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Why does developer need a specific temperature?

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All chemicals have temperature ranges in which they perform certain tasks optimally. Water, for instance forms ice/cubes when its temperature drops to 32 degrees farenheit; washes dishes and cleanses our bodies best between 72-100 degrees, and boils turning to steam at 212 degrees. As commented above, developers have optimal ranges in which they perform certain chemical reactions. Warmer temps accelerate the action and produce different results, just as colder temperatures result in longer development times. All just basic chemistry and physics.

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Many chemical reactions follow Arrhenius, which gives a relationship

between temperature and reaction rate.  Developers do this well,

so that one can make a chart that works for pretty much all developers.


For many years, the standard for black and white films has been 68F or 20C.


At higher temperatures, the gelatin softens too much. 

Films from about 1930's used a lower temperature.

From about the 1960's, they were allowed up to about 75F or 24C.


C41 and E6 are at about 100F or 38C.  The gelatin is hardened to

allow for that temperature.


Also, at higher temperatures, the times get too short for accurate

hand processing.  At cooler temperatures, they take too long, and

most won't want to wait around that long, especially agitating

every 30 seconds.

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-- glen

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There's also the complication that many developers use a combination of reducing (developing) agents - Metol and Hydroquinone or Phenidone and Sodium Ascorbate, for example. Those chemicals have slightly different temperature co-efficients, and a gross variation of temperature can cause their interaction to get out of balance; causing a change of characteristic. Resulting in too 'soft' or too 'hard' an image contrast. 

While colour film developed at the wrong temperature leads to a colour imbalance, since the colour layers develop at different rates. 

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Modern photo films consist of a transparent film base topped with what we call an emulsion. The emulsion coat is highly purified gelatin. Gelatin under the microscope resembles a tangle of spaghetti-like long chain molecules. The light sensitive compounds, silver bromine, silver chlorine, and silver iodine, consist as tiny crystals imprisoned by the spaghetti-like tangle. 

The gelatin binder is chosen because it is transparent, flexible, plus not very soluble in water. However, gelatin soaks up the waters of the developer (mainly water) and this causes the gelatin binder to swell much like a dry sponge plunged into the bathtub. 

The now swelled gelatin allows the solutions of the photo developing process to infuse and percolate about. This is important because fresh developer enters the structure and is quickly exhausted in areas where exposure is high. Because the wet film has a swollen binder, agitation supports the entry and exit of the fluids of the process. 

In the camera, the silver salts have received an exposure to light (the latent image). This light energy was insufficient to cause the latent image to be visible. The developer is a reducing agent that seeks out those silver salt crystals that have been exposed to light. The developer is selective; it reduces only the exposed crystals, fracturing them into their two component parts. The silver part is thus separated and become a tuft of metallic silver which is opaque -- thus it appears black. The other part, called a halogen (Swedish for salt maker) dissolves away in the waters of the developer. Again, unexposed silver salts remain unscathed.  The image you see in black & white film and prints is made up of countless tufts of metallic silver. 

Color film works much the same except additional steps will imbed dye adjacent to the tufts of silver. In the back & white process, we now remove the unexposed and thus undeveloped silver salts. This is the job of the fixer (to render permanent). In the color process, the silver image is chemically changed back to a sliver salt using a solution called a bleach, which leaves the dyes intact. Follow the bleach with a fixer and the color film consist of dyes imbedded in gelatin. 

To answer your question more completely: The swelling of the gelatin to allow the fluids in and out is a timed reaction based on the temperature of the fluid. Also, chemical reactions are accelerated by a warm environment. 

A remarkable fact about gelatin: it shrinks back to normal size as the film dries (like a sponge).

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Yes, it was gelatin that made photography what it is today.

I have only read about it, and mostly not thought much about it, but before gelatin we had albumen.

That is what really got photography going, but is much less convenient.

When albumen dries, it stays dry, and doesn't soften up and allow chemicals in.

So, albumen plates (before plastic base) had to be exposed and developed while wet.

People had to carry a darkroom around with them.

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-- glen

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