The silver solution for a negative bath is prepared in substantially
the same manner as for a positive.
Two thirds of the amount of silver nitrate intended to be used,
should be dissolved in the quantity of water required to make the
bath, and a grain or two of iodide of potassium added and placed in
the sunlight until the solution has changed color, become turbid,
and again clear and colorless. This change is caused by a minute
portion of the silver combining with the organic matter and with
other impurities in the water, if there are any; a molecular change
then takes [p030] places, oxygen is evolved and the infinitesimal
portion of silver changes to a metallic state and sinks to the
bottom, carrying with it the impurities, which induced the chemical
action or change.
The solution may now be filtered, the remaining portion of the
silver nitrate added, and with a few drops of pure nitric acid, the
solution will be ready for use. But if on trial it is ascertained
that it will not give satisfactory results, it will be found that it
has not been sufficiently excited, or an acid reaction has not been
induced.
If the first supposition is true, more iodide of potassium must be
used, as a silver solution must receive as much or nearly as much of
the iodide as it will hold in solution, before it will work
harmoniously with the collodion; the grain or two put in the
solution with the first portion of nitrate of silver have not been
sufficient.
A very little more may be added, which will combine with the silver,
forming a white curdy precipitate, which, on shaking the bottle,
will in a short time redissolve. If any remains, it should be
filtered out, as no more can be held in solution.
An other plan is rather more convenient, but requires more time. It
is to coat with collodion as large a plate as the bath will receive;
immerse it in the bath, leaving it there some hours, usually all
night.
After trying one of these remedies, if the bath refuses still to
yield good results, more nitric acid should be added, until the
solution will turn blue litmus paper slowly red.
I have never known a silver bath to fail to come to terms under this
treatment. [p031]
The silver solution, when in constant use, rapidly deteriorates, and
unless there is a very large quantity, it will soon become unfit for
use, and however much or little there may be, it is only a question
of time as to when it will cease to act satisfactorily.
This result is caused partly by the gradual contamination of the
solution by the alcohol and ether washed from the collodionized
plates that have been sensitized in it. When the bath is seriously
affected in this manner, it is indicated by the difficulty
experienced in getting the developer to flow evenly over the plate,
and also by the strong alcoholic odor of the silver solution.
The necessity for a change of the bath may be delayed for a time by
the addition of alcohol to the developer, which causes it to flow
more evenly.
Another source of evil to the bath is the continual absorption from
the surfaces of the plates immersed of minute portions of the salts
with which the collodion is excited. After the silver solution has
taken up all it can dissolve or assimilate, the surplus is held in
suspension and is called free iodide, which deposits itself on the
surfaces of the plates, and when in quantity causes the plate when
taken from the bath to appear as if fine sand had been sprinkled
over it. These small crystals prevent the action of the light on the
parts they cover, and when the plate has been developed and fixed,
every crystal has produced a small transparent spot or pinhole, as
it is termed, and many a lovely negative has been ruined by
pinholes.
This trouble may be cured by increasing the quantity of the solution
and adding more silver nitrate, which, if in sufficient quantity,
will dissolve the free [p032] iodide. But if it may not be
convenient to do this and no other solution is ready for use, then,
after immersing your plate, tip the bath dish back so that the face
of the plate may be inclined downwards, when the crystals will be
deposited on the back of the plate and do no harm.
The silver bath is also contaminated with organic matter, taken up
from the edges and backs of the many plates used, of which some
portion of the albumen is likely to be exposed to the action of the
solution, and also from dust, etc., falling into the bath dish,
which will be taken up and held in solution by the acid in the bath,
but when the acid can hold no more in solution, its presence will be
indicated by a greyish white vail forming on the surface of the
plate when developed; this is called fog, and may only partially
obscure the image, or it may completely veil it.
The fogging of the plates in this manner may be prevented for a time
by adding more nitric acid to the bath, which will dissolve the
organic matter and prevent its deposition on the plate.
When the silver bath has, from long or much use, become seriously
affected by any of these evils, it should be set aside and a fresh
bath put in use, until an opportunity offers to thoroughly cleanse
and renew the old solution, which can be done by one series of
manipulations, as thus:
Into a large bottle, which will hold the solution to be treated, and
somewhat more, put from a pint to a quart of water, according to the
quantity of the solution; into this bottle containing the pint of
water, pour in a small stream the whole of the old solution, when it
will be found to have turned to a milky whiteness [p033] or
opalescence, which is the iodide of silver set free by reducing the
strength of the solution; this must be filtered out, and the
solution neutralized with bi-carbonate of soda, which should be put
in, in small quantities until a deposit of carbonate of silver
begins to be formed; then place the solution in an evaporating dish,
and subject to heat until the bulk is reduced about one-half; test
it with litmus paper, and if acid, give it more bi-carbonate of
soda; add water and silver nitrate to bring to the required bulk and
strength, set in the sunlight for a time or until required for use,
then filter, and it will be found to be as good as new, but it may
require a few drops of nitric acid.
Pure water may be always had ready for use, by keeping a large
bottle full of the common water standing out-door in the sunlight,
adding a grain of silver from time to time as the bottle is emptied
and replenished.
The new "agate iron ware" pots are the best for use as evaporating
dishes. They will not break or crack, and they are not nearly so
expensive as porcelain dishes.