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Chemistry of Photography

3. Fixing the image. Unexposed AgX on the photography film is removed with thiosulfate (eq. 3). The soluble complex, [Ag(S O 2-] 3-, can be easily washed away to leave only the silver image.

Early Photography

The process above forms the negative in black-and-white photography; light shining through the negative produces the final photograph (the positive).

Spectral Sensitization

4.Toning (coloring) the image. The silver-based black and white photographs may be

altered, to produce different colored images. Ex. Ag with thiosulfate in acid solution produces sulfur that then reacts with the Ag-image to yield the brown Ag2S which creates sepia photographs.

Light

The silver halides located on the film are sensitive to:

  • Blue
  • Violet
  • Ultraviolet

  • Without sensitization, positive photographs make all of the other colours appear as dark grays or blacks.
  • For this an addition of certain dyes are added to the emulsion to increase the sensitivity.

Mechanism

2. Development. The small number of Ag metal atoms formed act as a catalyst and sensitizes the surrounding halide salt so the sensitized AgX is reduced, to produce black silver metal in the area exposed to light. Modern developers contain one of many reducing agents. The most common is hydroquinone, which reacts with Ag+ (in AgX).

  • Photon is absorbed by a fragment of silver halide, therefore an electron is ejected from the halide ion. It is briefly held at certain point within crystal. Since the atom isn't stable it can decompose( into a silver ion and electron). In its lifetime the atom can gain another electron. If this happens the 2nd electron remains there until another silver ion comes. The buildup of silver can continue as long as photoelectrons are accessible. The smallest group of silver corresponding to a stable latent image speck consists of 3 or 4 silver atoms.

Developer:"A solution for developing a film or photographic paper - i.e. for turning an exposed film's or paper's latent image into an image that can be seen"

Fixer:"The chemical solution used for fixation. It removes any photo-sensitive silver-halide crystals that were not acted upon by light or by the developer"

Collodion: "a colourless or yellow syrupy liquid that consists of a solution of pyroxylin in ether and alcohol: used in medicine and in the manufacture of photographic plates, lacquers, etc"

List of Chemicals Used in Photography:

  • Processes were created by Louis-Jacques-Mandé Daguerre and William Henry Fox Talbot in the 1830's
  • Daguerre's method consisted of copper plate coated with silver iodide that was then exposed to light. The silver iodide decomposed to metallic silver and iodine. An image was then seen once treating the plate with mercury vapor, then rinsing in a salt solution. "Daguerreotype".
  • Talbot's method starts with washing the paper successively in saltwater and silver nitrate or silver chloride solution. The wet paper is then exposed in a camera until an image appears in the areas hit with light. The remaining silver chloride was removed by washing with a salt solution. A negative sheet can be made , and then diffusing light through the negative onto another sheet creates a positive image.
  • Sodium Thiosulfate Pentahydrate (Fixer)
  • Ethanol 96%, denatured with 5% methanol(Multiple purposes)
  • Diethylether (Collodion)
  • Cadmium Bromide Tetrahydrate (Salt for Collodion)
  • Ammonium Iodide (Salt for Collodion)
  • Iron II Sulfate (For Developer)
  • Glacial Acetic Acid 99-100%(For Developer)
  • Calcium Carbonate(Plate Cleaner)

Film Structure

For colour film it is treated with three dye developers:

1) Cyan(combination of green and blue light)

2) Magenta(combination of red and blue light)

3)Yellow(combination of green and red light)

Improvements

Further in depth of Daguerreotypes:

  • Films in cameras commonly only have one emulsion layer, these are called single-emulsion films.
  • In typical radio-graphic film the base is made of a clear polyester material. It provides physical support for the other parts of the film. It does not participate in forming the image. Sometimes the base may contain a light blue dye for a visual effect.
  • The emulsion is the active part of the film, where the image is formed. It contains silver halide crystals that are placed throughout a gelatin. The gelatin supports and separates the silver halide crystals.

The methods created by Daguerre and Talbot were inconvenient because they required a long time of exposure, could be as long as 60 minutes. In 1840 Talbot improved by having a short exposure that left an invisible "latent" on the film. This could be developed into a visible image by treating it with a solution of Gallic acid and silver nitrate. The main difference between Talbot's method and the method existing today is the silver halide which is suspended in gelatin.

  • In the early 19th century photographs were created using papers with silver nitrate or silver chloride. The problem was the "photographs" would continue to darken as time passed.
  • In 1839, Louis Daguerre developed a method to create light-fast images relying on silver halide photochemistry and a process for making the image permanent.
  • The treatment of the photographic plate, which was made up with a piece of copper covered in AgI, consisted of mercury vapours, followed with a washing of sodium thiosulfate.

Silver-based photographic process:

Reverse Processing

Black and white film consists of one layer of silver compound.

Colour film consists of three layers.

  • First layer is sensitive to blue light(around 475nm)
  • Second layer is sensitive to green light(around 510nm)
  • The bottom layer is sensitive to red light(around 650nm)

"1. Forming the image by exposure to light : A small number of X- ions in the AgX crystals that are located in the film are oxidized to form X. The electrons that are released from this oxidation reaction reduces the Ag+ to silver metal in the surrounding AgX crystal."

1. Exposure of the film in the camera

2. Development of the negative image

3. Dissolution of the developed silver image by treatment with an oxidizing agent

4. Exposure of the remaining silver halide to light or to a chemical fogging agent

5. Development of the silver halide, producing a positive image

6. Washing and drying of the film.

http://chemistryexplained.com

Classic Processing

http://www.idc-online.com/technical_references/pdfs/mechanical_engineering/photographys01.pdf

Photochemistry:

  • Defined as the branch of chemistry concerned with the chemical effects of light.
  • First Law: states that light must be absorbed by a compound in order for a photochemical reaction to take place.
  • Second Law: states that for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.

Bibliography

BY Hannah Walton

http://www.chemistryexplained.com/Ny-Pi/Photography.html#ixzz3ZHSgrMpe

http://science.howstuffworks.com/innovation/everyday-innovations/instant-film.htm

http://www.sprawls.org/ppmi2/FILMSEN/#SENSITIVITY

http://www.idc-online.com/technical_references/pdfs/mechanical_engineering/photographys01.pdf

https://terpconnect.umd.edu/~Toh/ChemConference/ChemConf96/Bodner/Paper2.html

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/photchem.htm

http://www.photographytips.com/page.cfm/2011

1. Exposure of the sensitive material, usually a gelatin emulsion of silver halides on a cellulose acetate film, in the camera

2. Development in the darkroom by treating the film with a solution of organic reducing agents such as hydroquinone and N-methyl paraaminophenol. The reagents reduce to metallic silver those silver halide crystals that acquired latent-image silver clusters. The brighter the subject of a photograph, the darker is the image that forms in this development, so that one obtains a negative picture.

3."Fixing" the image so that the film will not darken on further exposure to light. This is accomplished by dissolving the undeveloped silver halide grains in a solution of sodium thiosulfate:

4. Washing away the dissolved silver salts and drying the negative.

5. Printing, that is, shining diffuse light through the negative onto a sheet of sensitive photographic paper (a gelatin emulsion on paper).

6. Darkroom development of the exposed paper using developer solution much like that used in the film development step. This step produces a positive image, in which the tones are like those in the original scene.

7. Fixing, washing, and drying the print as in the analogous film processing steps.

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