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The E-4 process is a now outdated process for developing color reversal (transparency) photographic film, which was introduced in 1966.
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- See also Ektachrome for full details of Kodak E-series processes.
The process is infamous for two reasons:
First, it uses the highly toxic boron hydride-based reversal agent tertiary butyl-amine borane (TBAB).[lower-alpha 1][1]: 379, Table LXVI Early releases of the consumer-sized version of the chemistry provided the TBAB in the form of a tablet, possibly to avoid the possibility of inhalation.[2] This was later changed to loose powder, likely as a countermeasure against inadvertent ingestion of the substance.
Second, the prehardener agent contains formaldehyde and 2,5-dimethoxytetrahydrofuran,[1]: 377, Formula 269 which when mixed generates succinaldehyde, a noxious gas which has been likened to tear gas.[2] Process E-6 films are hardened during manufacture, eliminating the prehardener step altogether and allowing them to be processed at 100 °F (38 °C).
Ektachrome film structure and exposure
Structure
Sample exposure to various colors
Ektachrome film has three separate light-sensitive layers; each layer is sensitive to a different group of wavelengths corresponding to red, green, and blue colors. When the film is exposed, each layer records a latent image based on its sensitivity. A yellow filter prevents blue light from exposing the green- and red-sensitive layers, which have some sensitivity to blue light.[3]
The E-4 process is faster than E-3; whereas E-3 required 15 steps and up to 70 minutes from start to finish,[2][4]: 30–31 E-4 was completed in approximately 50 minutes over 13 steps.[5] E-4 runs at 85 °F (29 °C),[5] about 10 °F (6 °C) higher than E-3. The temperature tolerance is ±1 °F for prehardener, ±1⁄2°F for the first developer, and ±2–5 °F for all other steps.[5] The ME-4 process was a motion picture variation of the E-4 process.
The major change for E-4 was the inclusion of a chemical reversal agent, which permits processing of the film without the manual re-exposure/fogging step required by the predecessor E-1 / E-2 / E-3 processes.[2][5]
Total darkness is required during the first four development steps; normal room light can be used for the remaining steps.[5]
| Step | Schematic | Time (min.) | Temp. | Description | ||
|---|---|---|---|---|---|---|
| 1 | Prehardener | 3 | 85 °F (29 °C) ±1 °F | Tempers film for high-temperature processing | ||
| 2 | Neutralizer | 1 | 83–87 °F (28–31 °C) | |||
| 3 | First developer |  |
7 | 85 °F (29 °C) ±1⁄2°F | Conventional black-and-white developer used to transform silver halide crystals exposed in all three layers as a negative image. | |
| 4 | First stop bath | 2 | 83–87 °F (28–31 °C) | Solution should not be reused for second stop bath (step 7) | ||
| 5 | Wash | 4 | 80–90 °F (27–32 °C) | Running water | ||
| 6 | Color developer |  |
9 | 83–87 °F (28–31 °C) | ||
| 7 | Second stop bath | 3 | 83–87 °F (28–31 °C) | Solution should not be reused from first stop bath (step 4) | ||
| 8 | Wash | 3 | 80–90 °F (27–32 °C) | Running water | ||
| 9 | Bleach |  |
5 | 83–87 °F (28–31 °C) | Convert metallic silver to soluble particles | |
| 10 | Fixer |  |
6 | 83–87 °F (28–31 °C) | Dissolve silver particles, which can be recovered after processing | |
| 11 | Wash | 6 | 80–90 °F (27–32 °C) | Running water | ||
| 12 | Stabilizer | 1 | 83–87 °F (28–31 °C) | |||
| 13 | Dry | var. | <110 °F (43 °C) | |||
E-4 processed film is color stable for about 30 years.[6]
The process largely was phased out in 1976 with the introduction of the E-6 process, which is more environmentally friendly due to its lack of toxic chemicals. E-6 avoids the use of TBAB by adding a separate reversal bath containing the tin salt stannous chloride.
The E-4 process has been discontinued since 1996; after 1976 it was used solely for Kodak IE color infrared film,[7] due to a legal commitment by Kodak to provide process support for 30 years after introduction. Kodak discontinued E-4 processing in 1985, but independent photofinishers continued to support the process.[8] The E-4 chemicals were reverse-engineered and substitute formulae were published in the British Journal of Photography Annual in 1977.[1]: 374
- Not to be confused with tetra-n-butylammonium bromide, which also is abbreviated as TBAB.
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