Mike mentioned the professional literature on dye sensitization associated
with the photo-hardening of colloids. Just recently I've been looking into
dye sensitized vinyl monomers as an alternative to dichromated gelatine. It
sounded very simple: a vinyl monomer in aqueous solution (7% acrylonitrile
in water), a photoreducible dye (rose bengal), a mild reducing agent or
electron donor (ascorbic acid) is exposed through the bottom of something
like a glass tray using a powerful slide projector, or contact printed
sitting on an overhead projector. Development involves washing away the
unexposed monomer with cold water. Unfortunately after I looked up the
toxicity data for acrylonitrile, it didn't seem quite so simple (8-(. There
may be other safer monomers (acrylamides, for example) that might also work,
so if there is anybody on the list who has any experience with this sort of
polymer chemistry I'd appreciate hearing from you.
The dangers of acrylonitrile have taken me back to dichromated gelatine.
Galina asked for some references:
F.J. Tritton, "A Method of Increasing the Printing Speed of Dichromated
Gelatin," The Photographic Journal 69 (1929) 281-285. The author, who worked
for the Autotype Co., reviews Eder's 1914 research recommending addition of
0.1 to 0.2 copper chloride to a 3% pot. dichromate sensitizing solution
(copper sulphate might be easier to get and was nearly as good) which
supposedly increased sensitivity fourfold. Tritton found only a 2 to 2 1/2
increase and achieved better results with cerium chloride (threefold
increase) or lanthanum chloride (fourfold increase but too expensive for
general use). He also recommends reducing the exposure by a third and using
a 2% solution for the initial soak rather than adding the cerous chloride to
sensitizing solution.
Jaromir Kosar. Light-sensitive systems : chemistry and application of
nonsilver halide photographic processes. New York : John Wiley & sons,
c1965. See esp pp. 91-94 for a now somewhat dated and sometimes
contradictory review. On the subject of developing the exposed DCG in a mild
reducing solution - apparently hydroquinone has been suggested; and
resorcinol and thiourea are supposed to incease sensitivity up to fifty
times.
G. K. Oster and G. Oster, "Photoreduction of Metal Ions by Visible Light,"
Journal of the American Chemical Society 81 (1959) 5543-5545. Primarily
about ferric ions, which may also have some application to Pt/Pd? Solution
of 5% gelatine containg 5% pot. dichromate, chelating agent as electron
donor 0.05% neutralized triethaolamine (or EDTA?) and dye (methylene blue
etc) rendered water insoluble when exposed to visible light.
T. Kubota, "Hologram Formation with Red Light in Methylene Blue Sensitized
Dichromated Gelatin" Applied Optics 15 (1976) 556-558. Apparently methylene
blue is easily precipitated in aqueous dichromate solutions unless you raise
the pH to 9 with ammonia.
A. Graube, "Dye-Sensitized Dichromated Gelatin for Holographic Optical
Element Fabrication," Photographic Science and Engineering 22 (1978) 37-41.
Discusses alternative dyes such as methylene green (available as a solid
which can be mixed into the sensitizing solution to partially overcome the
precipitation problem) and rose bengal (which sensitizes DCG to yellow-green
light).
J. Blythe "Methylene blue sensitized dichromated gelatin holograms: a new
electron donor for their improved photosensitivity." Applied Optics 30
(1991) 1598. Haven't seen this.
This is very haphazard and anybody who's done any dichromated gelatin
holography could undoubtedly make much better suggestions. (Is anybody on
this list also on a holography list?). I have absolutely no idea whether
methylene blue dye would wash out easily, or whether you could substitute a
similar non-coloured chemical compound. Anyway, if you wanted to have a go
yourself, instead of paying M. Berger for what probably isn't a once in 200
years discovery, here are a few things that might be worth trying.
Philip Jackson
pjackson@nla.gov.au