[Footnote: Being an abstract of the introductory lecture to a course on
photography at the Polytechnic Institute, November 11.]
By E. HOWARD FARMER, F.C.S.
Since the first announcement of these lectures, our Secretary has asked
me to give a free introductory lecture, so that all who are interested
in the subject may come and gather a better idea as to them than they
can possibly do by simply leading a prospectus. This evening, therefore,
I propose to give first a typical lecture of the course, and secondly,
at its conclusion, to say a few words as to our principal object. As the
subject for this evening's lecture I have chosen, "The Preparation of
Gelatine Plates," as it is probably one of very general interest to
photographers.
Before preparing our emulsion, we must first decide upon the particular
materials we are going to use, and of these the first requisite is
nitrate of silver. Nitrate of silver is supplied by chemists in three
principal conditions:
1. The ordinary crystallized salt, prepared by dissolving silver in
nitric acid, and evaporating the solution until the salt crystallizes
out. This sample usually presents the appearance of imperfect crystals,
having a faint yellowish tinge, and a strong odor of nitrous fumes, and
contains, as might be expected, a considerable amount of free acid.
2. Fused nitrate, or "lunar caustic," prepared by fusing the
crystallized salt and casting it into sticks. Lunar caustic is usually
alkaline to test paper.
3. Recrystallized silver nitrate, prepared by redissolving the ordinary
salt in distilled water, and again evaporating to the crystallizing
point. By this means the impurities and free acid are removed.
I have a specimen of this on the table, and it consists, as you observe,
of fine crystals which are perfectly colorless and transparent; it is
also perfectly neutral to test paper. No doubt either of these samples
can be used with success in preparing emulsions, but to those who are
inexperienced, I recommend that the recrystallized salt be employed. We
make, then, a solution of recrystallized silver nitrate in distilled
water, containing in every 12 ounces of solution 1¼ ounces of the salt.
The next material we require is a soluble bromide. I have here specimens
of various bromides which can be employed, such as ammonium, potassium,
barium, and zinc bromides; as a rule, however, either the ammonium or
potassium salt is used, and I should like to say a few words respecting
the relative efficiency of these two salts.
1. As to ammonium bromide. This substance is a highly unstable salt.
A sample of ammonium bromide which is perfectly neutral when first
prepared will, on keeping, be found to become decidedly acid in
character. Moreover, during this decomposition, the percentage of
bromine does not remain constant; as a rule, it will be found to contain
more than the theoretical amount of bromine. Finally, all ammonium salts
have a most destructive action on gelatine; if gelatine, which has
been boiled for a short time with either ammonium bromide or ammonium
nitrate, be added to an emulsion, it will be found to produce pink
fog--and probably frilling--on plates prepared with the emulsion. For
these reasons, I venture to say that ammonium bromide, which figures so
largely in formulæ for gelatine emulsions, is one of the worst bromides
that can be employed for that purpose, and is, indeed, a frequent source
of pink fog and frilling.
2. As to potassium bromide. This is a perfectly stable substance, can be
readily obtained pure, and is constant in composition; neither has it
(nor the nitrate) any appreciable destructive action on gelatine. We
prepare, then, a solution of potassium bromide in water containing in
every 12 ounces of solution 1 ounce of the salt. On testing it with
litmus paper, the solution may be either slightly alkaline or neutral;
in either case, it should be faintly acidified with hydrochloric acid.
The last material we require is the gelatine, one of the most important,
and at the same time the most difficult substance to obtain of good
quality. I have various samples here--notably Nelson's No. 1 and "X
opaque;" Coignet's gold medal; Heinrich's; the Autotype Company's; and
Russian isinglass.
The only method I know of securing a uniform quality of gelatine is to
purchase several small samples, make a trial emulsion with each, and buy
a stock of the sample which gives the best results. To those who do not
care to go to this trouble, equal quantities of Nelson's No. 1 and
X opaque, as recommended by Captain Abney, can be employed. Having
selected the gelatine, 1¼ ounces should be allowed to soak in water, and
then melted, when it will be found to have a bulk of about 6 ounces.
In order to prepare our emulsion, I take equal bulks of the silver
nitrate and potassium bromide solutions in beakers, and place them in
the water bath to get hot. I also take an equal bulk of hot water in a
large beaker, and add to it one-half an ounce of the gelatine solution
to every 12 ounces of water. Having raised all these to about 180° F., I
add (as you observe) to the large beaker containing the dilute gelatine
a little of the bromide, then, through a funnel having a fine orifice,
a little of the silver, swirling the liquid round during the operation;
then again some bromide and silver, and so on until all is added.
When this is completed, a little of the emulsion is poured on a glass
plate, and examined by transmitted light; if the mixing be efficient,
the light will appear--as it does here--of an orange or orange red
color.
It will be observed that we keep the bromide in excess while mixing. I
must not forget to mention that to those experienced in mixing, by
far the best method is that described by Captain Abney in his Cantor
lectures, of keeping the silver in excess.
The emulsion, being properly mixed, has now to be placed in the water
bath, and kept at the boiling point for forty-five minutes. As,
obviously, I cannot keep you waiting while this is done, I propose to
divide our emulsion into two portions, allowing one portion to stew, and
to proceed with the next operation with the remainder.
Supposing, then, this emulsion has been boiled, it is placed in cold
water to cool. While it is cooling, let us consider for a moment what
takes place during the boiling. It is found that during this time the
emulsion undergoes two remarkable changes:
1. The molecules of silver bromide gradually aggregate together, forming
larger and larger particles.
2. The emulsion increases rapidly in sensitiveness. Now what is the
cause, in the first place, of this aggregation of molecules: and, in the
second place, of the increase of sensitiveness? We know that the two
invariably go together, so that we are right in concluding that the same
cause produces both.
It might be thought that heat is the cause, but the same changes take
place more slowly in the cold, so we can only say that heat accelerates
the action, and hence must conclude that the prime cause is one of the
materials in the emulsion itself.
Now, besides the silver bromide, we have in the emulsion water,
gelatine, potassium nitrate, and a small excess of potassium bromide;
and in order to find which of these is the cause, we must make different
emulsions, omitting in succession each of these materials. Suppose we
take an emulsion which has just been mixed, and, instead of boiling
it, we precipitate the gelatine and silver bromide with alcohol; on
redissolving the pellicle in the same quantity of water, we have an
emulsion the same as previously, with the exception that the niter and
excess of potassium bromide are absent. If such an emulsion be boiled,
we shall find the remarkable fact that, however long it be boiled, the
silver bromide undergoes no change, neither does the emulsion become
any more sensitive. We therefore conclude, that either the niter or the
small excess of potassium bromide, or both together, produce the change.
Now take portions of a similarly washed emulsion, and add to one portion
some niter, and to another some potassium bromide; on boiling these
we find that the one containing niter does not change, while that
containing the potassium bromide rapidly undergoes the changes
mentioned.
Here, then, by a direct appeal to experiment, we prove that to all
appearance comparatively useless excess of potassium bromide is really
one of the most important constituents of the emulsion.
The following table gives some interesting results respecting this
action of potassium bromide:
__________________________________________________________
Excess of potash bromide. | Time to acquire maximum |
| sensitiveness. |
--------------------------+------------------------------+
0.2 grain per ounce | no increase after six hours. |
2.0 " " | about one-half an hour. |
20.0 " " | seven minutes. |
--------------------------+------------------------------+
I must here leave the _rationale_ of the process for the present, and
proceed with the next operation.
Our emulsion being cold, I add to it, for every 6 ounces of mixed
emulsion, 1 ounce of a saturated cold solution of potassium bichromate;
then, gently swirling the mixture round, a few drops of a dilute (1 to
8) solution of hydrochloric acid, and place it on one side for a minute
or two.
When hydrochloric acid is added to bichromate of potash, chromic acid is
liberated. Now, chromic acid has the property of precipitating gelatine,
so that what I hope to have done is to have precipitated the gelatine in
this emulsion, and which will carry down the silver bromide as well. You
see here I can pour off the supernatant liquid clear, leaving our silver
and gelatine as a clot at the bottom of the vessel.
Another action of chromic acid is, that it destroys the action of light
on silver bromide, so that up to this point operations can be carried on
in broad daylight.
The precipitated emulsion is now taken into the dark room and washed
until the wash water shows no trace of color; if there be a large
quantity, this is best done on a fine muslin filter; if a small
quantity, by decantation.
Having been thoroughly washed, I dissolve the pellicle in water by
immersing the beaker containing it in the water bath. I then add the
remaining gelatine, and make up the whole with 3 ounces of alcohol and
water to 30 ounces for the quantities given. I pass the emulsion through
a funnel containing a pellet of cotton wool in order to filter it, and
it is ready for coating the plates.
To coat a plate, I place it on this small block of leveled wood, and
pour on down a glass rod a small quantity of the emulsion, and by means
of the rod held horizontally, spread it over the plate. I then transfer
the plate to this leveled slab of plate glass, in order that the
emulsion on it may set. As soon as set, it is placed in the drying box.
This process, as here described, does not give plates of the highest
degree of sensitiveness, to attain which a further operation is
necessary; they are, however, of exceedingly good quality, and very
suitable for landscape work.--_Photo. News_.
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