Lots of bells ringing. In fact, I think SO-239 is what was in
my memory. It has been too long since I read the theory but I
have a source for brushing up on it. You reminded me that
Sabattier effect and solarazation are not the same. I think it is
just too long since I thought about this so only rough outlines
remained in my memory. I never actually forget anything.
On 10/16/2021 6:46 AM, BOB KISS (bobkiss) wrote:
DEAR RICHARD,
Yes, you are right.I have used Kodak SO-239 and X-Ray duplicating films (both direct positive films) to make enlargednegatives for printing on PT/PD and other alt processes...well before I learned to scan, image process, and print enlarged negs with my Epson.They use the phenomenon called solarization, (NOT to be confused with the Sabattier Effect, re-exposure during development, which is often improperly called solarization).The name came from the source of its discovery...black suns in images exposed for the landscapes.There was so much overexposure in the sun that it caused a reduction in density on the film yielding a darker or even black sun.
At RIT we did some experimenting with various film/developer combos exposing the crap out of them through step wedges.You can clearly see the reduction in density with increased exposure in some combinations.(For those who care, it has to do with surface developers vs. internal developers because the excess exposure causes some of the released electrons to migrate into the interior of the ag-x xtal ergo, internal developers reduced the effect and surface developers increased it.)
To make these films, they basically "flash" it up to the point where any further exposure will start decreasing density when processed in a surface dev.The film you mention here was probably made the same way.
CHEERS!
BOB
-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx [mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of `Richard Knoppow
Sent: Friday, October 15, 2021 4:02 PM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: ISO 4 Direct Positive Orthochromatic film
My memory is not what it used to be so I don't remember
details. You may know there is an effect where if an emulsion is
seriously overexposed it reverses the image. This effect was used
to make direct positive material. I am pretty sure Kodak and
others made such films for either document copying or for making
duplicate negatives or duplicate release prints where only the
original positive was availabe (or from a reversal film). I am
just drawing blanks on the name of the material or even the name
of the effect it made use of. I am hoping someone else here has
enough of a clue to remember. Meanwhile I will look at what
documentation I have to get a reminder. ISO-4 is in the range of
fast enlarging paper or positive films. If you developed a clip
what did you get? If I am right it should have come out black and
a clip exposed to strong light should come out clear.
It is the term "direct positive" that leads me to think it
might be this stuff. If, however, its just "positive" its
probably something like Kodak Fine Grain Release Positive, a film
originally intended to make release prints of B&W movies. This is
a very slow film, ISO-4 would make sense for it, with an emulsion
similar to fast printing paper like the late, lamented,
Kodabromide. There is likely still data on it on line and similar
film may still be made because it was also used to make masks (or
matts) for motion picture special effects and titles. It can be
processed in any fairly active developer like paper developer or
D-19.
On 10/15/2021 12:38 PM, Martin magid wrote:
> I developed a strip, and there is nothing printed on the edges.
>
> Now what?
>
> Marty
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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