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Monographs on the Theory of Photography from the
Research Laboratory of the Eastman Kodak Co.

No. 1



Copyright 1921
Eastman Kodak Company



The Silver Bromide Grain

of Photographic

Emulsions



% A. P. H.Trivelli and

S. E. Sheppard



ILLUSTRATED



D. VAN NOSTRAND COMPANY

NEW YORK



EASTMAN KODAK COMPANY



ROCHESTER, N. Y.
1921



00 A



Monographs on the Theory of Photography

Edited by

C. E. Kenneth Mees

and

Mildred Spargo Schramm



Monographs on the Theory
of Photography

The Silver Bromide Grain of Photographic Emulsions.
By A. P. H. Trivelli and S. E. Sheppard.

Other volumes soon to appear.
Gelatine. By S. E. Sheppard, D. Sc. 2 volumes.
The Theory of Development. By A. H. Xietz.



Preface to the Series



The Research Laboratory of the Eastman
Kodak Company was founded in 1913 to carry
out research on photography and on the pro-
cesses of photographic manufacture.

The scientific results obtained in the Labora-
tory are published in various scientific and tech-
nical journals, but the work on the theory of
photography is of so general a nature and occu-
pies so large a part of the field that it has been
thought wise to prepare a series of monographs,
of which this volume is the first. In the course
of the series it is hoped to cover the entire field
of scientific photography, and thus to make
available to the general public material which
at the present time is distributed throughout a
wide range of journals. Each monograph is in-
tended to be complete in itself and to cover, not
only the work done in the Laboratory, but also
that available in the literature of the subject.
A very large portion of the material in these
monographs, however, will naturally be original
work which has not previously been published.
The monographs are written by those specialists
in the Laboratory who are best qualified for the
task, each monograph being edited by the Direc-
tor of the Laboratory and by Mrs. Schramm,
who is the active editor of the series.

Rochester, New York
April, 1921



Preface

The fundamental units of the sensitive materials used in
photography are the small grains of silver halide which,
imbedded in gelatine, form the emulsion.

Since these grains are of very small size, and are, further-
more, precipitated in a colloid medium, they have usually
been treated simply as colloid aggregates.

As a result of a complete crystallographic study involving
photomicrographic work of a high order, it has been possible
not only to confirm the fact that the grains of high-speed
emulsions are definitely crystalline, but to identify their
crystalline form and to show that all the grains, though having
several distinct shapes, belong to the same crystalline class.
The grains being thus established as micro-crystalline, their
formation in the emulsion can be studied by the aid of the
recent physico-chemical theories of precipitation and especially
of the dispersion theory of Von Weimarn, according to which
the dispersity of the initial precipitate will be determined by
the concentration of the solutions and other physical condi-
tions. The changes in the dispersity of the original precipitate
during ripening, which will follow from the laws of surface
energy, is now found to be related to changes in the content of
adsorbed impurities, and in connection with this the effect
of ammonia upon the grains has been studied.

The catalysis of crystallization by nuclei is suggested as
an explanation of some of the effects produced by the
admixture of silver iodide with silver bromide in an emul-
sion, and the fact that traces of colloidal silver make the
grains color-sensitive is believed to be related to this.

A study of the relations existing between the sizes of the
grains and their photographic properties is reserved for a
future monograph.

Rochester, New York
April, 1921



The Silver Bromide Grain of
Photographic Emulsions

CONTENTS

Page

Preface 7~~

Chapter I. The influence of ammonia on photo-
graphic emulsions and a theory of
ripening 11

Chapter II. Von Weimarn's theory and the deter-
mination of the dispersity of silver bro-
mide precipitates 27

Chapter III. Accessory factors influencing the disper-
sity of silver bromide emulsions . . 35 ^

Chapter IV. Crystallization catalysis 52

Chapter V. Capillarity and crystalline growth . . 57

Chapter VI. Experimental study of the crystalli-
zation of silver bromide 75

Chapter VII. The classification of silver halide crystals 95

Chapter VIII. The silver bromide crystals of photo-
graphic emulsions 99"

Chapter IX. The directions of most rapid growth in
silver bromide crystals, and the occur-
rence of anomalous forms .... 107

Chapter X. The behavior of silver bromide and sil-
ver iodo-bromide crystals in polarized
light 121

Summary of crystallographic study of silver halide grains . 129

Alphabetical list of serial publications referred to, with the
abbreviations adopted in citations 132

Bibliography 133

Index of Authors 137

Index of Subjects 139



The Silver Bromide Grain of
Photographic Emulsions

CHAPTER I

The Influence of Ammonia on Photographic
Emulsions and a Theory of Ripening

The use of aqueous ammonia in the ripening of photographic
silver halide emulsions was introduced by Johnston 1 and is
well known to photographic technologists, particularly through
the later work of J. M. Eder. 2

Eder states that exposing a dry gelatino-bromide plate
for a few minutes to the vapor from strong ammonia imme-
diately before using in the camera results in a marked increase
in sensitiveness. On the other hand, Gaedicke 3 concluded
that fuming prior to exposure diminished the sensitiveness,
but that, subsequent to exposure and prior to development,
it increased the developability of the latent image, resulting
in an effective sensitizing. This action he considered to be
one on the latent image, not an acceleration of the action of
the developer. Sheppard and Mees 4 found that certain
plates gave a higher inertia, or lower speed, with ferrous oxalate
developer than with organic developers, while a larger group
gave practically the same speed with both developers. For
the latter, however, a slight fuming with ammonia increased
the inertia, i. e., decreased the speed, when ferrous oxalate
was used as developer. In addition to these relatively invisible
effects, the accounts of which exhibit rather contradictory
conclusions, it was observed by Eder that, if moist silver
bromide plates were exposed under a bell-jar to ammonia
vapor for a considerable time, they became more sensitive to
light and coarser-grained, ultimately forming a network of
coarse-grained silver bromide with relatively empty inter-
spaces resembling frost figures.

1 Johnston, J.. Gelatino-bromide of silver emulsions treated with ammonia. Brit. J.
Phot. Almanac 1877: 95. 1877.

2 Eder, J. M., Beitrage zur Photochemie des Bromsilbers. Sitzungsber. Akad. \Yi>>.
Wien. 81: 679. 1880.

3 Gaedicke, J., Ammoniakraucherung bei Trockenplatten. Jahrb. Phot. 27: 62. 1913.
■* Sheppard, S. E., and Mees, C E. K., Investigations on the theory of the photo-
graphic process.

11



MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

Englisch 1 found that a partial development of the latent
image was possible, for by treating an exposed plate with
strong aqueous ammonia the unexposed parts were apparently
more rapidly dissolved away than the exposed parts. He
attributed this to a lesser solubility of the exposed halide in
ammonia. Luppo-Cramer, 2 repeating the experiments, came
to a different conclusion. Under suitable experimental con-
ditions he found that the exposed portions at first apparently
dissolved out more rapidly than the unexposed, but that this
relation was reversed on continuance of the ammonia "devel-
opment." Liippo-Cramer modified Englisch's experimental
conditions by using ammonia vapor instead of aqueous
solutions. This has advantages in that the action is slower
and therefore can be better observed, and that there can be no
actual removal of dissolved silver bromide from the plate.
Proceeding in this way, and with the help of the microscope,
Liippo-Cramer concluded that ammonia development is
really a reaggregation or "ripening" process which proceeds at
different rates, according to the exposure to light of a given
part of the plate.

Luppo-Cramer considers that this supports the theory that
light brings about a certain disintegration of the silver halide.
He ascribed the "developability" with ammonia to the in-
creased "inner dispersity" of the silver bromide grains. He
finds that at first the exposed parts show a coarsened grain,
and concludes that, in consequence of disintegration, the
exposed silver bromide particles have on the whole a greater
solubility in ammonia, whereby at first an immediate Ostwald
ripening occurs. This, however, is reversed on further
treatment, the unexposed parts becoming later relatively
coarser-grained than the exposed parts. This he attributes
to the "disintegration by light" affording a greater number
of crystallization nuclei, whence, with greater number of
crystals formed, the ultimate size will be smaller, since the
mass of material per unit area is the same. Liippo-Cramer
later supported this view with experiments in which the
"chemical latent image" was completely (?) destroyed, but
could still be developed with ammonia.

It does not appear that this reasoning is either necessary
or sufficient. To begin with, if the first effect in the more
exposed parts is essentially an increased solubility and solution
of the disintegrated particles of the original silver halide
grains, whence come the subsequently invoked greater number

1 Englisch, E., Zeits. wiss. Phot. 2: 416. 1905.

2 Luppo-Cramer, Photographische Probleme, p. 83.

12



SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

of crystallization nuclei? And also, where, at the same time,
are the relatively larger crystals which, according to the
Ostwald ripening theory, must be present to increase at the
expense of the smaller crystals? There appear to be mutually
incompatible requirements here, since the increased crystal
fragments must disappear — by solution — to give the postu-
lated initial Ostwald ripening in the more exposed parts, and
yet, to explain the apparent reversal effect later, must also
have been there all the time.

The experiments now to be described show, we believe,
that the facts are capable of a less involved explanation.
They show:

(a) That there is no ammonia development of the latent
image, properly so called, but only an ammonia development
of the visible image, no effect being obtainable with exposures
to light much lower than those which give the least visible
photochemical effect;

(b) That the actual development can be more simply
explained by a simple recrystallization effect, not involving
directly, but only (if at all) as a very subsidiary factor, any
Ostwald ripening;

(c) That the development or ripening nuclei are due not
to disintegration, but to the photochemical decomposition
products of the silver halide — probably colloid silver adsorbed
to silver halide — and to similar decomposition products from
the reducing action of the ammoniacal gelatine on the silver
halide.

In addition to correcting what appears to us the incorrect
and unnecessary conclusions drawn by Luppo-Cramer in his
otherwise valuable and interesting papers, the experiments
are noteworthy because this ripening with ammonia affords
a cross-section of the ripening process in general, particularly
as convection currents within the emulsion are eliminated.
At the same time, it is believed that they indicate the causes
for some of the natural limitations and peculiarities in the
ripening process.

FUMING OF UNEXPOSED LAYERS

The experimental method followed was in the main similar
to that of Eder and Luppo-Cramer, namely, fuming with
ammonia vapor evolved from strongest aqueous ammonia.
Some side experiments with ammonia solutions applied direct
showed that far less control was obtainable in this way. The
aqueous ammonia — S. G. 0.90-0.92 — was contained in deep
crystallization dishes, the plates to be fumed being laid film

13



MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

down over them, so that an unfumed surround was obtained
in each ease. Care was taken that the distance between
the ammonia solution and the plate was kept constant, unless
purposely varied. For convenience the fuming was conducted
under a hood in a dark room. Illumination and inspection
during the experiments were facilitated by placing the care-
fully leveled dish over a dark-room lamp laid on its back, so
that red light was thrown upward through the dish and plate.
In all, some hundred experiments were performed, using Seed
23 and Seed Process Plates, Cine Positive film, and Lantern
Plates.

In repeating some of the experiments previously described,
an initial phase of the action of ammonia on the silver halide
emulsion was noted, which appears to have attracted little,
if any, attention. On fuming Seed Process plates, unexposed
to light and unmoistened, in the manner described above, it
was observed that the first visible differentiation of the fumed
from the unfumed area is a uniformly diminished opacity;
this was such that in one hour — the actual time varies both
with the kind of plate and with its relative moisture content-
the film had become almost transparent by transmitted light,
but showed a light bluish gray turbidity by reflected light.




Fig. 1 Fig. 2

Print through fumed plate on Print on fumed plate through

unfumed plate. Crescent shows unfumed plate. Crescent shows

untreated portion. untreated portion.

The extent of this induced transparency is shown by the
photograph in Fig. 1, which gives the result of printing a
negative through a plate so treated oh to another one unfumed.

14



SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

The part made transparent becomes, photographically, 1
greatly reduced in speed and density-giving power, as is shown
by Fig. 2, which gives a direct print from the negative shown
in Fig. 1 on the partially fumed plate. The unfumed sur-
round is overexposed long before the fumed part gives a devel-
opable impression. On development the image frequently
shows considerable irregularly distributed surface fog of a
dichroic nature. Microscopic investigation of the fumed
transparent area showed that in this state the emulsion has a
fine and very uniform grain, apparently considerably finer
than the original. The reduction of opacity, however, is



A 9k




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i S




-,."<■


r m
ft


' ft

^^ a ^ 1


• «


^H V W *m


**■#.,



Fig. 3
Ammonia fuming, early stage



due not solely to this diminution in grain size, but largely to
an approximation of the refractive index of the grains to that
of the gelatine.

The photographic speed- and density-giving power of an emulsion on exposure and
development must not be identified with the photochemical sensitiveness giving rise to a
visible image.

15



MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

It is very probable that at this stage a double compound
of silver halide and ammonia is formed. (See p. 25.) The
change of grain size is, however, not very pronounced as
compared with later stages. This is evident in that at this
stage the change is largely reversible. On being removed
from the ammonia atmosphere the emulsion regains opacity
to near its former value, this being accelerated by an air
current. The next phase (on continued fuming), is an irre-
versible ripening, in the sense of increase in size of grain, in
which large crystal aggregates are formed. They commence
at isolated points (see Fig. 3), and radiate from these until the
respective recrystallization circles or domains meet, when




Fig. 4
Ammonia fuming, middle stage

boundaries which tend to be straight lines are formed, so that
the original recrystallization areas become polyhedral, as
illustrated in Figs. 4 and 5.

It will be seen that the final stage is a complete filling up of
the area fumed with a number of polyhedral cells enclosing a

16



SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

sort of efflorescence of trichites and crystal aggregates. The
figures so far given are from natural size contact prints from
contact negatives made direct from the original preparations;
hence the appearance of the original preparations is reproduced
as accurately as possible. Photomicrographs dealing with cer-
tain aspects of this recrystallization process will be given later
in connection with the discussion of the theory. 1 At this
point it is necessary only to note that the beginning of nuclea-




Fig. 5
Ammonia fuming, final stage

tion, under the conditions given, is to a certain extent acci-
dental. In any case, it commences at the boundary where
the film is in contact with the ammonia container, but in the
fumed area dust particles or other casual nuclei seem to serve.
As scratching the sides of the container starts crystallization
from solutions, so a stress mark made with a glass rod on the
emulsion induces ammonia development along the trace. 2

1 Examples of this have been given by Eder (I.e.) and Luppo-Cramer, Kolloidchemie
und Photographic XII. Koll.-Zeits. 9: 240. 1911.

2 Cf. Luppo-Cramer, Kolloidchemie und Photographic I.e. This was confirmed in
the present investigation.

17



MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY
FUMING EXPOSED PLATES

The plates were exposed behind a scale negative — a sensi-
tometer strip — to a 100-watt lamp for definite times and at
definite distances. From density measurements of the scale
negative the relative exposures could be calculated, but it is
unnecessary to give these within this scale, because it was
found that precision in the matter of gradation within the
scale was neither important nor practicable to determine.
As has been pointed out under conditions of fuming, there
is no actual increase or decrease of material within a given
area, exposed or unexposed, but only changes of aggregation
or dispersity and of refractive index, which produce an appar-
ent change of density or opacity- Plates were exposed both
moist and dry. In preparing the moist plates the plate was
soaked for a brief period in water, and superfluous water
blotted off. The plates were weighed dry and wet to deter-
mine the amount of water absorbed.

The general effect of moisture was greatly to accelerate
the action of ammonia vapor. Although a definite propor-
tionality of effect could not be ascertained, it was evident that
excessive swelling in water produced more irregular effects.
The most marked difference between dry and moist plates, in
line with the acceleration, was the much coarser grain produced
in the wet or moistened plates, as will be evident from figures
to be given later.

EXPOSURE NECESSARY FOR AMMONIA DEVELOPMENT

A result of importance, in view of earlier pronouncements
on the "development of the latent image by ammonia," was
that the exposures to light necessary to obtain a developed
image were of an entirely different order, being very many
times greater than those required to obtain a developable
image by ordinary development.

Thus with Seed Process plates the exposure necessary to
obtain ammonia development of an image of the scale was
some 150 times that necessary to give an image developable
with pyro-soda. With Cine Positive film, the corresponding
figure was about 250 times as long an exposure; and similar
results were obtained with other emulsions. (With lantern
plate, 190 times.) Under these conditions, which imply
exposures well toward the ordinary solarization limit, it
appears incorrect to speak of a development of the latent
image by ammonia. And, in fact, close inspection showed
tha^ammonia development of an image is possible only from
an exposure which is either the same or but little below the

18



SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

threshold value of exposure to give a visible image. Of
course this threshold varies very considerably with the visi-
bility' conditions, and it is generally possible to detect with
the microscope definite evidence of photochemical changes
well below the visible threshold of image formation.

Fig. 6 shows the slight indication of image formation
after ammonia-fuming a Seed Process plate, dry, for some
seventeen hours, the plate having received an exposure 130
times that necessary to give a full scale with pyro-soda devel-
opment. The ammonia development here has proceeded to



hz


Uffl w


■ ^B


^ i


3300







0250
6400




3400




1720_
890






546






431






345







Fig. 6
Exposed plate, fumed dry; exposures in candle- meter-seconds

the stage of reversal already referred to, but has brought out
nothing further on the scale. An unfumed control plate as
well as the plate used showed a threshold visible image w r ithin
a field or two of the lowest developed by ammonia.

EFFECT OF MOISTURE

If the plate is moistened by swelling in water, fuming with
ammonia produces an effect earlier, but the developable

19



MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY

threshold exposure is much higher. Thus the result shown in
Fig. 7 was obtained after soaking a plate for one minute in
water and fuming fifteen minutes; but the ill-defined differenti-
ation or development covers only part of the scale covered in
Fig. 6 and implies about nine times as great an exposure, the
normal image being visible over a greater range. Further
development with ammonia in this case only filled the plate
with crystal aggregates and obliterated the primary differen-
tiation between exposed and unexposed portions. The faint
indication of an image obtained in this way is shown in Fig. 7.




Fig. 7

Exposed plate, fumed moist; exposures in
candle-meter-seconds



REVERSAL

The appearance of reversal observed by Luppo-Cramer is
clearly indicated in Fig. 8. It is to be noticed that more than
one type of reversal, as regards relative optical density, is
apparent in the process. The exposed portions, as compared
with the unexposed, appear at first more transparent, and

20



SILVER BROMIDE GRAIN OF PHOTOGRAPHIC EMULSIONS

later, less transparent, than contiguous unexposed portions.
This is due, first, to the new silver bromide-ammonia complex
having a lower refractive index than silver bromide; second,
to the varying stages of dispersity of the new and old phases.
Reversal with increased exposure to light for the same time
of development (fuming) indicates that the optical opacity
at first increases with the number of nuclei, reaches a maxi-
mum, and then diminishes. (See Fig. 8.) Reversal with
increase of time of fuming is more apparent than real, being




Fig. 8
Exposed plate, showing appearance of reversal



dependent upon the relations between the stages of recrystal-
lization in two contiguous fields. Finally, this is affected by a
third factor — partial or complete reconversion of the silver
bromide-ammonia complex into silver bromide, leaving
pseudomorphs of silver bromide by evaporation of ammonia.

21



MONOGRAPHS ON THE THEORY OF PHOTOGRAPHY
EFFECT OF AMMONIA-FUMING ON DIFFERENT EMULSIONS

As already noted, the rate and sensibility of ammonia-
fuming is very dependent in one and the same emulsion on
the actual state of the plate in respect of moisture content.
It is very difficult to bring different emulsions to the same
state in this respect, hence reliable comparisons between
different emulsions are anything but easy to obtain. It was
hoped at one time that ammonia-fuming might be used as a
method of investigation and control of the "grain" of an
emulsion when coated, somewhat in the manner of the etching
reactions in metallurgy; but it is evident that, even if it should
be possible, much more work on the control of conditions will
be necessary. Taken by and large, however, the results
showed that the finer-grained emulsions react, or rather
reaggregate, more rapidly on fuming with ammonia than the
coarser-grained ones. Their sensibility in the matter of the
development of an image by ammonia after exposure to light
appears to be entirely a matter of their photochemical sensi-
tiveness. The rate at which ammonia-ripening takes place
is a function of the size of the grain, the character of the
emulsion, and the moisture content.

THEORY OF AMMONIA DEVELOPMENT

Reference has already been made and certain objections


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Online LibraryA. P. H. (Adrian Peter Herman) TrivelliThe silver bromide grain of photographic emulsions → online text (page 1 of 11)