basic description of the E6 process

[richard p. t.]

Someone recently asked me in an email what the various baths in the E6 process do. Below is my reply (with a couple of minor additions).

All photographic emulsion involves molecules of silver-halide (which are a
creamy colour). These molecules form into crystal shapes. Clumps of
these crystals form the grains you can see in film. The molecules
comprise two atoms (this is a simplification) - one of silver, and one of
a halide like Bromide. These molecules are a bit unstable. When they are
exposed to light, they break apart again into the two atoms - silver and
bromide. Now the molecule in its natural state was a creamy colour.
Silver on its own, however, is a black colour (it needs to be polished to
be shiny like on a silver spoon). So when the molecule of silver halide
breaks appart into its constituents of Metalic Silver and the halide
(Bromide), then there is a transformation from a creamy colour to black.
O.k., so one molecule has broken into two atoms and there is a change of
colour, but you can't see it because it is so small. When you add
developer to the exposed film, a chain reaction happens and that tiny
change gets spread to all the other molecules on that crystal. The whole
crystal then converts to metalic silver. Now you can see it. That is the
magic of photography. And that is also why all film is first and foremost
negative - where light hit, the silver hailide (which was creamy coloured)
goes black. Light makes a change from cream to black. Where there is no
light, it stays creamy. Yes, when you look at unexposed film, it doesn't look creamy.
That is because the emulsion on normal panchromatic film has various dyes and other
ingredients added to it. The dyes are about spectoral sensitisation. Silver-halide molecules
are mostly just sensitive to blue (and uv) light. By adding spectral sensitisation dyes
the emulsion is able to be made sensitive to green (making it orthochromatic) and red (making it
panchromatic). These dyes make the emulsion look darker. There are also dyes added for
anti-halation purposes.
Back to the story. Colour film works the same way, except there are other chemicals mixed in
the emulsion besides silver and the halide (Bromide) and the spectral sensitising dyes. These other
chemicals (called colour couplers) are able to form a coloured dye when colour developer is added -
but the dye only forms where there is the chain recation going on that
converts silver-halide to silver and Bromide. So, the dye only forms
where the silver image forms. pretty clever hey. Those dyes are called
'chromagenic dyes' and they only form when the developer used has the
necessary extra ingredients. Process colour film with black and white
developer and all you get is a silver image - no dyes form.
So, with the colour reversal process, you want colour positive images. So
you don't want dyes to form from the first developer. The first developer
is just a strong black and white developer. You can use a developer like
D19 instead of the E6 first developer - you just need to use it for a long
time as it isn't as strong as the E6 first developer. But basically, the
E6 first developer is just a strong black and white developer. When the
film goes in the first developer, a normal silver negative of whatever was
filmed is developed on the film. So now the film has silver where light
hit, and it still has silver-halide where light didn't hit. Those bits of
remaining silver-halide is precicely what will be converted to silver and
colour dyes during the second, colour developer. As with all reversal processing,
the amount of silver developed in the first developer will determine how much
silver-halide is left to develop in the second developer. Since the amount of silver that developes
in a developer is dependent of not just the exposure of the film, but also the
temperature of the developer, the time of development, and the adgition of the developer
(which moves the liberated halide away from the surface of the film), the first development
phase of the E6 process is the critical phase for determining how bright or dim the reversal image
will be. The condition of the first development phase are thus critical in reversal processing.
The second developer (in both colour and black and white reversal) simply has to develop ALL of the
remaining silver halide. It is a development to completion. You can't over second develop.
The second dev is a chromagenic developer. This means it develops not ony silver, but has the
added ingredients to make the dye form on colour film. However those
remaining silver halide chrystals (the ones that didn't develop in the
first developer) still need to be exposed to light in order to develop.
otherwise, just as they didn't develop during the first developer phase,
they wouldn't develop during the second developer stage. You could pull
the fill out of the tank and turn the lights on as you do with black and
white reversal film. Instead, however, the E6 process uses a chemical
fogging agent. This basically chemically exposes the film to light. The
3 bath version of E6 like the Fuji or Tetenal you are using has this
chemical fogging stage mixed in with the second developer. The 6 bath
system has the chemical fogging agent as a separate bath. That is called
the 'reversal bath'.
So now, after second development, the film has the original silver
negative, it now also has a silver positive image, and also a positive dye
image. The two silver images - the negative and the positive - add up to
all the silver-halide that was on the film now being converted to metalic
silver. So there is black silver everywhere. No light would get through
the film if you projected it now. So that silver needs to go. That is
where the Blix comes in. The 'Blix' is a mix of Bleach and Fixer. In the
black and white reversal process, the Bleach is able to disolve metalic
silver, removing it from the film altogether. That would work here. But
the colour system uses a different kind of bleach instead. This is
because the black and white bleach is environmentally dodgy, but it is the
only way to get rid of the silver altogether without touching the
remaining silver halide (which is still there during the bleach phase of
black and white reversal processing). The colour bleach, on the other
hand, doesn't dissolve the silver, but rather converts it back to
silver-halide - ie, it mixes it with halide (Bromide) again and it becomes
creamy once more (it also becomes sensitive to light again but that fact
isn't used here). Then, as you know, fixer is a solvent for
silver-halide. Put undeveloped film in a bath of fixer and all the
emulsion comes off. So, put our second developed colour film that has
been colour bleached (ie, all the metalic silver converted to silver
halide) in fixer, and all the silver halide comes off. Now we are left with just
the dye. In the 6 bath E6 process, the bleach and the fixer are separate
baths. In the 3 bath system, they are mixed together for convenience. In the 6 bath
process, there is a pre-bleach bath. This speeds up the bleaching process,
as well as containing the dye stabilizing preservative (formaldehyde). In the 3 bath process,
the formaldehyde is in the wetting agent bath which comes at the end.
From the above, you can see that the primary difference between the 3 bath and the 6 bath
E6 processes is that in the 3 bath, the reversal agent is mixed with the second developer, and
the bleach and fix is mixed together in a blix, and there is no pre-bleach. However the
chemicals that determine the quality of the image are the same - the first developer and the
chromagenic second developer.

Here endeth the lesson

[jeddytrois]

Wow. This is my first post at filmshooting.com. I registered really just to say thanks so much for that lecture. I've been wondering about the chemistry behind the scenes and you covered all of the basics. Wonderful, thanks so much. Now that I'm here I'll check out the forum. Thanks again!

[richard p. t.]

Well thanks for taking an interest!
cheers,
richard