Originally Posted by Marco B
We now have a serious conflict. One description suggests the potassium iodide bath was done before, and the other after the exposure. A significant difference... :unsure:
Originally Posted by Igor Savchenko
In addition, the first description suggests the paper was darkened completely before the exposure and bleached during the exposure to reveal the image..., while the second suggests the darkening took place while exposing and the image revealed afterwards?!...
Anyone else who can elaborate on the direct positive processes Hippolyte applied (both the silver version and/or the cyanotype version) and especially the chemistry going on during the processes?
Interesting book by the way you referenced Igor (http://www.christopherjames-studio.c...d/thebook.html). I am surprised Christopher's book is not listed on the Alternative Photography website as well... might be a nice addition.
Ah... well, it is listed, but not on the "Books" main page, but under "Amazone books":
Well, as usual, even though I was not even searching for this specific Hippolyte Bayard process at all, browsing the internet I hit on the work of Tania Passafiume for the George Eastman House back in 2001...
Seems she dug into the direct positive process and Bayard quite deeply. Haven't had the time to really go through this 64 page PDF document, but it sure seems interesting enough:
Reading some comments though, it seems quite clear that the potassium iodide was applied before exposure, see the "What is the chemical reaction?" section on page 7.
Tannia also shows some detailed instructions for re-creating the process.
I think some of you that seem to be following my posts here on APUG are going to love this document! ;)
How did you know I was following this?
Originally Posted by mabman
what i did was just take a long exposure through a camera ..
my box camera has a time-setting, so i opened the shutter
and left it for about 1/2 hour-40 mins ..
the negatives ( or positives if i contact print a negative this way )
are kind of delicate. the ones i fixed were in a water bath first
then partially exhausted speed fixer and the images bleached quite badly.
i have a recipe for a weak hypo bath for sun prints someone gave me
that i will use next. i haven't had time to make more exposures to test
the hypo ...
Originally Posted by Marco B
thanks for this nugget !
it was a great thing to read about :)
(and maybe do? ) :)
IIRC, the H&D curve for certain emulsions eventually flattens out and then reverses direction with increased exposure. In this latter region density actually decreases with increased exposure.
if the paper was not pre-exposed black prior to exposure ..
but still coated with the iodide, do you think it would it be a faint negative image,
... like an ambrotype?
i'm relatively clueless
but ... it seems that what bayard did was make a reversed+fixed version of nicéphore niépce's
unfixable - lumen / retina prints.
You are correct. All emulsions today are treated in some way to emphasize or repress one of those images, neg or pos.
Hippolyte Bayard was not the only one to experiment with a direct positive process, particularly after the advent of the Daguerreotype. Fox Talbot thought that this may be the way to go, thus not realising at the time the full advantage of his own replication process. I would suggest from my own research in this area, that fogging with the first halogen was followed by expose with a second. This is a very interesting area for practical research and one which promises many possibilities for new forms of chemical imagery.
As am I. It has been decades since I have read anything on the subject. The illustration of the H&D curve sticks out in my memory but little else.
Originally Posted by jnanian