HOW TO KEEP THE SENSITIZING BATH IN ORDER.
Experience tells us, however strong we may make the bath solution to
coagulate the albumen on the paper, that a certain amount of organic
matter will always be carried into it. At first this is not apparent,
since it remains colourless in the solution; but after a time, after
floating a few sheets of paper, the organic silver compound gradually
decomposes, and the solution becomes of a brown or red tint, and if
paper were floated on it in this condition there would be a dark surface
and uneven sensitizing. It is, therefore, necessary to indicate the
various means that may be employed to get rid of this impurity. The
earliest, if not one of the best, is by the addition of white China
clay, which is known in commerce as kaolin. A teaspoonful is placed in
the bottle containing the solution, and well shaken up; the organic
matter adheres to it, and precipitates to the bottom, and the liquid can
be filtered through filter-paper or washed cotton-wool, when it will
be found decolourized. Another mode of getting the liquid out of the
bottle is to syphon it off by any syphon arrangement, and this prevents
a waste in the solution from the absorption of the filtering medium. The
accompanying arrangement (fig. 4) will be found useful for the purpose,
and can be applied to other solutions where decantation is necessary. A
is a wide-mouthed bottle holding the solution. B is a cork fitting the
mouth, in which two holes have been bored to fit the two tubes, D and C,
which are bent to the form shown. When the kaolin has subsided to the
bottom, air is forced by the mouth into the bottle through C, the liquid
rises over the bend of the tube D, and syphons off to the level of the
bottom of the tube inserted into the liquid, provided the end of D,
outside the bottle, comes below it.
[Illustration: _Fig._ 4.]
[Illustration: _Fig._ 5.]
To bend a tube, a common gas flame is superior to a Bunsen burner. The
tube is placed in the bright part of the flame in the position shown; by
this means a good length of it gets heated, and a gentle bend is made
without choking the bore, which would be the case were a point of a
flame used.
Another method of purifying the solution is by adding a few drops of
hydrochloric (muriatic) acid to it. Chloride of silver is formed, and
when well shaken up, carries down with it most of the organic matter,
but leaves the bath acid from the formation of nitric acid.[13] This
must be neutralized unless a little silver carbonate is left at the
bottom of the bottle as described at page 20. A camphor solution may
also be added for the same purpose. Make a saturated solution of camphor
in spirits of wine, and add a couple of drachms to the solution, and
shake well up. The camphor will collect the albumen, and it can be
filtered out. In case the first dose does not decolourize it, another
one must be added.
Another plan is to add potassium permanganate (permanganate of potash)
to it, till such time as the solution takes a faint permanent rose
tint. The theory is that the organic matter is oxidized by the oxygen
liberated from the permanganate, and falls to the bottom. It is not
strictly true, however, and the solution will never be as free from
organic matter as when the other methods are employed.
The final and best method is to add a small quantity of sodium carbonate
(say 5 grains), and expose it to daylight. When the organic matter
becomes oxidized at the expense of the silver nitrate, the metallic
silver with the oxidized organic matter will fall to the bottom. This
plan answers admirably when time is no object, but in dull weather the
action is slow. When once the precipitation fairly commences it goes on
quickly, and if a little freshly precipitated metallic silver be left at
bottom of the bottle the action is much more rapid. This is a wrinkle
worth remembering in all photographic operations where precipitation is
resorted to.
We have hitherto supposed that the only contamination of the bath is
organic matter, but it must be borne in mind that each sheet of paper
floated on the solution transfers a certain amount of nitrate of the
alkali[14] with which the albumen is salted.
[Illustration: _Fig._ 6.]
It will thus be seen that in an old bath there will be no need to add
the soluble nitrates given in page 17, since they will be already
formed. When they are in excess the best plan is to precipitate the
silver by some means,[15] but we select one which is easy of application,
since it requires no watching. Evaporate the solution to half its
bulk, and slightly acidify it with nitric acid (10 drops to the pint
of solution will suffice); throw some ordinary granulated zinc into
the jar or bottle containing it; the silver will now be rapidly thrown
down in the metallic state, and in the course of two or three hours
the action will be complete. Next carefully pour off all the fluid as
close as possible to the residue. Pick out all the lumps of zinc, and
add a little dilute hydrochloric acid to dissolve up all the small
particles of zinc which may be amongst the precipitated silver. Filter
the solution away, and wash the residue once or twice with water. Take
out the filter paper, and dry it before a fire, or in an oven, and then
detach the silver, and transfer it to a small crucible, which place,
with its contents, over a Bunsen burner or spirit lamp flame till it is
red hot. The heat will destroy all organic matter, leaving a residue of
carbonous matter behind, which, after subsequent operations, will be
eliminated by filtration. Next cover the silver with nitric acid,[16]
and in an evaporating dish slightly warm it over a spirit lamp or
Bunsen burner. Red fumes will appear, and when all action has ceased,
more acid must be added till such a time that very nearly (but not
quite) all the silver is dissolved up. Then evaporate off all the fluid
and allow it to cool, when water can be added to such an extent that
it is _over strength_ for the bath. Now measure the whole bulk of the
solution in a glass measure, and test by the argentometer for strength.
An argentometer is, in reality, an instrument for taking the specific
gravity of a liquid. It is as shown in the figure. A B is a glass tube,
inside of which is a graduated scale showing grains; C is a hollow
glass cylinder, which has a little glass ball filled with mercury. When
immersed in water, the instrument sinks till the scale reads 0--that
is, A B is deeply immersed. When any soluble salt is dissolved in the
water, the stem rises further. If the soluble salt be silver nitrate,
the scale is made to read grains per ounce. It is then evident, if the
bath contains any other soluble salt besides the nitrate of silver, the
readings will be untrustworthy. Supposing you have a total quantity of
10-1/4 ounces of solution, and the argentometer tells you it is of a
strength of 105 grains to the ounce, you must make a small calculation
to see how much water you must add. In 10-1/4 ounces of solution there
will be 10-1/4 × 105 or 1076-1/4 grains of silver nitrate. If you want
to make the bath 40 grains to the ounce, you must divide this quantity
by 40, which is very nearly 27. The original amount of fluid (10-1/4
ounces), when deducted from this number of (27) ounces, will give you
the amount (16-3/4 ounces) of water that is to be added to give you a
bath of the required strength. When the water is added, the solution
should be filtered from the carbonaceous matter, and the bath, after
neutralizing with sodium carbonate, will be ready for use.