487
However, it’s important to understand that whilst
every worker, then Company, has their own ‘secret’
emulsion recipe, the basic principles haven’t changed
since 1848. A mixture of silver halides is made or
mixed in a solution of a transparent substance which
when applied to the plate, dries to a stable light sensi-
tive coating. Precise details of selecting, preparing,
mixing, and spreading seem like witchcraft today, but
remain familiar in any modern factory using gelatin.
This is then exposed, developed and used to make any
number of prints.
With this key bit of technology established it soon
became apparent that an emulsion applied to paper also
greatly simplifi ed print making. A prior albumen ap-
plication made the paper both more rigid but also kept
the sensitive emulsion where it was most needed, on
the surface. The albumen in the emulsion also added
a sheen to the fi nal image and acted as a protecting or
“anti-scuff” surface.
In the same year 1848 that Niépce introduced albu-
men, R. J. Bingham of the Royal Institution, London,
gets the credit for fi rst suggesting gun cotton or col-
lodion for photography. As a synthetic material it had
many obvious advantages. Scott Archer prepared his fi rst
negatives that year, then disclosed his collodion process
in 1851. By 1855 collodion had largely displaced albu-
men when Mayall published the then most widely used
albumen system. The two systems ran in parallel for a
long time and albumen continued for decades after col-
lodion had been replaced in turn by gelatin.
Albumen had a number of practical disadvantages,
not least bacterial spoilage and uncertain quality but
when dry it also stood up to abuse much better than
the other obvious natural alternative, gelatin. It was
not for another 30 years that this problem was solved
and even then albumen prints remained a fi rm favourite
right up to recently, and for large glass lantern slides at
the turn of the century nothing today really excels for
tonal quality.
Like albumen, collodion emulsions found very wide
application in both negative and positives. Its main prac-
tical advantage was the ability to prepare and store stock
solutions without decomposition. Even after a century
and a half some samples from the period are still usable
and the technology might not have moved on much
were it not for a severe disadvantage of collodion, its
unsuitability for scientifi c measurements. When dried,
the emulsion became very insensitive, and nearly all
negatives were created from the “wet collodion” process.
For precise work needed for the Transit of Venus in 1874
the wet process was totally inadequate.
The Royal Greenwich Observatory gave W. de W.
Abney the task of solving this and he modifi ed and
combined both the emulsions of the time. He used thestability of dried collodion as a base and albumen as a
sensitive layer. He cured the decomposition problem
with an unlikely ingredient, stale beer. This beer albu-
men process paved the way for all modern photography
because it illustrated two fundamental clues to progress.
The fi rst was that prior chemical treatments of dry
plates could yield huge increases in sensitivity and gave
long-term storage as a bonus. Second, it was possible to
modify the colour sensitivity of the emulsion. Abney’s
two critical discoveries led to everything we know today,
high speed panchromatic, then colour fi lms.
Once this groundwork had been established the time
was ripe for another major step forward in emulsion
technology, better alternatives to infl ammable collodion
and decomposing egg white. Gelatin had been tried
since Talbot’s invention but the stuff was simply not
well enough defi ned to be reliable. With Abney’s initia-
tive, better gelatin consistency was sought and quickly
found to be vastly superior. By the 1880s gelatine had
become the major ingredient in emulsions but even as
late as 1896 R Child Bayley still hadn’t fully solved
the specifi cation problem in a paper to the Royal Pho-
tographic Society. Gelatin emulsions had become the
standard photographic medium for the next Venus transit
in 1882, and Abney’s special additives had increased
sensitivity way beyond anything thought possible a
decade before.
Emulsion technology hasn’t changed much from
that time because of a peculiarity of gelatin, it contains
natural components which actually sensitise the emul-
sion. No synthetic substitute comes anywhere near that
unique property. Making an emulsion falls into several
stages, laid down in the 1880s.
Gelatin has to be critically quality controlled for
consistency, then mixed with water to make the basic
ingredient. To this is added, or it is added to a solution
of halides. The ratio of bromide to chloride and iodide,
determines the photographic properties and the gelatine
controls the grain size when silver nitrate is added to
the mixture. Then follows a period known as “ripen-
ing,” still a very much mysterious situation when the
emulsion reaches an improved sensitivity. When that
process is over, the emulsion is coated onto the negative
or positive and allowed to dry. At all stages before use
many special ingredients can be added to confer special
properties, and there may be washings to remove some
or all of these. That is one reason why no two batches
of photographic emulsion behave exactly the same.
The only major change towards the end of the century
was to fl exible fi lm instead of glass plates introduced
by George Eastman. That led ultimately to an emulsion
expansion into the present mass market through ever
smaller formats and convenience in use.
Michael MaunderEMULSIONS
Hannavy_RT72353_C005.indd 487 7/5/2007 11:18:22 AM