Film Photography: where art meets chemistry

Emily
Feb 25, 2022
6 min read

Nowadays photographs are taken for granted, with instant digital displays on phones and cameras and the ease of printing the images straight from your device. But photography wasn’t always this easy and still isn’t! We’ve seen people in movies dipping paper in liquid whilst working in a red lit room developing their film photographs, but do we know the chemistry behind this process?

First Photographic Methods

Photography dates back to 1839 when the first photographic image was taken by Louis-Jacques-Mandé Daguerre, a French painter. This image, now known as a daguerreotype, was far from what we know today. The daguerreotype images formed on a polished, mirrored, silver-plated copper sheet which was exposed to iodine. The iodine reacted with the silver plating to form silver iodide (AgI), a pale-yellow photosensitive compound, which when exposed to light darkened. So, when the sheet was exposed to sunlight through the lens of a camera a photochemical process occurred where the Ag+ (from the silver iodide) was converted to Ag (elemental silver), which was a dark grey colour. After the image had been incident onto the plate it was developed using mercury vapour. The mercury reacted with the elemental silver (Ag) at the exposed sites of the image to form an amalgam, a mercury alloy (Ag2Hg3). This amalgam was a light silvery colour, so where the plate was exposed to light it was light in colour and unexposed areas were darker. The photosensitive silver iodide was still present in the unexposed areas, so this compound had to be removed as quickly as possible to prevent the plate from turning greyish all over. Daguerre initially used a sodium chloride (common salt) solution to remove the silver iodide, however, this was not that effective, and the images started to darken over time. He later adopted Sir John Herschel’s method, known as fixation, where sodium thiosulphate is reacted with the silver iodide to form silver thiosulphate, which can be easily washed off the image using water. Due to the fragility of the image sheet, daguerreotype images had to be protected with a glass layer to prevent scratches. Not only did this method involve highly toxic and harmful chemicals but it also required long exposure times. Improvements to the sensitivity of the image were made after it was discovered that coating the silver iodide surface in sulphur or gold caused the crystals to become more photosensitive. Progressions in photographic film started when flexible, transparent rolls of film were produced.

The Composition of the Film Reels

Gelatin was found to be a useful medium for silver iodide and other photosensitive materials. This gelatin emulsion was first used to coat glass and iron plates in photographic methods known as ambrotype and tintype, respectively. Henry Fox Talbot invented the calotype photographic method in 1841 which involved the use of silver chloride treated paper film. This paper film was translucent so the negatives could be used to form positive prints from contact printing. Film photography, as we know it now, with flexible and transparent films still uses this gelatin coating. George Eastman established modern photographic film back in the late 1800s, he developed the first commercial camera (the Kodak) which became available for sale in 1888. The film that Eastman had produced for the Kodak was comprised of celluloid nitrate sheets coated in gelatin-dispersed silver iodide crystals. The gelatin was used as it could bond to the celluloid film and evenly spread out the crystals. Multiple layers of the gelatin-halide emulsion can be coated on to film, making it easy to introduce different sensitive crystals to control the reflection/absorption of light. Generally, “the thicker the layers of the emulsion and the larger the silver crystals, the more light sensitive the image.” The light sensitivity of an image is rated using the ASA (American Standards Association), where a high ASA film is more light sensitive and requires less light to form an image. Different films are used for changing conditions. For instance, a 100 ASA film (100 speed film) is used in bright sunlight or with flash, whereas a 200 or 400 ASA film is used in lower light or indoors.

The use of celluloid nitrate in film was replaced by cellulose acetate in 1908 after several disastrous theatre fires were caused by the extremely flammable material. Eastman called this cellulose acetate film the ‘safety film’ as it is safer and more chemically stable than celluloid nitrate. In 1960, polyester film was introduced, this is more durable alternative to cellulose acetate. Both polyester and cellulose acetate films are used today. The use of transparent film means that prints can be made of the images, as light can be shone through and projected onto paper. To use the films in this way they must first be developed so that they are no longer sensitive to light.

The Developing Process

People that bought the first Kodak camera would fill their film rolls with photos and send the whole camera back to Eastman’s factory for developing. No skills were required when using this Kodak camera, in fact their slogan was: “you press the button, we do the rest”. In 1889, Eastman started to commercialise the film reels so amateur photographers could develop their photos themselves. What chemistry is involved in the film developing process?

“You press the button, we do the rest!” - Kodak's slogan (1888)

The developing process creates a visible image on the film, rather than the latent (invisible) image that is currently present. Due to the photosensitivity of the film the developing process is done in a dark room to prevent any damage to the image on the film. The film is submerged into a tray of developing chemicals. After discovering the harmful effects and toxicity of mercury, other developers were created. A developer is often comprised of the following components: developing/reducing agent, accelerator, preservative, and restrainer. The developing agent, often hydroquinone in black and white film, reduces the Ag+ to Ag. This reduction process has already been initiated when the silver iodide was exposed to light, hence development is accelerated in brighter areas compared with areas where there was little light exposure. Sodium sulphite is typically used as a preservative, which protects the developing agent from oxidising and becoming less effective. The accelerator, often potassium or sodium carbonate, makes the solution more basic which supports the developing process, whereas the restrainer, often potassium bromine, helps to slow down the process so that the image doesn’t get developed too quickly. The developer intensifies the exposed areas of the image on the film, so they are now visible.

After the film has been developed it is removed from the developer and placed in the ‘stop bath’, a bath of dilute acetic acid, to prevent further overdevelopment. A fixative, such as sodium thiosulphate, is used to dissolve any of the remaining silver iodide molecules left on the film. This helps to preserve the image and prevent any degradation if the film is exposed to light again. The film is then washed in water to remove the fixer, which can also degrade the image, and hung up to dry.

The Printing process

After the film is developed, the negative images on the reel can be transferred onto paper to form positive prints. There are two main methods that have been used to do this: contact printing and photographic enlargement.

Contact printing was mainly used with calotype images, where the developed paper film was placed emulsion side down onto some photographic paper and light was briefly shone through the film. The positive image then started to become visible on the photographic paper and was put through the development process to ‘lock it in’.

Photographic enlargement printing was made possible after the production of transparent cellulose nitrate/acetate and polyester films. This process allows the small film negative to be projected to a desired size onto a piece of photographic paper. To do this a photo enlarger is used, this is basically a box that shines light through the film to a lens which then enlarges the reversed image onto some photographic paper below. The paper is coated in photosensitive chemicals, often baryta (barium sulphate) for black and white photos, so when the positive image is shone onto the paper a latent image forms. After the enlargement process, the photo paper is put through the same developing process that the film was subjected to. Once the image on the paper is fixed using a fixative the photo can be taken out of the dark room and viewed under normal light.

With chemistry at the heart of film photography, it’s amazing how much it has progressed but also how it still uses the same basic techniques and chemicals. Despite the introduction of digital cameras and phones making photography easier, film photography is still used to this day with some artists preferring this old school feel!

Written by Emily Cuffin-Munday

Blog references:

A. Swan, C. E. Fioni, K. F. J. Heinrich, Daguerreotypes: A Study of the Plates and the Process [https://www.si.edu/mci/EarlyPhotography/references/Swan-Fiori-Heinrich%20SEM_1-1979.pdf]

D. Malcolm, Daguerre (1787–1851) and the Invention of Photography, Heilbrunn Timeline of Art History [https://www.metmuseum.org/toah/hd/dagu/hd_dagu.htm]

N. M. Witten, The Chemistry of Photography [https://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=1085&context=senior_theses]

M. Fineman, Kodak and the Rise of Amateur Photography, Heilbrunn Timeline of Art History [http://www.metmuseum.org/toah/hd/kodk/hd_kodk.htm]

How Products Are Made, [http://www.madehow.com/Volume-4/Photograph.html]

L. Mazoner, A Brief History of Photography and the Camera [https://www.thesprucecrafts.com/brief-history-of-photography-2688527]

P. Frank, The First Paper Photographs Were Made With Salt, And They Look Like This [https://www.huffingtonpost.co.uk/entry/salt-and-silver_n_6801416]