Last week I had the opportunity to lead a film developing demo at Artomatic, which has sadly now ended. In preparation, I attempted to compile as much science behind the process that I could find, particularly with regard to the chemical reactions occurring in each step (I was a chem major for six semesters, afterall). However, I felt that it was too nerdy and brain-hurty and scaled it down into more general terms (I'm more of a bio person, anyway). I am still interested in how the image appears like magic on the film and how all the solutions work together, so here's an overview:
Photographic film is a strip plastic-type material coated with a light-sensitive material.
Traditional black & white film consists of 3 main layers:
Film Base – plastic or polymer such as nitrocellulose, cellulose acetate, or polyester (PET); back in the day it was glass
Emulsion – light-sensitive silver halide crystals in a semi-permeablegelatin that is layered on the film base
Anti-Halation Layer – coating that absorbs light & prevents it from reflecting back into the emulsion to cause halos/ghosting
How does it work?
When film is exposed to light, it starts a series of reactions with the silver halide crystals within the emulsion. Each photon of light that hits a crystal separates the silver from the halide, which absorbs it and frees an electron.
Ag+Br- (crystal) + hv (radiation) --> Ag+ + Br + e-
The silver ion then combines (is reduced) with the election to yield a free silver atom (metallic silver):
Ag+ + e- --> Ag0
(Ag0 = latent image)
The latent image is similar to “latent fingerprints” that are there waiting to be found or imaged.
What does this look like on the negative?
Each crystal comprises the film grain and can contain many molecules of silver halide. The longer the exposure, the more free silver is produced on each grain, creating a darker area, or density variation. Dark areas of the negative have the highest density of opaque silver atoms (the greatest light exposure) and clear areas contain no silver (no light exposure).
Next week, we'll take a look at how the image is made visible on the film.