FREE ESSAY ON THE CHEMISTRY OF PHOTOGRAPHY

THE CHEMISTRY OF PHOTOGRAPHY

Photography is a very complex form of art, which uses standard chemicals and scientific
equations. It is a chemical reaction, which occurs when light hits a film or paper
emulsion. An emulsion is part of the film or paper, which is very sensitive to light.
This is also the part where the chemical reaction takes place. In an emulsion, are small
crystals of silver halide, silver bromide, silver chloride, or silver iodide [Eaton,
George]. These chemical crystals react with light when it is passed through a camera, to
create an image on paper [Foto Info]. The development of film production has a very
simple chemical equation, most widely known as:
Light + Ag(X) a Ag + (X)
This equation expresses how when any form of light is added to silver X, it creates
silver plus the X [Mad Scientist]. In most cases, the AgX represents one of the compounds
above.
In the photographic process, there are three main chemicals used. These are the
developer, the stop bath, and the fixer. The developer, or the reducer, is the chemical
that completes the reaction with the light exposure, by reacting with the silver, to
create metallic silver [Britannica Online]. This process is called reduction. Common
reducers used are hydroquinone, phenidone, metol, pyro, and absorbic acid. If you combine
two of these reducers, it creates superadditivity. Some common combinations are metol and
hydroquinone, which produce MQ, or phenidone and hydroquinone, which produce PQ. [Foto
Info]
Each of the chemicals used in the developer have their own specific qualities and
features. When hydroquinone is used alone, it can be a very slow reducer. When metol is
used, it brings out detail very quickly. Phenidone is almost never used by itself. If it
is combined with hydroquinone, it keeps detail, but at the same time, it increases
contrast. If used alone, it is very fast but has low contrast. One of the less commonly
used of these developers is pyro. This is an older developer, and is also known as
pyrogallol. Pyro leaves a stain on the negative while it develops it. Most people who use
this developer want that stain as part of an effect, because it ends up softening the
image. Finally, absorbic acid has been used for quite some time to develop film. [Foto
Info]
The Kodak DK-50 is a fast developer, which produces a clean image on the paper. A
breakdown of the chemicals involved looks like this:
Water 52 degrees centigrade 750 ml
Metol 2.5 grams
Sodium Silfite, anhydrous 30 grams
Hydroquinone 2.5 grams
Balanced Alkali 10 grams
Potassium Bromide, anhydrous 0.5 grams
[Kodak]
This shows the amounts of each chemical added. If you change the amount of these
chemicals, or add different ones, the developer will create different qualities in the
pictures it makes. An example is in the Kodak D-76 Developer. This developer is good for
shadow detail, and has normal contrast. The makeup of this developer looks like this:
Water 52 degrees centigrade 750 ml
Metol 2 grams
Sodium Silfite, anhydrous 100 grams
Hydroquinone 5 grams
Borax 2 grams
[Kodak]
A stop bath makes sure the developer does not develop any more than it should, by
stopping it [Britannica Online]. This is needed because there is always some extra
developer in the emulsion. A developer has a pH higher than seven, and is therefore a
base. Most photographers use acetic acid as their stop baths [Foto Info]. This acid
neutralizes the developer, causing it to stop. Another simple stop bath is water. The
only problem with this is that water will not stop the development of the photo
instantly. This is generally not a problem, unless the artist has a specific look that
they are trying to achieve.
The last chemical in the photography process is the fixer. The fixer breaks up the
un-exposed silver salts in the emulsion. Then, you will be able to view the photograph
outside of the darkroom, because it will no longer have a chemical reaction with the
light. There are six components of the fixer. They are the solvent, the fixing agent,
acid, preservative, hardener, and the buffer [Woodson, Roger]. The solvent is usually
water, which dissolves the chemicals, including the silver thiosulfate complexes that
were formed. The fixing agent is usually sodium thiosulfate or ammonium thiosulfate. The
difference between the two is that sodium thiosulfate is a powder. Ammonium thiosulfate
is a liquid, which is easier to mix. It also works much faster. Acetic acid is also used
in the development of pictures in the fixer to maintain acidity [Sun Spot]. The
preservative is usually sodium sulfite, or Na2SO3. This is used to make the decomposition
of thiosulfate weaker. The hardener prevents swelling or softening of the emulsion, which
might damage the photograph while you wash it. Finally, the buffer makes it so the fixer
does not become too basic. Once again, acetic acid is usually the buffer, when mixed with
some sodium thiosulfate. [Sucsy, Arthur]
When all this is complete, the photo should be soaked in a water bath. This will wash off
the fixer or silver from the emulsion. If this step is not done, fading and even a loss
of color can occur. Another reason the photograph may have problems is if you let the
temperature of the chemicals during the process go below 65 degrees. Below this
temperature, the chemicals will not function properly, and may not have a reaction. [Foto
Info] If there is no reaction, then there is no picture image displayed on the paper.
Bibliography
Woodson, Roger Complete Idiot's Guide to Photography 
MacMillan Publications, November 1996
Eaton, George T. Photographic Chemistry : In Black-And-White and Color 
Morgan & Morgan, June 1988
Encyclopedia Britannica Online
Accessed May 8, 2000
Eastman Kodak Company
Accessed May 19, 2000
The Mad Scientist Network
Accessed May16, 2000
Dr. Arthur C. Sucsy
Accessed May 8, 2000
Foto Info
Accessed May 19, 2000
Sun Spot Photography
Accessed May 28, 2000