[rollei_list] Re: Old film

----- Original Message ----- 
From: "John A. Lind" <jalind@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, May 30, 2005 9:09 PM
Subject: [rollei_list] Re: Old film


> At 12:00 PM 5/30/2005, Richard K. wrote (in part):
>
>>I don't know why Kodak has been trying to kill off 
>>Kodachrome but they
>>have for a long time. Probably supporting the special 
>>processing is the
>>problem. Kodachrome evidently has excellent dark storage 
>>properties while
>>Ektachrome type films have better resistance to fading 
>>under projection.
>>I've seen quite a lot of Kodachrome that still looks 
>>pristine after 60 years.

  The stability of incorporated coupler color films has been 
very substantially improved over the years. Modern color 
films probably have a lifetime of 50 or more years before 
dye fading becomes too great to get good prints or reproduce 
them. The early films had lifetimes of no more than 15 
years.
   All color films are B&W to start with. The color is 
generated by a complex reaction during development. 
Incorporated coupler films, often called chromogenic films, 
have dye generating chemicals called couplers in them. These 
couplers are anchored so that they can not diffuse either 
from layer to layer or within a particular layer. Agfa 
invented the method of attaching the coupler molecule to 
very large long chain molecules that are too big to move 
around in the gelatin. This method is the one currently used 
in all color films that have couplers. Kodak came up with a 
method of encapsulating the coupler molecule in a resin. The 
resin allowed penetration of processing chemicals but did 
not allow the coupler or the dye that resulted from it to 
escape. The resin itself could not move around in the 
emulsion.
   The couplers produce dye in combination with the reaction 
products of certain kinds of developers, thus there is a 
complex reaction: the developer reacts with the exposed 
halide to produce silver and also a reaction product. That 
reaction product further reacts with the coupler to produce 
the dye. In reversal films the first developer is of a type 
which does not generate the necessary kind of reaction 
product so no dye is generated. The second developer is of 
the right kind so a positive dye image is generated by it. 
In color films the silver image produced by the developers 
is unwanted and is removed by a bleach that does not affect 
the dyes. Thus, although the original image is a silver 
image just as in B&W film, none of the silver is left after 
processing. Large labs recover the silver. Nearly all the 
silver used in the manufacture of the film can be recovered 
after processing.
   The "amateur" photographers to whom Kodachrome is 
attributed were Leopold Mannes and Leopold Godowsky, both 
well known classical musicians. Mees, in his farewell book 
on retiring says that the joke at Kodak was that Kodachrome 
was invented by God and Man. In fact, in Joseph Fiedman's 
book on the history of color processes, he says that 
probably Mannes and Godowsky were responsible only for the 
differential bleach penetration process. I don't have the 
book in front of me and am not sure I remember names 
properly but the original idea for the generic process 
probably dates from work done in the 'teens. The name of the 
original investigator is right on the tip of my brain, how 
frustrating.
   The differential bleach process was extremely complex and 
required astonishing control over all sorts of variables. 
The film had to be dried between each step to insure the 
rate of diffusion would be predictable. Evidently, the dry 
film was floated on the surface of the bleach until it had 
penetrated the necessary layers. The bleach removed the dye 
and also converted the silver image back to a halide. It was 
probably a combination of ferricyanide and something else. 
The differential re-exposure process is the one currently 
used. Kodak changed from the original process to the current 
one perhaps a year and a half after releasing Kodachrome to 
the market. Initially, Kodachrome was available only as 16mm 
motion picture film. Later, probably after the conversion to 
the new process, it was made available in 35mm still film, 
various sizes of roll film, and sheet film up to 16x20, all 
processed in Rochester by Kodak.
   Because the couplers are in the second developers rather 
than the film Kodachrome must go through four developments. 
Kodachrome consists of three layers of B&W emulsion with a 
yellow filter layer made of colloidal silver under the top 
sensitive layer. It probably also has separator layers 
between the emulsion layers. The top layer is sensitized 
only to blue light. This is the blue recording layer. The 
yellow filter under it removes blue light from the rest of 
the film. the second layer is orthochromatic, sensitive to 
blue and green, but, because of the yellow filter it records 
only green light. The bottom layer is a specially sensitized 
emulsion that is sensitive to blue and red, but not to 
green. There are a number of sensitizing dyes that will 
produce this effect. In fact, if you look at the spectral 
response of most panchromatic films you will see a dip in 
the green region. In any case, the bottom layer is the red 
recording layer. In order to have a positive color image the 
three layers must be reversed and the proper dye generated 
in each layer. The top layer is yellow, the center layer 
magenta, the bottom layer cyan.
   In the current process the film is first developed in a 
plain developer so that there is a negative silver image in 
all three layers. This developer is such that it does not 
destroy the red sensitization of the bottom layer. The first 
reversal step is to flash the film with red light through 
the base. this exposes only the bottom layer because the 
other layers are not sensitive to red light. The film is 
then developed in a developer containing a cyan coupler. The 
developer is of the sort that generates reaction products of 
development which will further react with the coupler to 
make the dye. Since the only areas of the emulsion which can 
be developed are those which were not developed during the 
first development, a positive silver image and a coincident 
positive dye image result. The film is rinsed and then goes 
is flashed with blue light from the front. Since the yellow 
filter layer is still intact the blue light exposes only the 
top layer. This is then developed in a developer containing 
the yellow coupler. Now the process changes. the center 
layer can not be flashed because the top and bottom layers 
have now had all the silver in them developed so are quite 
opaque. So, the third developer contains a chemical fogging 
agent along with the dye coupler. Since the only developable 
halide left is in the center layer the fogging agent can not 
have an effect on the other layers. The center layer is 
developed with a coupler for magenta. At this point a 
complete positive dye image in complementary colors exists 
in on the film and all the halide has been developed to 
metallic silver. The next step is to remove the silver using 
a bleach which does not affect the dye image. This bleach 
also removes the yellow filter layer, which remember is also 
made of silver. After this step the film contains a full 
color positive image. The film is then washed and probably 
stabilized and is finished.
   The original process was much like this except that the 
three color developers could work in all three layers. After 
initial development the negative silver image and yellow 
filter layer was removed leaving the remaining halide. Then 
the entire film was fogged with light. Then the film was 
developed in the cyan coupler developer. This produced a 
cyan image in _all three layers_. The film was washed and 
dried. Then it was floated on a bath of a bleach which 
penetrated the two top layers, removing the cyan dye and 
converting the metallic silver image back to halide. Note 
that the bleach was so closely controlled that it did not 
penetrate to the bottom layer in which the cyan dye and 
silver image remained. The film was then developed in the 
second developer resulting in a magenta image in the two top 
layers. Again the film was washed and dried. Then it was 
bleached again, this time in a bleach which penetrated only 
the top layer. It was then developed in the final bath, the 
yellow coupler developer. After this a full color dye image 
was on the film along with the positive silver image. the 
silver was removed by a bleach which did not affect the dye 
and the film finished. Whew!  While both of these processes 
are fussy the original one worked mainly because some very 
dedicated people made it work.
   For a time Ilford made a reversal color film similar to 
Kodachrome but it used very thin layers of pre-fogged 
emulsion to separate the layers. During the first 
development these layers became opaque. The top and bottom 
layers could then be selectively fogged with white light and 
reversed in the color developers. The center layer, as in 
Kodachrome, was reversed by a developer containing a fogging 
agent or by the use of a separate fogging bath. The opaque 
silver separator layers were removed along with the image 
silver and filter layer. I don't think Ilford made this 
material for very long and I have no idea of what it looked 
like.
   There have been an enormous number of attempts at full 
color photography beginning almost as early as photography 
itself Hand colored Daguerreotypes are not uncommon and 
various color separation and chemical processes were tried 
well back in the 19th century. Very few of these processes 
had any success or even the possibility of success. Some 
were eventually successful but could not be made to work at 
the time of their invention because the technology and 
science if photography was not sufficiently developed. Now, 
of course, color is routine but I think we should remember 
the perfectly tremendous amount of effort by a great many 
men and women it took to get us here.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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