Behind the Polaroid Instant Imaging System…
htmlguru,
The encounter with those two papers leading to the Gevaert instant imaging system I hinted at above could only be traced back by me to this quotation:
“In 1938, André Rott, Gevaert R&D, observed a positive image on a baryta-coated base by the transfer of silver from a negative.”
This is far less accidental than I stated before. But, in case of a planned experiment, wouldn’t it be more apt to employ transparent film (with nuclei)? However it could be that outfixed paper still contains enough apt nuclei to let migrating halide form an image at the presence of developer…
(I’ve got a reproduction of one his results, looks a bit like lift-off transfer… A reproduction of an Edith Weyde result looks more like that accidental image transfer…)
Obtaining reversal images:
Land refers to 4 principle ways to gain a reversal picture:
-) exhausted developer (eg. Polacolor)
-) oxidized developer (eg. Cibachrome)
-) soluble silver complex (eg. Polapan)
-) coupler (which is rather a variation of the above)
-) I would add the Waterhouse reversal process
where a standard process competes with a fogging process. The bromide released by the initial development which should start at the exoposed halides hampers fogging. Thus development of the unexposed and unrestraintely fogged areas should prevale.
The Polaroid techniques can all be classified under,
`Image Diffusion Transfer´ or `Diffusion Transfer Reversal´
The B&W
process is based on the subsystem of
`Silver-Halide-Diffusion´
The idea behind it is to produce a negative, dissolve the remaining halide, letting it migrate into a receptive layer, preparing it for development, developing it and gaining a positive on this second layer.
For these processes often a mono-bath is used containing the developing agent, alkali and a halide solvent. The developing agent and the solvent can even be incorporated in the negative emulsion.
This technique makes for the second development use of a variant of solution-physical-development. Its more known counterpart, the chemical development, means that, starting from a nucleus caused by radiation exposure (or fogging), a reducing agent transfers all the silver-ions in that grain! into metallic silver. The solution-physical-development, however, means reducing nuclei after the very grain has already been greatly dissolved by a subtle halide dissolving agent. The resting silver is thus not longer delivered from the grain directly, but from the dissolved halide. (The name is due to a wrong theory on the kind of silver precipitation.)
In the silver-halide-diffusion process there are of course no nuclei, as the halides are originating from unexposed image areas. In order to make them developable in the receiving sheet fine spread metallic particles are incorporated which will form nuclei with the diffusing halides. Thus a solution-physical-development can take place.
Images made out of diffused silver have higher covering power than images originating of directly developed, which means less silver has to employed for the final image.
The Polaroid technique added to this all the feature of instant development
(fast, chemicals in pods etc., later even self-controlled with SX-70).
The history behind the silver-halide-diffusion
1857 B. Lefévre
1898 R.D. Liesegang
1938 André Rott (Gevaert) B.P. 614,155 / 1939
1941 first commercial product ( “Transargo”, Gevaert)(two sheets)
1941 Edith Weyde (I.G. Farben /Agfa) D.R.P. 887,733 and US Pat. 2,875,052 (noble metals as nuclei)
4 mono-sheet processes (The receptive layer is positioned between the base and the negative; after development the obsolete negative layer is taken off.):
1942 air-reconnaissance film, Agfa
1942 “Veriflex”, Agfa
1947 “Diaversal”, Gevaert
1947(?) “Contargo”, Gevaert
1947 André Rott (Gevaert) US P. 2,665,986 1947 (where he describes silver-halide-diffusion yielding mono- and tri-chromatic positive images)
1947 Edwin H. Land (Polaroid) US P. 2,698,238 (lead sulphide, cadmium, lead and zinc salts as nuclei) this patent can be used as cooking book. In the Neblette, 7ed. Land describes a lot of the problems he had to overcome in precipitating the silver.
(Off topic: Image transfers based on migrating developing agent have been described 1898 and 1922)
The Colour
processes fall in two categories:
The Polachrome
is just a variation on the silver-halide-diffusion mono-sheet process combined with a filter screen
It employs a panchromatic layer coated on a linear additive screen. It lacks a separate receptive layer; but this layer is coated between the screen and the negative. For processing a second film strip is laid against the first film with processing chemicals being spread between them. After processing and forming of a positive image behind the screen by those migrated and developed halides, the second film is torn off and takes with it the obsolete negative sticking to it.
(The Polavision
seems to have been the same process however the negative is said to be left in place but to be bleached out somehow(?))
The Polacolor
is a dye-diffusion two-sheet technique
A developing agent fixed to the very layer is employed, with a dye coupled to it. Being of a colour complementary to the colour the adjacent halide layer is sensitive to. This agent being oxidised, after reducing the exposed halide, has become soluble and migrates with its attached dye into the receptive layer, where they are fixed to form an image in the kind of the imbibition technique.
The developing agent is Hydroquinone which is soluble in alkali but oxidises to quinone which is insoluble. In common development processes the latter is overcome by the presence of sulphite. This is lacking in the Polacolor process. The addition of the dye does not change these characteristics. To avoid diffusion of the hydroquinone-dye couple into neighbouring layers the moment the alkali is brought upon, spacing layers are employed.
However, in the literature it is not stated how it is overcome that those dyes do not form a filter layer which would spoil the exposure of the three light sensitive layers.
The integral films SX-70 etc.
employ a dye-diffusion mono-sheet technique
In contrast to the other mono-sheet techniques the negative remains and is not bleached either. In principle it is based on the Polacolor process. The negative emulsion is coated onto a black base, the receptive layer is transparent. To avoid post-exposure when outside the camera for development, a TiO2 layer is squeezed between the layers with the chemicals. It will form the optical base for the image. As there is no parting of the layers it must be self-controlled.
Just prior to uploading I found a hint at a book I never had in my hand, nor was it referred to in the literature I got (actually I just realized I overlooked the hint in the Neblette)…:
Photographic Silver Halide Diffusion Processes
by Rott / Weyde, 302 pages, Focal Press (1972), ISBN-10: 0240507266, ISBN-13: 978-0240507262
