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Thanks,
John
A brief history of this page: I had to come up with an idea for a final
project in my chemistry class, and so naturally I turned to my favorite hobby/job:
Photography. I decided to make my own photographic paper, but when I searched
the web for information I came up empty handed. Only minorly fazed, I emailed
around, and encountered a few very helpful people. To save future experimenters
some hassle, I present this site as a compelation of the process and results
of my paper-making experiment.
Before we get to the results, here is the process I used to make the emulsion:
(1) I prepared a 10% solution of gelatin using distilled water gelatin.
I added the gelatin crystals to the water slowly and at a temperature of
about 70 degrees Celsius. (You can prepare anywhere from a 3-10% gelatin
solution without any quality difference)
(2) In another beaker I prepared a 0.6 Molar Silver Nitrate solution. (using
distilled water, of course)
(3) I cooled this solution back down to about 40 degrees Celsius
(4) After the gelatin was completly solubized, I added sodium chloride
untill the solution was 0.7 Molar.
(5) In a darkroom under RED light only, I mixed solutions (2) and (3),
and stirred for a little over a minute.
(6) I quickly coated a test strip to check to make sure that there was
no fogging (If there was, then I would have to start over, making sure not
to expose the mixture and making sure to mix the two together at a lower
temperature, so as to prevent too much grain growth).
(7) I exposed another coated test strip for 10 seconds at f/5.6 (No negative).
When I put this into the developer, it only SLIGHTY turned grey. If you
get a good, deep black when you do this, you have got it perfect on the first
try. Good job! If you got a result like mine, your emulsion needs more Ostwald
Ripening (the sliver chloride needs a chance to grow into grains, making the
emulsion more sensitive.) To do this, heat the emulsion back up to 50 degrees
Celsius and stir for about a minute. Then make a test strip with this emulsion
and expose and develop like before. Do this untill you get a good black test
strip. You dont want to go too far, however, or there will be an overall
fog on your photograph.
(8) I coated the gelatinous solution onto thick water color paper using
a foam brush and then used a blowdryer to dry the emulsion. I used this method
for all of my test strips as well (There are many different methods of coating,
but none has stood out a clear winner, so pretty much any way you have of
putting an even coat of emulsion on the paper will work.)
In order to test the quality of the emulsion, I picked a photgraph with
good highlights, shadow areas, and mid-tones. This way I could evalutate
all of the aspects of the paper's performance.
This is the picture printed on Ilford paper and processed normally:
Here is the picture I processed using my paper (standard development):
While it is not nearly as high a quality picture as the one printed with
commerical paper, it is still recognizable and interesting both as chemistry
experiment and a method of creating a mood.
Summary of information gathered about photographic chemistry:
Almost all black and white photographic paper commercially available today
is made from two main ingredients: silver nitrate and a halide (sodium chloride
and/or potassium bromide.) The halide and other ingredients (i.e. hardeners,
restrainers, etc.) are first suspended in a solution of gelatin.
Gelatin has many properties that make it especially useful in photographic
emulsions. First, it serves as a protective colloid for the light-sensitive
silver halide crystals that make up the active part of the emulsion. Second,
it is transparent and very pure, allowing light to expose the emulsion without
distorting it. Third, it is water permeable, so that the developing solutions
can react with the exposed silver halide crystals to form the final image.
Fourth, it has good adhesion to its base and is easy to coat uniformly on
that base. Many attempts have been made to use inorganic replacements for
gelatin, but none have been found to be effective, and there is not a single
commercially available paper that does not use gelatin.
At this point, the emulsion maker has two solutions, one of silver nitrate
in distilled water and the other of a halide in gelatin (I will use sodium
chloride for this example.) The following steps have to be carried out under
red light only, as the solution becomes light-sensitive at this point. The
two solutions are then mixed, forming silver chloride and sodium nitrate
(AgNO3 + NaCl = AgCl + NaNO3). The silver chloride precipitates, but is held
suspended by the gelatin. The sodium nitrate is soluble and can be washed
away.
The emulsion then goes through a process called Ostwald Ripening.
This is a very interesting process and it determines whether the emulsion
will be a fast large-grained one or a slow fine-grained one (faster emulsions
require shorter exposures, slower ones need more light to be exposed properly).
At the beginning, the grains (groups of particles of silver chloride) are
fairly small, so small that the grains are hard to see with the naked eye.
If the maker of the emulsion wants a slow, fine-grained emulsion, he would
stop the process right away (by cooling the emulsion.) However, if a faster
emulsion is desired, he would let the process continue. At this point, the
smaller grains dissolve into solution and re-form themselves on the larger
grains. Thus, the large grains get larger and the small grains get smaller.
The emulsion can then be coated onto the base. Most commercial papers have
hardeners added to them at this point for several reasons, but mainly because
they prevent the gelatin in the emulsion from liquefying and coming off
of the base when the paper is developed. Depending on the specifics of the
desired paper, other ingredients may also be added at this time. They serve
a variety of purposes including slowing down the emulsion, speeding up the
emulsion, adjusting the pH to a desirable level (6.0-6.5), increasing contrast,
and more.
After the dried emulsion is exposed, either in a camera or the darkroom,
it is time for it to be developed. The developing agent must be a reducing
agent that can discriminate between the exposed and the unexposed grains
of the emulsion. Two organic compounds have been proven to perform this task
exceptionally well, and they are used almost exclusively, either alone or
in combination, in photographic developers. They are hydroquinone and p-menthylaminophenol
sulfate (Kodak Elon developing agent). The developing agent in the developer
solution is ionized, and these ions supply electrons to the silver ions of
the exposed silver chloride grains, reducing them to solid silver. The solid
silver is what forms the final and permanent black image. The chlorine and
the developer are simultaneously oxidized. Sodium sulfite is present in the
developer solution and reacts with these byproducts to form clear and colorless
compounds that will not stain the final image.
After development, the image is not yet stable. It has two more steps to
go through before becoming permanent: stop bath and fixer. Stop bath serves
to stop the developing. The stop bath is an acidic solution that reduces
the pH of the developer remaining on the emulsion enough so that the development
stops. The fixer makes the print permanent - one can spot a print that was
not fixed properly by noticing that it has faded and the white areas have
turned brownish. While silver halides are not soluble in water, they are
soluble in sodium thiosulfate, which is the main component of fixer solutions.
The sodium thiosulfate dissolves out the halide (chlorine in our example)
and then reacts with the silver ion in the following way: Ag+ + 2(S2O3-2)
= Ag(S2O3)2-3. A hardener is usually also present in the fixer to prevent
swelling of the emulsion during subsequent washings as well as to make it
more durable overall. The film or paper is then washed to ensure that no
chemicals are remaining on it. Any remaining chemicals would cause discoloration
over time. Many of the stains that can be seen on old black and white photographs
are a result of improper fixing or washing following development. However,
with the knowledge that we now have about the permanence of black and white
emulsions, a photograph can last almost indefinitely if it has been processed
correctly.
If you have any questions or comments about this page or photographic
chemistry in general, feel free to email
me.
This experiment and page produced by: John Wiemann
My website: http://www.duke.edu/~jmw22/
Special thanks to: Doug Corbin (Kodak) and Scott Williams (RIT)