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I'm interested in knowing about the various processes that have been used in chemical photography. As I understand it, most are based on silver. Are there any major ones which have not used silver? What other metals have been used? Are there any processes which don't use metals at all?

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    $\begingroup$ TEM film is/was barium based. $\endgroup$ – Jon Custer Sep 7 '18 at 14:04
  • $\begingroup$ Thanks but can you clarify that or provide a reference? I'm assuming TEM means "Transmission Electron Microscopy"? And the barium was part of a chemical reaction, not an inert substrate layer? $\endgroup$ – Metamorphic Sep 7 '18 at 18:44
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I wanted to contribute what I know so far:

  • Most photography processes are based on silver halides, which are light sensitive, decomposing into metallic silver and halogens. Here is a claim that silver salts are the most light-sensitive, but it is not well-supported. Silver processes include black and white, as well as more complex color processes like E-6 and C-41 which use dye couplers and color-sensitive layers. I don't know what makes the layers color sensitive, but the reaction is like $$\ce{AgCl + photon -> Ag(s) + {1\over 2}Cl_2(g)}$$ Apparently, after a photon is received, freeing a silver atom, this atom must combine with another free silver atom through diffusion over the lattice of the silver halide crystal before it can become part of the image. There may be a better reference, but this fact is mentioned in the photographic developer Wikipedia article.

  • Platinum and palladium photography exists. These are also based on metal halides, but they usually include ferric oxalate as a sensitizer. Also, they appear to be based on alkali salts like sodium tetrachloropalladate. Palladium is above platinum on the periodic table, and was used as a cheaper alternative. I saw some old black-and-white photographs in a museum recently, which were labelled as platinum or platinum-palladium prints.

  • Other metals with light-sensitive salts include mercury and gold, but I am not aware of these being used in photographic paper (although mercury saw early use as a developer in the Daguerreotype process). These undergo a different reaction than silver halides, at least in the case of mercury - undergoing disproportionation rather than releasing free halogen.

  • The Cyanotype process is based on iron, using both ferric ammonium citrate and potassium ferricyanide. It is the source of the name "blueprint". The light-sensitive compound is ferric ammonium citrate; the citrate reduces the ferric ion under exposure to UV light. The resulting ferrous ion then reacts with potassium ferricyanide to form insoluble ferric ferrocyanide (Prussian blue).

Light-sensitive compounds outside of chemical photography:

  • In biological systems, chlorophyll is mostly organic, but contains magnesium in a key role. This contrasts with the opsins such as rhodopsin which are responsible for vision. They are proteins bound to cofactors, I think the cofactor is always retinal which is organic - so no metals in vision as far as I know.

  • Light-sensitive synthetic organic molecules exist, such as aryl phthalate esters. However, I am aware of no photographic process based on organic molecules.

  • Glow-in-the-dark toys are usually based on metallic phosphors such as zinc sulfide and strontium aluminate.

  • Wikipedia has a category of light-sensitive chemicals which seems under-populated, but includes many inorganic compounds, as well as hydrogen peroxide and LSD.

  • Digital photography of course uses metals, with CCDs containing capacitors formed from metal-oxide-semiconductors. Whether this can be usefully related to the chemistry of chemical photography I do not know.

I'm not sure what the overall conclusion is. Why metals are important but not necessary; why silver seems to be uniquely sensitive to light, and how to classify and understand this uniqueness...

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