[alt-photo] Re: direct positive paper

[Richard Knoppow]

----- Original Message ----- 
From: "etienne garbaux" <photographeur at nerdshack.com>
To: "The alternative photographic processes mailing list" 
<alt-photo-process-list at lists.altphotolist.org>
Sent: Saturday, October 23, 2010 2:10 PM
Subject: [alt-photo] Re: direct positive paper


> Richard wrote:
>
>>I wonder what its intended purpose is, the high contrast 
>>sounds like a document copying paper although I don't know 
>>why anyone would need one these days.
>>I have not looked at the specs, is it a reversal paper or 
>>a direct positive based on a pre-fogging and controlled 
>>solarization? The later is developed in a single step 
>>while reversal development requires two developments and a 
>>bleach step.
>
> The contrast is not that high.  It's intended for 
> pictorial work, but (I'm guessing) all attempts to 
> lengthen the scale fell somewhat short.
>
> It is processed in a single step, but judging by the poor 
> keeping properties of the latent image I wonder if the 
> pre-fogging may be done chemically (I don't know how to 
> make a pre-flash with light any more stable than a latent 
> image).  I saw some Russian literature about a decade ago 
> discussing a direct-positive technique based on emulsions 
> that were chemically pre-fogged.
>
> Best regards,
>
> etienne
>
     I don't remember the full theory of the direct positive 
material I am thinking of but its designed to take advantage 
of the reversal of the image under high exposure. Most 
modern emulsions do not have such a reversal region because 
its not desirable in normal negative and positive materials. 
The effect can be exagerated to the point were an exposure 
of reasonable amount will cause the reversal. The material 
will be dark if developed without exposure. Its also 
possible to do this with dye as in the Ciba/Ilfochrome 
process but this is probably silver halide. Its probably a 
fixed contrast process. Reversal materials _can_ be made to 
have low contrast and some motion picture intermediate and 
duplicating materials are made that way so that normal 
contast duplicate negatives can be made in one step but most 
reversal materials are made for projection or direct viewing 
and are quite high contrast.
     It is possible in principle to reversal process any 
negative emulsion including paper, but getting acceptable 
results is not easy.

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