Re: Light sensitive compounds for alternative daguerreotypie

----- Original Message -----
From: "Jonathan Danforth" <jonathan@danforthsource.com>
To: <alt-photo-process-l@usask.ca>
Sent: Friday, July 28, 2006 10:57 AM
Subject: Re: Light sensitive compounds for alternative
daguerreotypie

> Ryuji,
>
> Thanks for your response.
>
> What I fail to understand is why things like
> platinum/palladium prints and chrysotypes work. None of
> those three processes use silver unless I'm missing
> something. If a piece of off-the-shelf B&W paper is just
> a silver slurry smeared on paper and a piece of handmade
> platinum paper is the same thing but substituting platinum
> for silver, what's the difference?
>
> I'm frankly not sure what the gold chloride step does
> chemically but it certainly makes the image on the dag
> plate more durable and enhances contrast. If you're not
> careful and heat the solution too long on the plate,
> solarization starts to occur in highlights. The whole
> gilding process is unrelated to the photosensitive part of
> what's going on with a daguerreotype though.
>
> -Jonathan
    I don't have Ryuji's expertise in chemistry but will try
to give you a simple answer. There are a number of chemical
compounds which are sensitive to light. For instance, both
postssium dichromate and some Iron compounds are changed by
exposure to light and affect other materials with which they
are associated, for instance, a collegen like gelatin or
glue for dichrmate. The differece between these and an
emulsion made of a silver halide is the sensitivity. Both
dichromate and iron require a very large exposure, silver
halides require only a very small exposure, enough to change
the structure of the individual crystals a little in a
certain way. The image is produced by a developer which is
selective of those changed crystals and converts them to
metallic silver. The developer is really an amplifier
supplying energy to the process and this is the key to the
difference. No such amplification takes place in iron or
dichromate processes, or where a silver halide is converted
to metallic silver directly by the action of light as in
printing out paper. The use of a silver compound is not
necessary to get sensitivity to light but they appear to be
if the amplication is desired. While much research has been
carried out over perhaps a century or more, nothing has been
found to replace silver halides.
   As far as light sensitive materials, your skin reacts to
light in a complex and rather slow way by releasing melanins
cause tanning. If you tape a negative to your skin and lay
in the sun you will get a print on your very own body!
Bathing suit marks are an example of this process, no silver
involved. Again, this takes a lot of energy where a silver
halide emulsion takes very little.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com