The Chemistry of Photography

Black & White Film

Black and white film is composed of 4 layers. *An upper protective coat. *A layer of gelatin that contains silver halide (AgBr, AgCl, or AgI) crystals. (The type and proportions of the different silver halides determining the speed of the film)

*The film base, usually made from a flexible polymer. *And the anti-halation backing to prevent light from reflecting back onto the emulsion.

Color Film

The Color film “emulsion” is actually made up of 3 different layers of emulsion.

* Each is sensitive to different wavelengths of light. *The emulsions still contain silver halide crystals but are now coupled with dyes. *The dyes are the compliments to the colors too which that layer is sensitive. *There is a yellow filter between the first and second emulsion layer to prevent blue light from getting through to the lower layers because all silver halides are sensitive to blue light. *The film base is an orange color to reduce the contrast of the negative and to correct for sensitivities in the red and green layers. *The anti-halation layer in color film serves the same purpose as in black and white film

Exposure, Development of Black and White Film - Overview

• A. Unused film in camera

• B. Exposure of film to light (photons)

• C. Formation of silver ions (latent image)

• D. Development changes silver ions to metallic silver

• E. Fixing – removes unreacted silver halides from the emulsion.

• F. Wash – rinsing with clean water. Removes all by-products of development process.

Converting Silver Halide Crystals to Metallic Silver

Ag+Br- (crystal) + hv (radiation) ® Ag+ + Br + e-

Ag+ + e- ® Ag0

Silver Crystals – Sensitivity Centers

• The silver halide crystal contains imperfections called sensitivity centers.

Effects of light on the film

• Within a crystal the Silver atoms have a positive charge and the halide atoms a negative.

• Light (photons) striking the halide atoms within the grains causes excitation of electrons which move within the crystalline structure.

• Those electrons are attracted to the Sensitivity Centers.

Ag+ Br - (crystal) + hv (radiation) Ag+ + Br + e-

Latent Image Formation

• The silver ions are attracted to the negative charge of the electrons at the sensitivity center.

• As more light (photons) hit the halide atoms silver ions build up on the sensitivity centers.

• The silver ions acquire and additional electron and become metallic silver.

• These sites form development centers and make up what is called the “latent image”.

Ag+ + e- Ag0

Developing the image• All of what we’ve discussed so far has gone on

within your camera.• Now we’ll go to the process of “developing” your

film.

• Black and white film is handled in complete darkness as the film is sensitive to all wavelengths of light.

• The Steps of processing/developing film are:

Development

Stop

Fix

Wash

Hardening bath (optional)

Development• Photographic Developers are generally

Reducing agents. The silver ions are reduced to silver metal. The developer donates electrons to the positive silver ions.

• The greater the number of silver nuclei attracted to the sensitivity centers the faster the developer will reduce the silver ions to silver metal. So the more light a crystal is exposed to the faster it will develop and the darker it will be.

• Developers need to be somewhat selective so as not to turn unexposed silver dark. A process known as fogging.

• Photographic developers contain carefully balanced levels of the developing agents, “accelerators” such as Sodium or Potassium Hydroxide, and Sodium or Potassium Carbonate. There are also restraining agents built in such as Potassium Bromide. These restrainers slow down development in areas that received less exposure.

Stop Bath• Photographic developers are generally of a pH

greater than 10.• A “Stop bath” usually made from a weak acid

such as acetic acid is used to stop the development, and prevent fogging of the unexposed silver.

Fixing

• Undeveloped silver halide crystals remaining in your film will darken with time if exposed to light.

• To prevent this, film is “fixed” or has the undeveloped silver halide crystals removed from the film.

• Sodium Thiosulfate, usually referred to as “Hypo” is one of the most common fixing agents though others are used depending on the specific characteristics wanted in the fixing solution.

• The silver halides have a low solubility in water. To remove them they need to be turned into more soluble forms that can be removed in the water wash.

Fixing ….. Cont.

• The chemical reactions of the fixing process follow this general form:

AgBr + S2O3-2 --> AgS2O3

- + Br-

which is followed by

AgS2O3- + S2O3-2

--> Ag(S203)2-3

Then:

Ag(S203)2-3 <--> AgS2O3

- + S2O3 –2

And the final step

AgS2O3 <--> Ag+ + S2O3-2

Washing

• The final wash of a photographic negative needs to be lots of fresh clean water to remove any residual developing agent, fixative or silver complexes as these can cause degradation of the image with time.

• The ability of a film to withstand this degradation is referred to as it’s Archival Quality.

• Depending on the film, and processing methods film can remain unchanged for many decades.

• An optional hardening bath can be used after the wash to try and minimize scratches to the dried emulsion.

Developing Color Film

The processing of color film is very similar to processing black and white film with some notable exceptions. Color film also starts with silver halides. But added to the film are dye “couplers” thatare sensitive to various wavelengths the silver halides are not. The absorbed energy is transferredto the silver halides to form the latent image. So it is the dyes that are absorbed onto the silver halide crystal that need to absorb the radiation in the desired wavelength.

During “development” the silver halide crystals are reduced by the developer, resulting in “oxidation”of the developer. The oxidized developer then reacts with the dye couplers to form the colored dye.

Color processing has an added “Bleach” step which converts all of the silver back to a silver halidewhich is removed during the fixing stage. Some processes combine these two steps into a single bleach/fix. So there is no silver left in color negatives.

Film vs Print Paper

As we discussed above film has a transparent, clear or in the case of color film colored base. Thisis so that light can be shone through the negative film and onto photographic papers to produce prints.

The primary difference is the base material. For prints it is an opaque paper base. With the general concepts of the emulsion and processing steps carrying over from film to paper.

B&W papers however are not sensitive to red light so when printing B&W prints you can work under a red light. Color papers are sensitive to all light and must be handled in complete darkness. Mostcolor print processes use the combined bleach-fix step rather than separate bleaches and fixers.

Conditions effecting rate of development

• User Controlled Dilution Temperature pH

• Non User Controlled

Diffusion rate of developer solution in gelatin Adsorption of developing agent Oxidation products of developing agents Accumulation of by products Purity of manufacturing chemicals

Sample processing time vs temperature chart for B&W films

Film Type Temp/Time

20oC(68oF) 22oC(71.5oF) 24oC(75oF)

125 PX 8:30 7:00 5:30

100 TMX 10:00 8:00 6:30

400 TMY 11:30 9:15 7:30

3200 TMZ 17:00 14:00 11:15