I know that black and white film uses silver bromide. Could another combination be used; such as magnesium bromide? Why silver? I'm really interested in how magnesium could be used to produce photographic images.

Response:
The short answer is I'm afraid, for all practical purposes, "No". This is partly because silver salts were observed to have unusual properties, very early in the history of photography. This apparently special behaviour concentrated peoples' efforts on silver halides and as a result, their behaviour as photographic materials rapidly improved. Perhaps if they hadn't been so much better than anything else, the search would have been wider for longer and other materials may have been found and persevered with. My feeling though, is that even if other systems had been worked on for longer, silver-based systems would still have come out on top.

It is worth reminding ourselves what silver halide crystals are now expected to do. We will then appreciate how difficult it would be for other materials to perform the same combination of tasks. Firstly, they have to respond to very low light levels. Even in practical systems, many crystals will respond to fewer than 10 photons, perhaps even as few as 3 or 4. Having responded by producing a permanent change which can be detected somehow, this so-called "latent image" must be stable until detected, possibly for months. The detection process must distinguish between exposed and unexposed crystals. Also, the materials must be stable for years before being exposed. Let's look in more detail at these requirements by looking at the mechanisms involved.

I realise that other materials could operate with different mechanisms, but whatever is used there has to be a permanent, detectable change as a result of the absorption of very few photons. This means that within each responding element, a crystal, in the case of silver halide at least, there must be communication between parts of the crystal. Sometimes the crystals are too large to be considered as a single point in space as far as the absorbed light is concerned.

The communication is largely electronic but some ions also move about. The process of absorption of light results in the production of an electron in the conduction band of the crystal. Clearly the availability of a conduction band, ie a set of de-localised orbitals with band of energy levels into which an electron can be readily promoted by the light-absorption process, is a primary requirement. This is a property common to photosensitive semiconductors such as silicon, gallium arsenide or zinc sulphide. These are examples of 4-valent, 3-5 and 2-6 type semiconductors and silver halides can be considered to be 1-7 semiconductors !

The silver halide crystals need to be sensitive to well-defined spectral regions. Even for B&W photography they must have a spectral response which is nearer to the human eye's response than the silver halide absorption spectrum in order to render the luminance information in the scene accurately. This requires the use of dyes which adsorb onto the crystal surface. The light is usually absorbed by the dyes and the resulting excited state of the absorbing dye molecule interacts with the crystal to produce an electron in the conduction band. Although this ability to be spectrally sensitised is a requirement of the silver halide system, it is not unusual for semiconductors to be sensitised by dyes in this way.

The photo-electron in the conduction band is soon trapped, usually by a dopant introduced during manufacture of the finished photographic emulsion ( the term "emulsion" is used in photography to describe the coated layers or in photographic science to describe the dispersion of silver halide crystals ). The dopant used for this purpose is, for example, sulphur which provides a suitable trap depth. Once the electron has been trapped it can combine with a mobile silver ion to produce a metallic silver atom at the trap site.

Although this is another requirement, there is a wide choice of chemicals available to provide traps so other systems could be doped for this purpose. The availability of mobile silver ions within the crystal structure is however very important. A single silver atom produced from the trapped electron and silver ion, is not very stable and could lose the electron and therefore the effect of light absorption. However, if another photon is absorbed and another electron is trapped at the same site, the process of silver ion migration and reduction leads to a cluster of two silver atoms which appears to be relatively stable. The stability of these very small latent image specks is vital to the effectiveness of the crystal as a sensitive recording medium but if the metallic state were too stable relative to the ionic state, the reduction process might happen without light and the material would not be useful.

The last requirement is that the pattern of latent image which reflects the original pattern of light must be detectable. We cannot see the latent image in any normal sense and its presence must be revealed somehow. In the case of silver halide photography, the detection process relies on the difference in the rates of reduction of an exposed crystal and an unexposed crystal. The reduction process involves treatment of the crystal with a solution containing a reducing agent ie. a "developer" solution containing a "developing agent". The developing agent provides electrons to the crystal and the process of silver formation proceeds. Crucially, the kinetics of the process depend on the presence of the latent image and must not proceed at a significant rate if the crystal is unexposed. In other words, crystals with only a few atoms of silver metal amongst the millions of silver ions in the crystal, must be reduced to silver in often less than a minute while crystals with no photolytic silver must remain unaffected. This alone is close to a chemical miracle. It's no wonder silver halides are unique !

The modern silver halide crystal used in photography can contain a number of other dopants, often metal ions. The function of these is to control the response to light in a number of ways, not just to improve sensitivity, but, for example to control contrast (the dependence of the photographic response to total exposure ) or to reduce the dependence of the response to light intensity. The halides used are also important design parameters, as are precipitation conditions.

Other aspects of the design of photographic materials have evolved around the needs of silver halides. This makes it increasingly difficult to find alternatives and provided silver-based systems continue to improve as they seem to be doing still, there is no strong incentive to look for alternatives. Catching up with the silver halide technology after the long process of the refinement of the formulations would be extremely difficult.

Despite these arguments, there have been other candidates for example cuprous oxide ( a 2-6 semiconductor ). Lead has also been investigated. I don't know whether magnesium has ever been considered. For systems similar to silver halide, there needs to be a reasonably stable M+ state, a requirement not met by magnesium.

Sorry to be so negative ( excuse the pun ! ) about non-silver systems. None has yet been found which comes close to the performance of silver halides. What about digital photography ? you might ask. Imaging chips use semi-conductor sensors in a completely different way. In some ways they have advantages over silver halide but in others, affordable systems are still deficient in image capture capability. But that's another long story.