This must be filtered out when the remaining pint and a-half of water
is added. The solution is now ready for use, and, being of the simplest
character, is not to be excelled, though the addition of some soluble
salts may be advantageous, particularly in dry climates or in very dry
weather. Such salts are found in sodium nitrate, or ammonium nitrate, as
much as equal weights of either of these substances being added. Thus
our formula would stand as follows were these additions made:--
_Original Solution._
1.--Silver nitrate 50 grains
Water 1 ounce
_Modified Solution._
2.--Silver nitrate 50 grains
Ammonium nitrate or
sodium nitrate 50 "
Water 1 ounce
The reason of the addition of the ammonium or sodium nitrate is that
prints are better obtained on paper which is not absolutely free from
water. When very dry, the liberated chlorine (see page 32) is apt to
attack the albuminate, whereas it is deprived of much of its activity
when it is able to be absorbed by water, which, in the presence of
light, is decomposed into hydrochloric acid and oxygen.[10]
Hydrochloric acid can attack the silver nitrate present in the pores
of the paper, and produce fresh silver chloride. If the paper were
quite dry, the liberated chlorine would scarcely be able to attack
even the silver. Moisture, though very little, is desirable. In the
excessively dry climate of India, &c., in the summer, one or other of
these deliquescent salts should be invariably present for the purpose
indicated, unless fuming be resorted to.
The sensitizing bath should also never be allowed to be acid with nitric
acid, since the resulting prints would invariably be poor.
The best way of securing this neutral state is by keeping a little
carbonate of silver at the bottom of the bottle in which the solution is
kept. A few drops of a solution of sodium carbonate added to the bottle
over-night will secure this. The reason why nitric acid is to be avoided
is shown by placing a print in dilute nitric acid. It is well known that
darkened silver chloride is unaffected by it; but the print will be
found to change colour, and to become duller and redder than if washed
in water alone. The nitric acid evidently attacks the albumen. Nitric
acid decomposes the carbonate of silver (which, be it remembered, is an
insoluble body), forming silver nitrate, and liberating carbonic acid.[11]
Alum in the printing bath has also been recommended for preventing the
bath from discolouring, and it is effective in that it hardens the
surface of the albumen; but the ordinary explanation of its effect is
defective. If a solution of common alum be added to the silver nitrate
we get silver sulphate (which is best out of the bath, and it is
slightly soluble in the solution), and aluminium nitrate is formed.[12]
The same effect would be produced if aluminium nitrate were added to the
bath solution. We, however, give a means of adding it as recommended by
some writers. When filtering the solution, put a small lump of alum in
the filter paper, and pour the solution over it, or add one grain of
alum to every ounce of solution, and then filter.