0% found this document useful (0 votes)
199 views137 pagesBaker
Uploaded by
LudwikCopyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF or read online on Scribd
0% found this document useful (0 votes)
199 views137 pagesBaker
Uploaded by
LudwikCopyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF or read online on Scribd
/ 137
PHOTOGRAPHIC
EMULSION 'TECHNIQUE
By
T, THORNE BAKER, F.R.P.S.,
AM.LE.E., F.Insr.P.
1941
AMERICAN PHOTOGRAPHIC PUBLISHING CO.
BOSTON Corvaicin, 1941, #Y
Awenicay Puotocariie Pustisinx Co.
Made and printed in the United States of America
by the Plimpton Press, Norwood, Massachusetts TABLE OF CONTENTS
InrRopuction ee eR oeain
I. Narore or Puorocrapnic Exurstons or
Introductory —Light-sensitive Silver Com-
pounds —The Nature of an Emulsion —The
Function of Ammonia —Sensitizing Properties
of Gelatin — Ostwald Ripening — The Crystal
Structure of Silver Bromide — Iodide in Emul-
sions — Grain Size and Emulsion Characteris-
tics — The Effect of Light on Silver Bromide
I, Marerrats ror Excursion Maxinc . a1
Gelatin —Chemicals and Their Choice —
Analyses and ‘Tests — Storage — Methods of
Handling Bulk Materials
TH, Lawoxarory Equipment 37
Layout for Experimental Work — Commercial
Production and Its Requirements — Ventilation
—Safelights —Digesting Apparatus — Ther-
mostatic Control — Washing and Filtering of
Emulsions — Making up —Cold Storage
IV. Neoarive Emvistons . 63
Their Structure and Composition — Types of
Formula — Preparation of the Reacting Solu-
tions — Emulsification — Ripening — Setting —
Washing — Final Digestion or Finishing —
Making up — Anti-halation Methods — Re-
versal Emulsions
V. Stow Emvrsions . = ESS,
Sensitive Material for Copying, Commercial
Work and Transparencies — Methods of Pro-
ducing High Resolution and Fine Gre vi
Vi.
vit
VL.
XI.
TABLE OF CONTENTS
Chloro-bromide Emulsions — Chloride | Emul-
sions — Mixed-jet_ Emulsificati
‘ment of Warm Tones
CoLOR-SENSITIVE EMULSIONS
Action of Color-sensitizers —
and Panchromatic Plates and Films — Self-fil-
tering Emulsions — Hyper-sensitizers — Bi
packs and Tripacks — Three-layer Color Films
— Dye-couplers and Color Formers
X-Ray AND ULTRAVIOLET .
X-ray Emulsions —Intensifying | Screens —
Lippmann Emulsions — Ultraviolet Plates
Coat EMULSIONS ON Gass - e
Preparation and Cleaning of the Glass — Sub-
stratums — Drying Cupboards and Drying
Problems — Coating Heads — Ventilation and
Heating
BRoMIDE AND CHLORIDE PAPERS
Nature of the Raw Paper—Paper Tests —
Baryta Coating — Emulsion Formulas —Labo-
ratory Methods of Coating and Drying — Com-
mercial Coating Machines — Drying Tunnels —
Non-stress Coating —Drying, Reeling and
Packing — Variable Contrast Papers
. Fitms, Neoarive anp Positive
Types of Base — Substratums — Substratum-
ing Machines — Negative Film Emulsions —
Positive Film Emulsions — Experimental Film
Coatings on Roll and Cut Film — Film Coating
Plant —the Drying ‘Tunnel — Air-condition-
ing — Films for Imbibition Emulsions — Static
—Commercial Defects — Packing
PrinTiNo-our EMULSIONS «
Salted Paper — Sensitizing Silk — Printing-out,
103
128
175
192 TABLE OF CONTENTS
Emulsions — Gelatino-chloride Papers — Collo-
dio-chloride Papers — Self-toning Papers — Sil-
ver Phosphate Papers
XII. Testrnc EMuisionep Propucts
Testing Equipment — Estimation of Speed and
Quality — Interpretation of the Characteristic
Curve — Measurement of Color-sensitivity —
Photometers and Density Meters — Keeping
Tests
XII, Various Merartic Processes
Carbon and Carbro Tissue — Gum-bichromate
—Iron Printing Processes — Ferroprussiate —
Cyanotype — Kallitype — Platinotype and Pal-
ladiotype —Diazotype Papers — Bleach-out
Color Processes
XIV, Extreme-speeD EMULSIONS .
Super- and Hyper-sensitizers — Sulphur Com-
pounds in Gelatin —Anti-fogging Agents —
Speed Characteristics
Es a so ales
202
236
253
259 INTRODUCTION,
SHE pioneer days of photography were noteworthy for the
fact that the sensitive material which recorded the camera
image was in a state of continual evolution. ‘The dramatic
change from collodion to gelatin emulsions due to Dr. Maddox
in 1871, introduced a new era in the art of photography because
of the mysterious increase in speed which the use of gelatin
brought about. Although so many years elapsed before the na~
ture of the gelatin sensitization was discovered, empirical work
with gelatin emulsions brought about a constantly increasing
sensitiveness to light, which culminated in the early part of this
century in a speed of about goo H and D, or its equivalent of
1o Weston. Twenty years later the speed of photographic
plates jumped up to half as much again, and after another
twenty years modern color-sensitive emulsions with their super-
sensitizers made another remarkable jump to something between
five and ten times the old figure.
In the meantime, the application of photography to technical,
industrial and scientific problems has expanded on a truly amaz~
ing scale which needs no emphasis here. Physical chemists,
physicists and mathematicians have been attracted by the fas-
inating nature of the phenomena associated with sensitive
emulsions into a new field, with which this book attempts in
some part to deal.
During an experience of more than thirty-five years in the
emulsion-making field, the author has been approached on nu-
merous occasions with requests for sources of information on
photographic emulsions, In many branches of experimental
work and scientific research, it is felt that it would be of un- x INTRODUCTION
be made on a laboratory scale, and suitably coated. As time
proceeds, many newcomers enter the field of commercial pro-
duction of one kind or another, and in the following pages the
problems of the investigator and the manufacturer are fully
discussed. The investigator is obliged to buy in the open mar-
ket the available material which happens to possess character-
istics nearest to his requirements. The factory laboratory, on
the other hand, makes a considered choice of materials and
checks up on that choice with manufacturing scale tests. Apart
from actual manufacture, the making of emulsions provides a
scope for experiment pregnant with possibilities. Some knowl-
edge of emulsion chemistry is of the greatest value in the general
study of photographic processes.
In the pioneer days, photographers had to make their own
‘emulsions, and it was by the combined efforts of a small coterie
of experimenters at the end of last century that the modern “ dry
plate” came into being. Some of these men were pure ama-
teurs, others were practical chemists and men of scientific train-
ing. Out of this little band of experimenters came the founders
of most of the original commercial emulsion-coating, factories.
‘The inevitable then happened. ‘The businesses came into com-
petition with one another, and further advances gradually be-
came guarded as trade secrets. ‘There came a spectacular lull
in the hitherto prolific literature, and the textbooks of Dr.
J. M, Eder and Sir William (then Captain) W. de Wivesleigh
Abney were the chief fodder on which the would-be experi-
menter had to feed
‘The interést in emulsion chemistry was renewed in 1920 and
onwards. The classic work of The Svedberg and other dis-
tinguished physicists introduced new methods of attack on the
morphology of the silver halide grain, A little later, at a Paris
‘Meeting of the International Congress of Photography, Dr. S.E. INTRODUCTION xi
Sheppard announced his discovery of the effect of the presence
in gelatin of allyl isothiocyanate — the secret of speed had been
revealed. But the millennium had not arrived, for speed could
not be increased ad libitum by just increasing the proportion of
sensitizer used in the preparation of the emulsion,
‘The sensitiveness to light of the emulsion depends to quite an
important extent on the particular gelatin used, and its inher-
ent “impurities.” But it is also dependent on the method of
precipitation of the silver halides, on the method and degree
of ripening, and on the digestion which the emulsion receives
after the by-products and excess solvents of silver bromide have
been removed by washing. Modern technique, however, makes
it possible to control within fairly definite limits the character
of an emulsion, so that by predetermined conditions the manu-
facturer can produce a plate or film having a long scale of grada-
tion for the pictorial photographer, a process film suitable for
ine work, an emulsion specially responsive to X-rays, a soft-
working bromide paper for enlargements, or a contrasty chloride
paper for the photo-finisher. Add to this the power which
Vogel's discovery of color-sensitizing gave to the industry, and
we find a further field for experiment of vast potentialities,
which has helped of course in large measure to provide the
various solutions to the problem of natural-color photography,
and to improve the graphic arts.
While the amateur emulsion-maker cannot compete with the
manufacturer of sensitive materials, the latter cannot be ex-
pected to interrupt works routine by supplying small coatings
of experimental emulsions made to some specification outside.
his own range of products. This book is thus intended not only
to be a guide to practical emulsion making, but as a textbook
for technical students, industrial chemists and photographers
generally, who are anxious for their own reasons to prepare
‘emulsions of various types and speed. With careful work, ex- xii INTRODUCTION
cellent results of repeatable quality can be made on a small
laboratory scale, and the apparatus involved need be neither
elaborate nor costly.
‘The great majority of formulas given in the following pages
have been quoted from published communications, but it has
been the special aim of the author to set out the fundamental
lines upon which such formulas are based, so that the emulsion-
maker can construct his own for any desired purpose. New
needs for special characteristics crop up with great frequency,
and there is definite scope for the manufacturer of sensitized
products who is able and willing to meet these new demands,
which may not necessarily run into immense production figures.
‘The manufacturer is helped today very largely by modern air-
conditioning equipment, improved means for temperature con-
trol, excellent emulsion-making plant and coating machinery,
and above all by the splendid researches in emulsion chemistry
which have been carried out in recent years. With the further
help of modern sensitometric work, special packing materials,
checking by trained technicians, and above all the ever-growing
intelligence of the photographer himself, the sensitive materials
industry has reached a high state of efficiency, and its products
have assumed the character of precision instruments.
Grateful acknowledgments are given to the Research Labora-
tories of Eastman Kodak Company, and Kodak, Ltd., for photo-
graphs kindly supplied; also to Messrs. T, H. Dixon and Co.,
Ltd., and Messrs. W. Watson and Sons, Ltd., for illustrations of
plant and apparatus; to Burt H. Carroll and Donald Hubbard,
whose research papers have been frequently quoted; and to Mr,
Charles A. Silver for his assistance with the proofreading, Cuapter IT
NATURE OF PHOTOGRAPHIC EMULSIONS
Introductory —Light-sensitive Silver Compounds —The Nature of an
Emulsion — The Function of Ammonia — Sensitizing Properties of Gela-
tin—Ostwald Ripening —The Crystal Structure of Silver Bromide —
Todide in Emulsions—Grain Size and Emulsion Characteristies—The
Effect of Light on Silver Bromide
‘HE image formed by the lens of a camera is recorded on
a light-responsive surface which in present-day photog-
raphy takes the form of sensitive silver salts in a film of gelatin.
No visible action takes place in this film during a normal ex-
posure. But it is capable of providing ultimately a visible and
permanent image, obtained in practice by developing the in-
visible or Jatent image formed by the action of the light-rays
in exposure. Daguerre showed that by exposing in the camera
a plate made of polished silver treated with iodine vapor, an
invisible latent image was formed which could be rendered vis-
ible by development with mercury vapor. ‘The mercury de-
posits itself on the parts of the surface exposed to light, but
does not adhere to the unexposed parts. The first reference to
photographic sensitometry was probably made by Arago in his,
statement that “to the portions which represent the halftones
the mercury affixes itself in greater or less quantity proportional
to the action of the light upon these parts.”
‘The thin film of silver iodide formed by the action of iodine
vapor on silver, according to the equation
Ag+ 1= Ash,
was in no sense an emulsion, but merely a stain similar to the
discoloration of a brightly polished silver article exposed to
the sulphurous atmosphere of a city. ‘The approach was nev- 2 PHOTOGRAPHIC EMULSION TECHNIQUE,
ertheless being made towards the emulsion, but it was years
later when Niépce de St. Victor coated glass plates with a mix-
ture of albumen and starch in which were suspended the sensi-
tive salts. Three years later came the wet-collodion process.
In this, a solution of guncotton dissolved in ether and alcohol
and containing soluble bromide and iodide was poured over a
glass plate and sensitized by flowing a solution of silver nitrate
over the set, but still damp, collodion film. ‘The plate was ex-
posed immediately in the camera, so that the soluble excess salts
should not crystallize out.
In an effort to cut out the sensitizing process (still largely
used today), the first actual emulsion was produced. This was
a suspension of silver bromide or iodide in collodion, made by
Bolton and Sayce. ‘Then came the use of gelatin by Dr. Mad-
dox, and today we find that the great bulk of sensitive photo-
graphic materials consist of gelatin “ emulsions,” or suspensions
of silver salts in a solution of the protective colloid gelatin, ap-
plied to glass, celluloid or cellulose acetate film, paper, and more
recently to thin aluminium alloy sheets.
It will thus be gathered that a modern photographic emulsion
consists of a suspension of silver halides in gelatin, which is
applied to a suitable support. It should be noted that emulsions
have been made by Staud and Connelly by dispersing the silver
halide in a mixture of gelatin and a water-soluble cellulose ester
such as cellulose acetate or lactate,’ while Sheppard and Houck
have used the potassium salt of cellulose acetate diphthalate.*
Agar-agar has been employed with little success. In spite of
wide research, no compound or substance has yet been dis-
covered which is anything like as sensitive to light as bromide
of silver, yet extreme sensitiveness can be obtained only when
it is subjected to the influence of gelatin, and if chemical fog
is to be avoided on development, a small proportion of the
iodide of silver must accompany the bromide. ¢
NATURE OF PHOTOGRAPHIC EMULSIONS — 3
‘The salts are formed by double decompo:
ample:
ion, as for ex-
eee gt KBr = AgBro +
silver potassium silver
nitrate +t bromide "bromide
While from the chemical equivalents it would seem that 170
parts by weight of silver nitrate would react with 119 parts of
potassium bromide, it is necessary in practice to have present
‘an excess of soluble bromide, for reasons which will be discussed
later. If an excess of silver nitrate were present, then the emul-
sion would print out on sufficient exposure to daylight, giving
a visible image of metallic silver; for this purpose, however, the
chloride of silver in combination with an organic silver salt is
commonly used. Silver chloride without excess of silver nitrate
gives, on the other hand, a comparatively insensitive emulsion
of the type used in making chloride or “ gaslight ” papers and
slow transparency plates, which yield on alkaline development
black and white images having great contrast and brillianc
Thus the three halides, silver bromide, chloride, and iodide,
suspended in gelatin or other suitable colloid medium, provide
us with the light-sensitive emulsions of which this book will
largely treat, Trivelli and Sheppard * describe a series of mer-
uric iodide emulsions, their sensitiveness and density-giving
ower being “ much inferior to silver bromide.” For printing
Processes on paper, certain salts of iron, copper, thallium, etc.,
can be used, as also can certain diazo compounds. The out-
standing feature of a silver bromide emulsion containing iodide,
however, is the fact that by the use of ammonia and the ap-
plication of heat under controlled conditions, a remarkably ex-
tensive range of speed, density-giving power, gradation, and
contrast can be obtained, which is repeatable with considerable
exactness,
Reduced to its simplest form, the making of a sensitive emul- 4 PHOTOGRAPHIC EMULSION TECHNIQUE
sion has a good deal of resemblance to cooking. Solutions of
the reacting salts are weighed out and mixed, one of them con-
taining a little gelatin which acts as a protective colloid and
as a means of preventing sedimentation of the halide precipitate.
Any amateur cook knows the difficulty of making a good
mayonnaise or hollandaise sauce. Some of the tricks of the
emulsion-maker are comparable to those of the adept culinary
artist, and the sensitometric characteristics of the finished emul-
sion largely depend on the exact method of mixing and cooking.
‘The mixed’ emulsion is kept warm for a certain time, during
which it is said to “ripen,” more gelatin is then dissolved in it,
and it is cooled until gelled.
Reference to the equation of the chemical reaction given
above will show that soluble nitrate (KNO, in this case) is
formed along with the silver bromide, Any such nitrates would
crystallize out on drying, if allowed to remain in the emulsion
when coated on glass or film base, and such by-products must
be removed. To effect this, the emulsion is broken up in the
jelly stage into small pieces, known as shreds, worms, or noodles,
and these are suspended in water until the soluble by-products,
together with any excess of ammonia and bromide used in the
making, are washed out by diffusion. ‘The washed gel is then
dissolved by heat, fresh gelatin is usually added, and the emul-
sion is again cooked, this time at a higher temperature which
is more or less critical, as is also the precise time of the cooking,
digestion, or “ finishing,”
Tt is the variations in the proportions of ingredients, the ex-
act method of precipitation, the time allowed for ripening, the
time of cooking after washing and the temperature employed,
and the character of the gelatin, which influence the final char-
acteristics and give us the immense range of modern emulsions
‘These vary from the very slow silver chloride paper emulsions
used for handling in Mazda light; to the exceedingly fast films NATURE OF PHOTOGRAPHIC EMULSIONS — 5
that record images in the millionth part of a second. A rough
idea of the relative sensitiveness of various types of emulsions
is given by Clerc * as follows —
Ultra-rapid negative emulsions ........... 75,000 to 100,000 *
Positive emulsions, black tones 1000 to 3,000
Geclatino-bromide papers, warm tones 300 to 1000
Gelatino-chlorobromide papers .. +. Too to 200
‘Transparency plates .....- ote rto 95
Gelatino-chloride papers . Zio eos
Silver bromide emulsions made with gelatin are very much
higher in sensitiveness than those made with collodion. The
digestion of the silver bromide grains or crystals with the gela-
tin is obviously the prime cause of this difference. Clerc tells
us® that, following from an observation by R. F. Punnett in
1924, S. E. Sheppard found that the differences in sensitive
properties are due to the presence in gelatin in varying propor-
tions (from 1 in 200,000 to 1 in 1,000,000) of sensitizers, among
them being thiosinamin (allyl thiourea), and mustard oil (allyl
isothiocyanate). The presence of these sensitizers, occurring
naturally in gelatins, and in quantities varying with different
makes or different batches of any one brand of gelatin, explains
the great variation in quality of any given emulsion made to
one formula with different samples of gelatin —a behavior
which greatly perplexed the early emulsion makers and for a
long time made impossible any assurance of uniformity in their
products. Experiments made with gelatins rendered chemically
inert, to which have been added known sensitizers, such as allyl
thiocarbamide, sodium. thiosulphate, etc., have given results
qualitatively very similar to those made with untreated ‘ ac-
tive” gelatin, though the characteristics of active gelatin can
not be explained by allyl thiocarbamide alone.’ A great deal
of research has been conducted in recent years on the chemistry
* This figure must be increased today as the result of recent advances. 6 PHOTOGRAPHIC EMULSION TECHNIQUE
of these sensitizers, and any real advance on present speeds will
be due to new discoveries in this field rather than to modifica~
tions in ripening, digestion, etc. Elaborate efforts have been
made, also, to produce some standard type of gelatin, with
Tittle success. As gelatin is an amphoteric colloid, capable of
combining with either anions or cations depending on the hy-
drogen ion concentration of the solvent medium, a good deal of
work has been done, by Sheppard and others, with emulsions
made with iso-electric gelatin, the pH of w1 4.7. The
treatment to which gelatins are subjected by certain manufac
turers is kept secret, but as a general practice the choice of a
batch from a number of samples by actual experiment is the
accepted guide of its suitability for a particular purpose.
‘As an example of the versatile nature of emulsion grain sen-
sitizing, the suggestion may be quoted of a mononuclear or poly-
nuclear heterocyclic compound containing a hydrogen atom
linked to nitrogen but replaceable by silver. The pyrimidenes,
pyrazols, and purpurines are quoted as examples.
The great majority of speedy emulsions are made with am-
monia, which assists in the ripening or crystal growth of the
precipitated grains of silver halide and their subsequent sen-
sitivity. In another type of emulsion, however, the suspension
in gelatin is “ boiled,” which means actually that it is heated
at a temperature somewhere between 160° F. (70° C.) and a
few degrees below boiling point. Such an emulsion is an inter-
esting one to consider at this point, because the absence of am-
monia enables us to watch the process with less chemical com-
plications, Trivelli and Smith? outline such a formula. ‘Two
solutions are prepared as follows:
165 g Silver nitrate 200
sg Witeraee 2000
Gelatin . 658 ‘Temperature, 72° C.
Water 1700 ce
‘Temperature, 70° C. NATURE OF PHOTOGRAPHIC EMULSIONS — 7
In a series of mixings, the silver nitrate solution was added by
pouring it through different sized nozzles into the salts solution,
the times of precipitation varying from 31 seconds to 85 min-
Lutes, ro seconds, ‘The emulsion was ripened for twenty minutes
at 70°C., then cooled quickly to 45° C., when 250 grams of
gelatin previously washed in water was added and the whole
stirred for twenty minutes at 45°C, After standing overnight
in a cold storage room, it was washed and remelted to 42° C. and
made up to a weight of 6.3 kilograms by the addition of 100
grams of fresh gelatin soaked in the requisite amount of water.
‘The final pH was 6.54.
‘The relation between the time of precipitation and the aver-
‘age grain size is interesting, and is shown in the table below:
‘on | Time of precipi- |, ing | Total surface of
Bmadsion | Time Beeb | No, of grains | Foie ase oe
umber |) tin, See. | 10" PO" | Cubic centimeter
8 a 685,
9 4 2.09)
10 Roca | obs
1 19 30 | OB
2 2 on7
13 pecs | 0.09)
4 85 10 0.04
Figure 1 shows the relation between grain size and time taken
for precipitation, reproduced from the paper quoted. It was
found by Southworth that there is a quantitative relation be-
tween the H and D speed (p. 208) and y (maximum contrast.
on complete development), and this was found by TTrivelli and
Smith to hold good for a number of emulsion series with dif
ferent times of development.
Let us now examine the making of the emulsion after the
initial precipitation of the silver bromide, usually termed the
emulsification, As already stated, a great proportion of speedy. 8 PHOTOGRAPHIC EMULSION TECHNIQUE,
RELATIVE GRAIN SIZE
TIME OF PRECIPITATION
Fic. 1
emulsions are made with ammonia, In many cases the whole
of the silver nitrate in the formula is treated with sufficient
concentrated ammonia to redlissolve the precipitate of silver
hydroxide first formed. The temperature of mixing is then
usually much lower. ‘The AgBr precipitate formed in emul-
sification appears as minute shapeless grains under the micro-
scope, but as ripening proceeds they grow in size and a definite
crystalline shape is recognized. The crystals continue to in-
crease in size, as time proceeds, owing to the presence of the
silver bromide solvents —ammonia and the excess of soluble
halide.
ilver bromide, according to von Weimarn’s theory, would
ive a precipitate on mixing the reacting substances, the “ grain
size ” of which would be controlled by the concentration of the
reactants. The fact that the precipitation takes place in the
presence of a colloid, however, must be taken into considera~
tion, in addition to the excess of AgBr solvents. To obtain
uniformity of grain, the emulsion is stirred during precipitation,
this tending to reduce the size of the crystals. In some cases
of ripening, the crystals are allowed to grow without agitation,
especially in those cases where a mixed selection of grain sizes
is needed to give photographic latitude, or a long scale of
adation ro PHOTOGRAPHIC
ULSION TECHNIQUE,
In the early stages after emulsification the system tends to
reduce its surface energy through any colloidal silver bromide
becoming transformed into crystalling AgBr. By the so-called
Ostwald ripening, the larger crystals grow at the expense of
the smaller ones, and the longer the ripening proceeds, the
bigger do the crystals or grains become. A certain amount of
aggregation eventually takes place, unless stirring is given. A
drop of emulsion can be taken on a glass rod, smeared over a
clean glass slide, and thinned out with a few drops of hot water.
‘The slide is then sharply shaken free of excess fluid, leaving
only a single layer of grains adhering to the surface. ‘The spe
‘men, as soon as dry, can be examined with a twelfth-inch oil-
immersion objective, using the concave mirror and small con-
denser aperture. It will usually be observed that a wide variety
of grain sizes is present, appearing as hexagonal plates, tri-
angles, needles and unresolvable particles (see Figs. 2 and 3).
All of these are actually octahedra. Dr. Sheppard states that
in 122 different emulsions examined at a magnification of 2,500
diameters, only octahedra could be positively identified. ‘The
equilateral hexagons which so soon become familiar to anyone
examining emulsion grains with the microscope are plates de-
veloped in two directions, while needles appear which are de-
veloped principally in one direction — all, however, octahedra.
If a few drops of the ripening emulsion be smeared over a
strip of glass and looked at by transmitted Mazda light, the
color after emulsification will be orange. As ripening proceeds,
this turns to yellow, then greenish-blue and then to blue, at
which stage the maximum useful ripening will usually have
been reached. If ripening be allowed to proceed further, the
transmitted light may appear bluish-violet or gray; at this stage
the emulsion will have overgone the mark and will probably
show fog on development without having gained any advantage
in speed. This method of watching the progress of ripening 12 PHOTOGRAPHIC EMULSION TECHNIQUE
was much favored by the old school of emulsion chemists, a
trained eye being able to detect with considerable accuracy
when the optimum stage had been reached. As the time can
vary from twenty minutes or so to three or four hours with
an unfortunate change in gelatins, it is a useful check in the
factory when new batches come into use, even though they may
have been passed as O.K. in the laboratory.
So far we have said little about the iodide in an emulsion,
Reference to any formula for a fast emulsion will reveal that
small amount of silver iodide is used. Whether this is formed
at the time of emulsification or at a later stage, the iodide ap-
pears to take a definite part in the molecular structure of the
bromide, Iodine has a greater chemical affinity than bromine
for silver, and at whatever stage it is introduced, it will replace
it. ‘The term iodo-bromide is frequently used. X-ray crystal
measurements of the lattice spacings indicate a complete homo-
geneity throughout the crystal, but the presence of the iodide
ions changes the spacing, making the distance between them
larger than in the pure state, This would indicate, according to
Friedman,’ that a crystal which contains iodide as well as
bromide ions exists in a strained condition, the degree of strain
being dependent on the concentration of the iodide ions within
the crystal. To a slight extent this corresponds to the sen-
sitivity of the resultant grain, for it has been determined that
the maximum sensitivity is attained when the concentration of
the iodide is approximately four mol per cent of the total halide
content. Beyond this concentration the sensitivity falls off
again.
‘The range of diameters of the crystals tells a good deal about
the character of the emulsion that is to be. A wide range in-
dicates good gradation and latitude, while if the crystals are
all of similar size, a high gamma with early reversal and lack
of latitude ordinarily results, except in cases of very slow emul- NA
URE OF PHOTOGRAPHIC EMULSIONS 13
sions which have had special treatment, While the grain
growth in ripening is almost invariably accompanied by an in-
crease in speed of the finished emulsion, the final sensitivity
is determined more by the conditions under which emulsifica-
tion took place than by the ripening itseli, provided always that
the final cooking or finishing is carried to completion, The
early literature on emulsion making was concerned chiefly with
the endeavor to get all the speed before washing. It is now
an open secret, since the paper of Carroll and Hubbard was
published in the Bureau of Standards Journal of Research in
1931, that a great deal of the final sensitivity is obtained in
commercial practice by the cooking given to the washed emul-
sion, Trivelli and Smith disclose this fact still further in Com-
munication No. 704 from the Eastman Kodak Laboratories,
‘The nature of the gelatin undoubtedly plays a very impor-
tant part, as well as do the concentration of the reacting solu-
tions and the method of pouring and stirring. As an example
of the extraordinary effectiveness of emulsifying conditions, one
may take a formula, built on lines originally published by J. M.
Eder, where the silver nitrate is merely wet with water, redis-
solved with ammonia, and in this very concentrated condition
is literally “ flopped” wholesale on to the salts solution in the
jar. This, of course, gives an instantaneous precipitation, the
grains being of a very uniform size, No ripening time what-
ever is allowed, the bulk gelatin being added immediately and
the emulsion set in ice water as soon as it is dissolved. Such
an emulsion yields a film of very high gamma but of poor lati-
tude, specially suited to copying black and white work. It will
thus be gathered that the emulsion chemist must find out for
any emulsion formula the optimum time of ripening and the nec-
essary physical conditions to obtain the results he wants, as
also the exact method of emulsification, in addition to selecting
the most suitable brands of gelatin, 14 PHOTOGRAPHIC EMULSION TECHNIQUE
Let us assume now that an emulsion has been ripened and that
sufficient extra gelatin has been added to give it the necessary
viscosity for coating —say a total of eight or nine per cent of
the volume, This viscosity is of course arranged to suit the
type of plate, film or paper, and the particular coating ma-
chine and climatic conditions. The soluble by-products to-
gether with any excess of ammonia and the excess alkali halides,
are next removed by washing. While this is ordinarily done in
the manner described in detail in Chapter III, other methods
have been suggested, such as pouring the emulsion in a fine
stream into a large bulk of alcohol, thereby coagulating or pre-
cipitating the gelatino-bromide, the water with its soluble salts
being taken up by the spirit, Many variations of the spirit
washing method were described in the early literature, espe-
cially in the British Journal of Photography, towards the end of
the last century,
‘The washed emulsion, then, is a dispersion of silver halide
grains in more or less pure gelatin, ‘The gelatin nevertheless
contains sufficient sensitizers —or they have been already ad-
sorbed to the silver bromide crystals — to insure that when the
emulsion is remelted and heated the maximum sensitivity is
obtained. ‘That some sensitization has been effected within the
grain in the case of mixed-grained emulsions has been shown by
the fact that while after exposure the smaller grains can be
rendered undevelopable (the latent image destroyed) by treat-
ment with chromic acid, some of the larger grains still remain
developable. ‘The final cooking, when the speed of the washed
emulsion can be increased many hundreds or even thousands of
times in the course of thirty minutes or so, is known as digestion
or finishing, and is fully dealt with in Chapter IV.
Owing to the lack of emulsion-making literature, particular
interest attaches to the paper by Trivelli and Smith ‘" in which
are described experiments to determine the influence of grain NATURE OF PHOTOGRAPHIC EMULSIONS — 15
size on the final digestion, ‘These experiments indicated two
things; (a) the H and D speed of the smaller-grain emulsions
increased to a greater extent than that of the large-grain emul-
sions; (b) the gamma of the smaller-geain emulsions increased
less than that of the larger-grain emulsions. As an example
of (2), the figures below are quoted from the paper of these
authors:
Avene gain saci | ons | om | 295
Time of finishing at
Tee Increase in speed
10 34 | 38 | 08
20 46 | 30 | 0
3° s2 | 44 | 09
2 so | 47 | 10
Average grain size in
Time of finishing at
60° C. in minutes
[ex | om | 208
Increase in gamma
10 M4 | 16 | 25
Admitting that grain size need not be the deciding factor in
the speed of an emulsion, it has been established that within
experimental limits there is a direct proportionality between
the average grain size and the H and D speed in certain emul-
sion series. On the other hand, as the gamma of very fine-
‘grained emulsions does not increase as much on final digestion as
it does in the case of coarser-grained ones, it is usual practice to
secure the high contrast (where desired) of a slow fine-grained
emulsion in the mixing and ripening rather than to trust to the
digestion after washing. Speed being due largely to the sen- 16 PHOTOGRAPHIC EMULSION TECHNIQUE
sitizing of the silver halide by the gelatin, it is obviously more
difficult for the gelatin to function in the case of fine grains,
which have a very large surface as compared to the coarse
grains, Asan example in the increase in surface area of a grain
with decrease in diameter, the illustration may be given of a
cube of 1 cm side, which has a free surface of 6 sq.cm. If di
vided up into a corresponding number of small cubes each of
o.or thousandth of a millimeter side, the free surface of the
6 sq. cm would be expanded to an aggregate of 60,000,000
sq.m.
Reference may be made here to the effect of very small
quantities of sodium sulphite in emulsions. If this substance
be introduced in quantity sufficient to reduce a few tenths of
one per cent of the silver bromide present, and the emulsion
be then digested, it can act as a powerful sensitizer, approach-
ing in large measure the effect of the natural sensitizers in gel-
atin, As stated by Carroll and Hubbard," the sensitivity nu-
clei formed are of metallic silver, in amounts similar to that
of the silver sulphide nuclei of normal emulsions. ‘The rate of
change of sensitivity increases with increasing alkalinity, and
decreases with increasing bromide-ion concentration. ‘The rate
of after-ripening with sulphite is less affected by hydrogen ion
concentration than the corresponding process in active gelatin,
‘The authors state that while many of their emulsions sensi-
tized with sulphite were of “commercial quality,” none were
equal to the best obtainable by standard methods from the
same type formulas,
After the emulsion has been coated on its support, a certain
amount of after-ripening or ageing can go on, especially if the
ripening after emulsification has not been carried to, or near
to, an optimum point, or if the finishing was not carried to fi-
nality. This gradual rise in speed or gamma in a coated plate
on keeping is not to be confounded with the after-ripening NATURE OF PHOTOGRAPHIC EMULSIONS — 17
which Carroll and Hubbard define as “ the increase in sensi-
tivity of photographic emulsions after washing,” which we have
seen depends on the effect of heat treatment upon the initially
ripened grains.
‘The ageing of plates and films may be regarded as a con-
tinuation of digestion in the dry state and at storage tempera-
ture, but for our purpose it should be regarded more as deterio-
ration. A coated product, after keeping for a short time to get
into complete equilibrium and for the hardening agent to com-
plete its work, should maintain the state in which it is put out
for two to three years, and much longer in the case of slow
materials. On the other hand, there may be a tendency in the
case of films for the gradual lowering of pH, due to acid libera-
tion, when speed will be lost. This is a matter to which due
attention must be paid in commercial film manufacture. As
an instance of the excellent keeping of glass plates, the case
may be quoted of star drift measurements at Oxford Observa-
tory, England, where special rapid plates were exposed in an
astronomical camera on a group of stars, and without develop-
ment were stored for fourteen years, and then given a second
exposure at the completion of the orbit. ‘The displacement be-
tween the two images, one having been latent for fourteen years
before development, was the measure of the drift.
When light is absorbed by a grain of silver iodo-bromide, the
silver halide molecule becomes dissociated, and atomic silver
and free halogen are formed, the halogen being absorbed by
the acceptors in the surrounding gelatin, ‘The new system can
be described as a crystal of silver iodo-bromide upon which
atomic silver is adsorbed. Such a system is easily reduced to
metallic silver by the action of the developer, as originally
established by Carey Lea, That the reduction starts from the
sensitive nuclei was shown in 1922 by the brilliant photo-
micrographic researches of The Svedberg. 18 PHOTOGRAPHIC EMULSION TECHNIQUE
‘The metallic silver grains formed from the exposed silver
halide by reduction during development have been regarded as
tiny coke-like masses, which link up to form aggregates or
larger “ grains,” but it has been shown by recent work with the
electron microscope that the grain formation is actually ribbon-
like in character. Photomicrographic work done in the ordi-
nary way up to magnifications of 2000 and upwards (diam-
Fic. 4
eters) has never revealed, nor indicated, such a formation,
But with the immensely greater magnification of X 50,000, pos-
* sible with the electron microscope, definite results have been se~
cured which may necessitate reconsideration of some of our
present ideas. ‘The image grain has now been shown (Fig. 4) to
consist of a ribbon of silver crumpled up like a string, which
‘would appear to occupy a part only of the total area of what we
have regarded as the grain and to possess light transmitting
properties rather than being completely opaque. It is more than
likely that subsequent studies along these lines will show the
structure to vary as regards its nonopacity with different condi- NATURE OF PHOTOGRAPHIC EMULSIONS — 19
tions of emulsification, and thereby be responsible to some ex-
tent for characteristic variations.
It is beyond the scope of this book to deal with the subject
of the latent image and the theory of development, but it
should be emphasized that one reason why sensitive silver emul-
sions have made modern photography possible and of such im-
mense value to scientific research, is that over a long range of
light intensities, depending on the individual emulsion, equal
‘increases in exposure result in equal increases in opacity of the
developed image, or equal increments of “ developability.”
Equal effective exposures produce equal densities under con-
trolled development (Bunsen and Roscoe reciprocity law), ex-
cept for a correction indicated by Schwarzschild which makes
it necessary to substitute for the exposure I< T (intensity
multiplied by time) I x T*, p being the Schwarzschild factor.
‘An example is given by Strong,* showing to what extent the
Bunsen and Roscoe law fails in the case of motion picture posi-
tive film, For a range of illumination intensities from 1 to
33,000, p varies from 0,68 to 1,00, the maximum intensity being
131 lumens per square meter and the exposure time varying
between 18.2 hours and 2.5 10 second.
‘The covering power and maximum density of an emulsion
depend to some extent upon the size of the reduced silver grains,
their aggregation, and their number per unit area, the ratio of
gelatin to silver having some effect where density measure-
ments are made by specular light. Clerc states that an opti-
cal density of 1 (transmission ten per cent) on a photographic
negative, corresponds to a mass of silver of about ten milli-
grams per square decimeter, a mass which is variable with the
grain size, conditions of development, and the wavelength of
the radiations used, Trivelli and Smith showed " that the de-
crease in gamma of an emulsion is proportional to the square
root of the total number of grains. ‘They state that this de- 20 PHOTOGRAPHIC EMULSION TECHNIQUE
crease is considerably greater than the changes observed in the
resolving power by diluting the emulsion, For recording pur-
poses, a good black which will give sufficient differentiation in
printing or on projection must be combined with high resolu-
tion, and the latter must therefore depend largely on suitable
emulsion characteristics rather than on thin coating, In fine-
gtain emulsions, however, Trivelli and Smith find that the re-
solving power increases exponentially as the grain size is
minished arithmetically.
Much of the foregoing has applied to “ rapid ” emulsions of
silver iodo-bromide having H and D speeds ranging from 25
to 2000 or more. In materials for printing, lantern-slide emul-
sions, and copying, speed is of litle consequence. Where extra-
fine grain, high resolution and negligible fog are the chief de-
siderata, physical treatment rather than chemical is involved,
and in chapters dealing with such emulsions it will be seen that.
the general technique of making is considerably modified.
Chapter References
- Staud and Connelly, USP. 2, 127, 621, 1936.
Sheppard and Houck, US.P. 2, 127, 573, 1936.
Trivelli and Sheppard, The Silver Grain of Photographic Emulsions,
. L. P. Clerc, Photography, Theory and Practice, edition 1937, p. 358
= Idem. p. 132.
Carroll and Hubbard, Bur. of Stands. Journ. of Research, 7, Aus
1951
‘Trivelli and Smith, Communien. 699, Kodak Research Labs,
|. Southworth, Brit. J. Phot., 84, 611, 1937.
J.S. Friedman, Amer. Phot.y 34) 45292.
10. Trivelli and Smith, Phot. Journ, 70, 609, 1939.
11, Carroll and Hubbard, Bur. of Stands. Journ, oj Research, 11, Dee.
1933.
12, J. Strong, Procedures in Experimental Physics,
13. L. P. Clere, Photography, Theory and Practice, p. 139,
14, Trivelli and Smith, fo. cit. Cuapter IT
MATERIALS FOR EMULSION MAKING
Gelatin —Chemicals and Their Choice — Analyses and Tests—Storage
— Methods of Handling Bulk Materials
ELATIN is the material which presents the most prob-
lems to the emulsion chemist. Its physical and chemical
properties can be tested in the laboratory, but the physical char-
acteristics are apt to change according to the treatment it under-
goes, and the chemical properties from an emulsion-making
standpoint depend to an important extent on certain constitu-
ents present only in extremely minute quantities. Bogue
states * that the gelatin solution used in an emulsion must be
so made that the dried film will have just the right porosity to
electrolytes, for in all stages of development where chemicals
are used it is necessary that they penetrate and impregnate the
gelatin layer with considerable ease, but no trace of the pre-
ipitated silver must be permitted to escape. Gelatin acts both
as a protective colloid and a vehicle for the silver haloids, and
it is in fact quite astonishing what a mass of silver haloid can
be suspended in a very weak solution of gelatin with compara-
tively slight sedimentation,
‘The best gelatin for emulsion-making purposes is probably
that extracted from calves’ hides, but other hides and other
parts of the animal are used, and bones also, Hides having a
high fat content are not suitable for emulsion gelatin, accord-
ing to Huzii,? as it causes spots on the plates. If the tempera
ture in extracting the gelatin is too high, the product will have
high light sensitivity and fog-giving properties; increase in the
number of extractions has similar effects. 22 PHOTOGRAPHIC EMULSION TECHNIQUE
‘The preparation of the gelatin involves a number of opera-
tions, ‘The raw materials are steeped in lime water, after hav-
ing been thoroughly washed free of dirt and blood. The liming
process dissolves out the albuminous and mucinous constitu-
ents, and, as it is a process of alkaline hydrolysis, caustic alkali
is sometimes employed, ‘The alkali is later neutralized and a
process of digestion or boiling given, followed by concentration
of the liquor, bleaching, washing, etc. ‘The final jelly is cut
into sheets and dried on strings or wire nets, and here a certain
amount of bacterial contamination can be contracted. Leaf
gelatin is sold in the form of thin sheets, of which in the photo-
graphic quality 100 to 110 go to the pound. It may be cut into
thicker sheets which are ground to powder when dry, or turned
into flakes which are sometimes sold as such, The ground
gelatin when sieved gives a powder of about 50 per inch mesh.
Such powder is liable to contain traces of iron, Photographic
gelatin, which may be looked upon as glutin and a mixture of
glutin and chondromucoid, is the purest form made, being su-
perior in quality to culinary gelatin,
A good gelatin should give a clear and nearly neutral solu-
tion. The moisture content should not be more than twenty
per cent and lower in the case of “ soft” gelatins. There are
roughly three classes of photographic gelatins, hard, medium
and soft, which can be differentiated by their setting and melt-
ing points and the quantity of water they will absorb. ‘The ash
content of a good gelatin should not exceed two per cent. Bases
and heavy metals should not be present in more than traces;
these may be sodium, calcium, zine, iron, copper, arsenic, nickel.
Phosphates, sulphates, sulphites, chlorides, borates and sul-
phur dioxide may also be present,
It is interesting to note that as early as 1847 Nipce em-
ployed gelatin as a vehicle for coating silver iodide on glass,
though this was soon dropped owing to the effect of the acid MATERIALS FOR EMULSION MAKING 23.
nature of the silver bath. Eight years previously, Mungo Pon-
ton had rendered paper sensitive to light with potassium di-
chromate, and later Becquerel showed this sensitivity to be
due to the size in the paper. It was Dr. R. L, Maddox who,
in 1871, first employed gelatin as a vehicle for silver bromide
and so prepared the first emulsion, Its use gave an entirely
new measure of speed, but it at the same time introduced a
great many perplexing phenomena, some of which are still not
fully understood.
Dr. S. E. Sheppard quotes in his monograph on the theory
of photography, ‘ Gelatin,” a statement by V. B, Storr ® in the
Second Annual Report of the Society of Chemical Industry on
the progress of applied chemistry, as follows: “The physical
properties are a very insufficient guide to the suitability of
gelatin for making photographic emulsions. ‘There are certain
chemical differences between different types of gelatin and even
between different batches of the same type which are more ef-
fective in determining speed, freedom from fog, and such qual-
ities, . . . It is very possible, if not probable, that they (chem-
ical differences) are due to the presence or absence of very
small quantities of specific substances rather than to varia-
tions in the proportions of the main constituents of the gelatin.”
This statement tells a good deal of the story of gelatin, and
every emulsion chemist will add to it that the only satisfactory
way of testing a new sample is to make with it a semi-works-
size lot of the particular emulsion for which it is to be used.
It is a common occurrence, for example, for a sample which
has been turned down by one maker to be accepted as excellent
by another. Where results are to be repeated over long periods,
or where works-batches of emulsion are being made, it is sound
Practice to use a blend of two or three different lots or even
makes, taking care that only one of these is allowed to run out
at one time, so that when a new batch of gelatin is worked in, 24 PHOTOGRAPHIC EMULSION TECHNIQUE,
only a proportion of the total amount used for each making is
changed.
‘The chemical test of most importance in selecting gelatin is
for silver reduction. A one or two per cent solution of the
sample is made up with distilled water, and to a measured vol-
ume is added an equal volume of ten per cent silver nitrate solu-
tion, to which has been added just sufficient concentrated am-
monia to redissolve the precipitate of silver hydroxide first
formed. The ‘ure is well stirred and is then left in the dark
for a fixed time, along with a similar control test of a gelatin
of known good quality. Any darkening or formation of a pre-
cipitate will give a good indication of the quality of the sample.
A good gelatin will contain a relatively small proportion of non-
glues. According to Stelling,* the non-giue extracted from gel-
atin by alcoholic precipitation of the glue is 3.39 per cent, 5.73
per cent from hide glues, and r0 to 16 per cent from bone glues.
Arsenic has some effect on speed and fog production, and has
been found to occur in eight out of twelve samples,” in the pro-
portion of 0.0005 to 0.003 per cent. Copper, zinc, tin and lead
may be found by the usual methods. ‘The most important im-
purities are probably the sulphur compounds dealt with else-
where. The presence of sulphur dioxide is undesirable, espe-
cially if the sample is to be used for highly color-sensitive
emulsions or for the making of light filters. Where leaf gelatin
is to be used for the latter purpose, it can be washed in several
changes of water before use, the leaves being kept well sepa-
rated, when soluble impurities will be removed.
Viscosity can be measured in a number of ways. A one per
cent solution may be tested with an Engler or Ostwald visco-
simeter, at a temperature of 120° F, Running the solution
through a water-jacketed pipette and comparing the time taken
with that of plain water is another and cruder test, but one
which will give useful practical information. ‘Thermal hys- MATERIALS FOR EMULSION MAKING 25,
teresis or hydrolysis may upset physical tests, for it must be
remembered that previous treatment such as heating up rapidly
and cooling, gelling and remelting and so on, may alter the be-
havior of a gelatin solution, ‘The viscosity of most gelatins can
be very largely controlled in an emulsion by modification of the
concentration, by the addition of alcohol or acetone, or by the
addition of chrome alum, formalin or sodium sulphate.
For Stirrer
Ze
B
Fic. 5
‘The melting and setting points of a sample are of some im-
portance. Emulsion gelati should have a melting point some-
where between 70° and 78° F, (22° and 25°C.) and a setting
Point of 79° to 86°F, (26° to 30°C.). R. Child Bayley’s 26 PHOTOGRAPHIC EMULSION TECHNIQUE
method of estimating melting point was to cast small discs of
gelatin upon a flat metal surface, which is afterwards placed
in a vertical position and heated. At the temperature of the
melting point, the discs begin to slide downwards. The ap-
paratus takes the form shown in Fig. 5. This is a copper tank
having a plain front say twelve by eighteen inches in size, with
a smaller back about nine by twelve inches, the depth AF being
about ten inches. The bottom is inclined as shown in the dia-
gram, so that when heat is applied to BG none gets to the test-
ing surface ABCD. Two or three thermometers are fitted in
corks placed in the top ADEF, the tank being filled with water
and gradually warmed with an alcohol lamp or Bunsen burner
after the discs have been cast. The discs can be made by pour-
a
Fic. 6
ing the gelatin solution into a paper cylinder or mold set up on
the surface ABCD, which is placed first in the horizontal posi-
tion (see Fig. 6). ‘Tests should be made with solutions of some
uniform strength. E. J. Wall recommends ten per cent. The
gelatin solution may be introduced into the molds with a pi-
pette, each one being cast of the same height, about one-half
inch, The tank should then be left in the cold for eight hours
until the gel is thoroughly set and in equilibrium. ‘The tank is 26 PHOTOGRAPHIC EMULSION TECHNIQUE
method of estimating melting point was to cast small discs of
gelatin upon a flat metal surface, which is afterwards placed
in a vertical position and heated. At the temperature of the
melting point, the discs begin to slide downwards. The ap-
paratus takes the form shown in Fig. 5. This is a copper tank
having a plain front say twelve by eighteen inches in size, with
a smaller back about nine by twelve inches, the depth AF being
about ten inches. The bottom is inclined as shown in the dia-
gram, so that when heat is applied to BG none gets to the test-
ing surface ABCD. Two or three thermometers are fitted in
corks placed in the top ADEF, the tank being filled with water
and gradually warmed with an alcohol lamp or Bunsen burner
after the discs have been cast. The discs can be made by pour-
a
Fic. 6
ing the gelatin solution into a paper cylinder or mold set up on
the surface ABCD, which is placed first in the horizontal posi-
tion (see Fig. 6). ‘Tests should be made with solutions of some
uniform strength. E. J. Wall recommends ten per cent. The
gelatin solution may be introduced into the molds with a pi-
pette, each one being cast of the same height, about one-half
inch, The tank should then be left in the cold for eight hours
until the gel is thoroughly set and in equilibrium. ‘The tank is MATERIALS FOR EMULSION MAKING — 27
then set in the upright position, the paper cylinders cut round
with a sharp penkife and removed, and the tank filled with wa-
ter. A small motor stirrer is advisable to keep the whole front
surface uniformly heated. As soon as the melting point has been
reached, the little discs will begin to slide down the face of the
tank, and the average temperature of all of them will give the
melting point desired.
_ Another method, used a good deal by the author, is to intro-
duce the gelatin solution into four little glass tubes one and one-
quarter or one and one-half inches long, and about three-
sixteenths-inch bore. ‘These are set and kept cold, at about
45° F. for four hours, and are then tied around the bulb of
an accurately graduated thermometer, ‘The bulb and tubes are
then suspended in a beaker of thin petroleum oil of about 0.90
specific gravity, the beaker standing in an outer water bath.
A small mechanical stirrer keeps the oil in motion. The water
jacket is heated, and as the oil approaches the melting point
of the gelatin, the gelatin slips out of the glass tubes and drops
to the bottom of the beaker. An average reading of the four
tubes will then give a fairly reliable figure for the melting
point.
‘The setting point, which is on the average eight to ten degrees
Fahrenheit higher than the melting point, can be ascertained in
various ways. Sheppard * describes one method which is stated
to give results of the accuracy commonly required in emulsion
making, which is as follows: Equal amounts of the solutions to
be tested are placed in similar test tubes of one-inch diameter.
‘These are then cooled in ice water and are examined at definite
intervals, until the approach of solidification is indicated by the
fact that the solution is hardly disturbed when the tube is
tilted. When this stage is reached, the tubes are inverted at
frequent short intervals. When the meniscus of the inverted
sample no longer sags, a thermometer is thrust into the jelly 28 PHOTOGRAPHIC EMULS
ION: TECHNIQUE
and the stationary temperature is taken as the setting point.
‘The rate of setting is, of course, of considerable importance
when emulsions are to be coated on any kind of machine in the
factory. Generally speaking, it may be said that the rate of
setting is reasonably gauged by the viscosity, or the jelly
strength.’ While the rate of setting and the setting point can
be controlled to a large extent by additions of hardeners (alum,
formalin, etc.) or alternatively by softeners (glycerin, etc.), if
a gelatin can be selected which resembles previous stock with-
out alteration of the additions, it is far better. ‘The use of
chrome alum, for example, is cumulative, and plates or films
coated with an emulsion containing it will go on hardening for
several months, during all of which time the rate of penetra-
tion of the developer will be progressively retarded, with a
consequent alteration in the apparent speed unless carefully
checked.
It is very desirable in all tests of the physical characteristics
of gelatins to submit the sample to the same initial treatment,
‘Thus any change in pH will be responsible for wide variations
the swelling capacity. The ions of many salts have a pro-
found effect on physical properties. As an example of labora-
tory practice, in making a five per cent solution, 5 grams should
be weighed out and placed in small pieces in 90 cc of distilled
water (or tap water if the test demands), and allowed to swell
for one hour at room temperature, 70° F. ‘The water is then
warmed, the gelatin dissolved by stirring, and the volume
brought up to t00 cc. ‘The temperature should not be allowed
to exceed 160° F. It should then be stood on the bench
and allowed to cool until it attains the temperature that is re-
quired for the test, for which a suitable temperature would
be 105° F.
An example of a commercial test is tabulated on the next
page. MATERIALS FOR EMULSION MAKING — 29
Guixrix Test. Lot No.117.
Grease .ce.ceeeesevs es seve None
Clarity ee - Complete
Engler viscosity at 10%, temp. 35° C. .....+ 33
Setting point of 10% solution ... Boe ara
Melting point of 10% solution, after two hours..... 31.9° C.
Reduction by ammo! + None
Copper, lead, zine None
Iron salts .. ~ Traces
BRE : 36
No. of cc of 5% chrome alum required to precipitate
10 cc of a 10% solution at 50° C. ...2...+ 59
For more detailed information on gelatin, the reader is referred
to Dr. S. E, Sheppard’s monograph “Gelatin,” Vol, 1; Kiss-
ling’s “Leim und Gelatin” (Stuttgart, 1923); and the text-
books of Alexander, Bogue, and others,
‘The brands of gelatin largely used in emulsion making are
Nelson’s No. 2 and Nelson’s X-opaque and S. E, emulsion leaf,
(made at Warwick, England), Simeon’s Winterthur (Swiss),
Heinrich’s and D.G.F. (German), Bertrand’s (France), and
last, but not least, the excellent products of the Atlantic Gel-
atin Co, and American Agricultural Co. Most makers are gen-
erous in the extreme with regard to supplying samples, but it
is a useful guide to them when ordering to specify the approx-
imate viscosity and hardness desired, if possible to send samples
of those brands in use, and to state the type of emulsi
which the gelatin is wanted —fast negative, slow positive,
bromide or chloride paper, and so on, Prices average about
85 cents to $1.25 per pound, and as an example of likely quanti-
ties for commercial work on a moderate scale, about twenty-
five to thirty pounds may be estimated for each day’s run of
3,000 feet of sensitized material forty-two inches wide. Gel-
atin will keep almost indefinitely if stored in a cool and dry
place, and it is the experience of many firms that if kept for
n for 30 PHOTOGRAPHIC EMULSION TECHNIQUE
six months before use it gives the best results. As already
stated, bulk gelatins are best made up of mixtures of two or
three batches or makes, which are arranged to run out at dif-
ferent times. A couple of weeks should be allowed to make a
thorough test of a new sample. In making the test, a word of
caution may be given. In packaging leaf gelatin it is the manu-
facturers’ usual practice to draw from two or three bins, so that
a one-pound packet may contain three or more different va-
rieties. If fifty grams are required from a pound packet, a cross
section of sufficient thickness should be cut on the cutter so
that a piece of each sheet is included in the test solution.
‘The reader may wonder why gelatin remains the only col-
loid which is successfully employed as a vehicle for sensitive
silver salts. Its sensitizing properties are sufficient reason, but
its vehicular power, porosity and protective powers combine
to make it unique. Collodion is still used in process work, and
for special types of emulsion of a slow speed. Silver halides
can be precipitated in a solution of pyroxylin in alcohol-ether,
and collodio-bromide and collodio-chloride papers have long
been in commercial use, ‘They possess one advantage, that if
coated on a base first coated with gelatin, the developed image
in collodion can be removed or stripped off the support by
treating the print with hot water, which dissolves the gelatin
terleaf but does not affect the collodion, An example of this
is the Defender Chromatone bromide paper. The image, after
toning, can be floated off the support and mounted in layers
for the production of a three-color print. Stripping papers can
be similarly made for a variety of purposes, although it is a
doubtful point whether a coating of hardened gelatin emulsion
on top of a gum dammar or wax coating of the support does
not yield a better material.
We have seen in Chapter I how excessively small need be
the proportions of ingredients or impurities in gelatin to be MATERIALS FOR
M
SION MAKING 31
responsible for important changes in emulsion quality. Tt is
therefore easy to understand that all chemicals used in emul-
sion making should be of a high order of purity. There are
three types of chemicals generally available, One is the stand-
ard type sold for amateur and professional photography. An-
other is the so-called chemically pure or C.P, variety, and a
third type is sold for the laboratory for more exact work, de-
scribed by the term analytical reagent or A.R. For emulsion
making in small quantities, A.R. chemicals are worth the extra
cost. At least silver nitrate and ammonium bromide should
be A.R. quality. In general, other chemicals may be of the C.P.
type. In all formulas given in the following pages, water may
be taken to mean distilled water unless otherwise stated. Emul-
sions are usually washed to best advantage in tap water, or
supplies from artesian well
Of the ordinary emulsion-making ingredients, ammonium
bromide is one of the most important, For extreme speed
emulsions this should not contain more than 0.2 per cent of
chloride. The finest quality is probably made by combining
purified bromine, free from chlorine or hydrobromic acid,
pure ammonia liquor. Less pure samples can be improved by
re-crystallization, ‘The chloride content can be checked by
boiling a weighed sample with concentrated pute nitric aci
driving out the bromine, and titrating the residue with centi-
normal silver nitrate solution.
Silver nitrate is made by dissolving silver metal free from
copper and lead in nitric acid. ‘The carefully re-crystallized
product is usually sold at a price calculated on the ruling price
of silver at the time of order. It must be free from nitrite. Its
formula is AgNO,; that is, it is a salt without water of crystal-
lization; molecular weight 170. For convenience, it may be
kept in the form of a concentrated solution, such as 2N or
24N, in stone jars. In the following text, however, many 32 PHOTOGRAPHIC EMULSION TECHNIQUE,
formulas will be found in which the silver nitrate is “ wetted ”
with a small quantity only of water in order to convert it into
a highly concentrated solution of the ammonio-nitrate. Here
the dry salt will in most cases be found more convenient.
‘There is much to be said in favor of keeping chemical stocks
in liquid form. It is probably more accurate and definitely
more convenient. In the case of silver nitrate, if a 2N solution
containing 340 grams per liter is made up, the ammonium bro-
mide may conveniently be made up also as 2N, having 196 grams
per liter. Equal quantities then react completely. On the
other hand, such a reagent as ammonium bromide, which is
used in slight excess of the equivalent weight of silver, may
be found more convenient to handle in the form of a ten per
cent solution.
Ammonia must be free from iron and pyridine compounds.
‘There is an advantage in breaking down the concentrated am-
monia to 0.920 specific gravity, as at this strength each cubic
centimeter will almost exactly re-dissolve one gram of silver
nitrate or be equivalent to one gram of AgNO,. It will be
found a great convenience to have it delivered in this form, al-
though the gravity must be checked from time to time. Am-
‘monium and potassium iodides are best made up in the form of
solutions and kept in the dark when not in use. A discoloration
con keeping, due to the liberation of some free iodine, need not.
be feared. Indeed, some emulsion makers think that the solu-
tion in this form is preferable. Chrome alum should be se-
lected free from contamination by small orange crystals of
dichromate, and should be made up with water not above
100° F., as otherwise it may decompose and lose its hardening
properties. It should not be neutralized. Chrome alum is most,
usually handled in the form of a five or ten per cent solution,
which should be re-filtered if at any time, on keeping, it shows
a deposit. MATERIALS FOR EMULSION MAKING 33
It is assumed that a reasonably accurate balance or pair of
scales is available, and a larger balance where works-scale quan-
tities are to be weighed. A balance sensitive to one or two milli-
grams is needed, since in many emulsion formulas the reacting
compounds are used in proportions closely approximating to
their combining weights. ‘The type of chemical balance which
Fie. 7
depends on a moving weight sliding over a scaled bar is not
recommended, but a balance of the laboratory type shown in
Fig. 7. A set of weights ranging from one gram to five hundred
grams with fractions to ten milligrams is suggested.
‘The following is a list of the chief chemicals which will be
needed in the course of the work described in this book:
Acetone
Acid, acetic, glacial
Acid, citric
Acid, hydrobromic
Acid, hydrochloric 34 PHOTOGRAPHIC EMULSION TECHNIQUE
Acid, nitric
Acid, phosphoric
Acid, sulphur
Aloohol, denatured, photographic quality
Alcohol, pure
Alum, chrome
Ammonia, concentrated
Ammonium bromide
Ammonium chloride
Ammonium iodide
Barium chloride
Calcium chloride
Carbitol acetate
Cupric chloride
Distilled water
Formali
Gelatins, various
Glycerin
Litmus paper
Methyl alcohol
DH indicators
Nessler reagent
Phenol crystals
Phenolphthalein
Potassium citrate
Potassium dichromate
Potassium iodide
Potassium thiocyanate
Rochelle salt (sodium potassium tartrate)
Sodium chloride
Sodium citrate
Sodium oleate («lycerin substitute)
It is assumed that the usual photographic chemicals are avail-
able. Calcium chloride or sulphuric acid will be required for
the desiccator. MATERIALS
FOR EMULSION MAKING 35
A word may be said here about alcohol, Denatured spirit
to be used in emulsions must be free from mineral oil, aldehydes,
pyridine or any coloring matter. A special type of industrial
spirit is made for the photographic industry which consists of
ethyl alcohol (C,H,OH), with a small percentage of wood spirit
or methyl alcohol (CH,OH). This, however, can only be ob-
tained with a permit. A form of denatured alcohol containing
about one per cent of gasoline which has been used by the
author with most types of emulsion with success, can be ob-
tained without a permit in any quantity. This is Synasol, sup-
plied by Carbide and Carbon Chemicals Corporation, 30 E.
4and Street, New York City. Where emulsions are to be used
for commercial purposes it would not be recommended, but for
experimental work it may be found a great convenience, Where
only small quantities of spirit are likely to be used for laboratory
scale experiments and cost is of no object, the reader is advised
to use pure grain alcohol or ninety per cent alcohol, both of
which are available in most laboratories. In works practice, a
permit will be required for the necessary amount of industrial
spirit, which will run into considerable quantities. Spirit is
used in commercial emulsions in proportions varying from five
to ten per cent of the total volume. In factory routine, frequent
checking up of the spirit is necessary to look out for the presence
of aldehydes or other deleterious impurities.
While a still for the preparation of sufficient quantities of
pure water is an essential on any works-scale production, what
is more important in selecting a site for an emulsion laboratory
to determine whether or not the available water supply is
suited to the work.
Chapter References
1. R. H, Bogue, The Chemistry and Technology of Gelatin and Glue,
p.srt 36 PHOTOGRAPHIC EMULSION TECHNIQUE
. M. Huzii, Reports Imp. Ind. Res. Inst., Osaka, 18, 1, 1937.
- B. V. Storr, and Ann. Report, Soc. Chem. Ind., 37, 405, 1918,
- C, Stelling, Chem. Zeit., 20, 461, 1896.
Kopke, Arb, Kais. Gesund., 38, 290, 1911.
S.E, Sheppard, Photographic Gelatin, p. 203.
Bogue, The Chemistry of Gelatin and Glue, p. 416. Cuapter IIT
LABORATORY EQUIPMENT
Layout for Experimental Work — Commercial Production and Its Re-
‘quirements — Ventilation — Safelights —Digesting Apparatus —Thermo-
static Control —Washing and Filtering of Emulsions— Making up —
Cold Storage
‘HE accommodation required for emulsion making depends
primarily on the quantities involved and type or types of
product to be made. As already hinted, one of the first things
to be ascertained in contemplating commercial production is the
suitability of the water supply or the provision of good water,
and freedom irom atmospheric contamination. While mod-
em air-conditioning plants make it possible to work in towns
and cities, a new plant is generally started in a vicinity where
the air is clean and fresh and space is plentiful.
We shall endeavor in this chapter to discuss plant and ap-
paratus both for the experimentalist and for the production of
commercial quantities, In many instances, of course, the larger
plant is a multiplication of the smaller unit, But where the
coating is to be done by any type of machine, conditions are
necessarily very different from experimental work where the
coating is by hand. The laboratory type of work will be dealt
with first, but it will be understood that much of what is said
While small quantities of emulsion of excellent quality can
be made in the laboratory, even in speedy varieties, the emulsion
maker must have the right tools to work with, light-tight
accommodation, reasonably good ventilation, an ample water
supply, and above all, a room free from dust for coating, where
heat up to 70° or 75° F. can be more or less controlled. Very 38 PHOTOGRAPHIC EMULSION TECHNIQUE
small scale work can be done in one room, used alternatively
for mixing, washing, coating and testing. In many instances
of experimental work this is the only condition under which it
can be carried on. The ideal accommodation for small-scale
work, however, is to have at least four rooms or subdivisions of
a large room, where (2) the solutions can be weighed out and
prepared, (b) the emulsification, ripening, washing and digest-
ing can be carried out, (c) the finished emulsion can be coated
and dried, and (4) the final product can be tested or used for
photographic work. The latter is obviously the average photog-
rapher’s darkroom,
‘The coating and drying room should be as free from dust as
possible, with sufficient and suitable ventilation which involve
Teaks. Cleanliness and order are above all things to
be desired, the utmost care being taken to keep all graduates,
crocks, stirring rods, beakers, dishes, thermometers, etc. scrupu-
ously clean. Cupboards are better than shelves for storing
chemicals and apparatus, bottles and glassware being first class
dust catchers and requiring frequent rinsing and wiping with
a clean damp cloth,
Any room to be used for coating and drying sensitive material
must be thoroughly light-tight, this usually involving rather
more investigation than would suffice in ordinary darkroom
work. By shutting oneself in the room in complete “ darkness”
for ten or fifteen minutes, it is often astonishing to find how
much stray light there is in a room previously thought to be
absolutely dark. Black or dark brown paper and paste or glue
is generally a sufficient remedy, or the filling up of cracks with
mortar or plaster of Paris, A strip of felt or rubber beading
along the bottom of doors is often a help, or a wooden lath
screwed to the floor to cover the clearance space, Benches about
three feet high are more convenient than tables, as practically
all work must be done standing and not sitting. This does not LABORATORY EQUIPMENT 39
WEIGHING-UP EMULSIFIGATION
ano AND
CHEMICAL T RIPENING
Room
WASHING
AnD
BINISHUING
TESTING ce
Room
COATING
AnD
ORYING
Fic. 8
apply to benches with sinks for washing jars and crocks, where
a more convenient height is two feet to two feet, three inches.
Sinks should be of ample size, and if of porcelain or enamelled
iron and not of wood, wooden slats should be placed over the
bottom to prevent breakage
A small layout is shown in Fig. 8, in diagrammatic form. The
general idea, as is often suggested for commercial laboratories,
is that the work should take place in a cycle, one operation fol-
lowing another in the ordinary sequence, so that chemicals and
solutions are prepared at one end, and the testing of the finished
product can be carried out at the other. It is a great conven-
ience to have a light trap between the coating room and the
emulsioning room, as shown at a in Fig. 8, as one can then go
freely from “ light ” to “ dark ” If two people are doing
the work, a small hatch 1 with ‘tiding panel is useful for han- 40 PHOTOGRAPHIC EMULSION TECHNIQUE
dling crocks, solutions, etc., from the making-up room to the
coating room,
A word may well be said here about illumination, It is a
common idea to have black walls in the darkrooms. This is
bad practice. If the source of light is properly filtered, the walls
and ceilings may be white, when they will reflect a maximum
ee
‘SLOW BROMIDE TRANSMISSION OF
RESPONSE YELLOW SAFELIGHT