Sciencey questions on POP

I'm trying to understand the chemistry behind the spectrum with POP on one side and develop on the other side better. When I started out I somehow thought it was the halide, chloride vs bromide or iodide but I've made salt prints using KBr and know I know a bit more about it doesn't make sense...

I now assume the presence of excess silver in the paper during exposure is the difference that makes something POP or not?
Is there anything I'm missing here or is it just a factor of things: expected exposure times + excess silver or other donors + choice of halide which you engineer in something POP or develop.

- Is it specifically correct to say that POP processes actually undergo physical "development" caused by the excess silver present in the paper during exposure?
- Is the darkening one sees after prolonged exposure to daylight with commercial photographic materials a low amount of free silver or other metals in the paper that cause physical development?
- Is it practically impossible to get POP to be true black because you can't take this "hidden" development to convert sufficiently large silver grains?

All good questions.....

I have one for you. Have you ever prepared a salted paper without silver nitrate in excess (preferably salt/nitrate in 1:1 stoichiometric ratio,) exposed and developed using a standard developer. I wonder what the outcome would be.

I havent tried it, i would assume it's difficult to get enough contrast between a developed to visibility latent image and chemical fog? (I guess this adds to the question list, and the experiment list!). Maybe very weak/restrained developer... I read some work on developed albumin although it was all more like, someone did that in the 90ies (1890 or 1990 pre internet you choose...). I haven't found any clear instructions on how to "do" developed albumin. But i guess the albumin makes it somewhat more similar like a gelatin based emulsion where all the things you want to happen are nice and close together in the albumen layer. Not sure if this develop out albumin involves a precipitation step where you add the AgNO3 before you coat the albumin either... so well now I'm even more confused

Internet has everything....

http://albumen.conservation-us.org/library/monographs/sunbeam/chap34.html

hi C-F

have you looked at the articles at the unblinking eye ?
liam lawless has writen specifically about POP paper,
modern manufacture, and the science behind it/
http://unblinkingeye.com/Articles/POP/pop.html

steve anchell has an article at alternative photography as well
http://www.alternativephotography.com/wp/pop-printing-out-process/printing-out-processes

a fellow in NY state named victor
makes POP paper, well something sort of like it called aristotype paper ( colloidio-chloride )
Dead Link Removed

i have attempted to convert regular DOP ( plain out develop-out-paper photo paper )
into POP paper by a method a fellow appuger named lars figured out,
by soaking the paper in potassium nitrate. it didn't quite work the way i wanted it to
so i just made retina prints instead. NedL has made these lars-inspired DOP >> POP conversions
quite a bit, and gotten some good results from his experiments.

if you want to make retina prints or lumen prints, or solargraphs, NedL has also figured out a way to
fix them too: (there was a url link here which no longer exists)

good luck !

I had a little more to think about the subject and here is what I can come up with, FWIW. Caveat: I am not a photo-physicist nor a long-term practitioner of the art, alternative or otherwise. Others more knowledgeable on the subject are welcome to flush out any discrepancies or adding nuances to my gross generalizations.

• Is it specifically correct to say that POP processes actually undergo physical "development" caused by the excess silver present in the paper during exposure

I think you are on the right track here in pointing out the similarities. The way I would describe is that in the case of the POP with excess silver nitrate, when a photon splits a chloride into silver metal and a chlorine ion, the latter has a silver nitrate molecule readily available nearby to form an additional silver chloride which then on continued exposure forms another pair of silver metal and a chlorine ion and so on. As a result of this chain reaction, the density is built up in a non-linear fashion from the low to higher exposure areas, thereby enhancing the contrast in the POP image as we see after full exposure. There is no increase in the photonic efficiency here, i.e. no. of metal atoms produced per photon. As you suggested, perhaps that could be looked as an in-situ, de facto "development" of what would otherwise be a flat and faint latent image.

In case of the classic development, one takes the latent image formed by photo-conversion to silver metal in more or less proportional quantity to the intensity of the photons and amplify it to a higher order of magnitude by use of a reducing agent with the silver atoms acting as a catalyst. Where there is more initial concentration of silver there is more development. The ensuing self-catalyzing action of the developer ends up creating the contrast seen in the final image. So here the process involves increasing the photonic efficiency which allows one to get away with shorter exposures to obtain the same final density.

• Is the darkening one sees after prolonged exposure to daylight with commercial photographic materials a low amount of free silver or other metals in the paper that cause physical development?

The presence of excess silver only speeds up the formation of silver metal during exposure. In absence of excess silver, the metal formation is much slower but on prolonged exposure all of the halide will eventually convert and darken the paper. On the flip side, in the POP the silver conversion will actually diminish or even stop at some point due to self-masking effect. So theoretically you should still have silver nitrate and halide left over after fully exposing the paper.

• Is it practically impossible to get POP to be true black because you can't take this "hidden" development to convert sufficiently large silver grains?

If by true black, you mean the tone of the print, the answer is in the initial size of the silver halide particles which are thought to be in colloidal state in the way they are formed with the average size in the range of the yellow to red wavelengths (0.5 – 0.7 microns) which governs the final image color. There is no “growing” of these particles as is done in case of photo-emulsions during manufacturing to larger sizes. Toning does increase the effective size of the particles by adding more atoms (more so in selenium or sulfide than in gold toning) which moves the image towards more neutral spectrum.

If by black you mean the density, then it is controlled by the self masking phenomenon as alluded above where the presence of sufficient silver metal hinders absorption of any further photons, kind of opposite of self-catalysis.

UV and visible light are sufficiently energetic to cause silver halides to darken due to the formation of silver metal. This has been known since the Middle Ages when it was found that naturally occurring silver salts would darken after they were mined. You can make contact prints from ordinary photo papers without development. Due to the small size of the silver grains such prints may be brown or have a lavender hue. However the prints are not stable to light and will continue to darken. They can be fixed normally but there is a reduction in density and the special colors are lost.

AFAIK the principle use for POP papers was to provide temporary prints for the subject to review before conventional prints were ordered from the photographer. Since they were not meant to last the customer was not getting something for nothing. However many people saved these proofs for many years. They were reasonably stable when briefly viewed in dim light.

C.-F. Hollemeersch said:

I havent tried it, i would assume it's difficult to get enough contrast between a developed to visibility latent image and chemical fog? (I guess this adds to the question list, and the experiment list!). Maybe very weak/restrained developer... I read some work on developed albumin although it was all more like, someone did that in the 90ies (1890 or 1990 pre internet you choose...). I haven't found any clear instructions on how to "do" developed albumin. But i guess the albumin makes it somewhat more similar like a gelatin based emulsion where all the things you want to happen are nice and close together in the albumen layer. Not sure if this develop out albumin involves a precipitation step where you add the AgNO3 before you coat the albumin either... so well now I'm even more confused

Click to expand...

The way to get DOP salt print would be to coat the silver nitrate first and then float in either plain salt, gelatin+salt or albumen+salt solution. Allow all nitrate to be converted so now you have excess salt. Then wash thoroughly to remove the excess salt. I suspect this step is much easier if plain salt is used. In any case, you end up with a silver halide without excess of nitrate. Apparently Talbot did this initially so he can have salted paper that can be kept longer that he "re-sensitized" by giving it a coat of low concentration silver nitrate (for excess) just before exposure.

A fellow named Jalo from Finland came by and showed us how he converted regular DOP to POP by brushing on some weak AgNO3 ( I think it was 4 or 5% solution ). Here's (there was a url link here which no longer exists), and his page with examples. I've been meaning to try it.

I'm not exactly sure what "physical development" means or if the term is used consistently. The kind of calotype I've been making is washed after sensitizing, so it does not have excess AgNO3. When it is developed in gallic acid, the image does not form strongly until some AgNO3 is added, and you can intensify the negative to almost any degree by continuing to add more silver nitrate. I think this is often called physical development. I think the "recombination" described by nmp that happens in a salt print to make new silver halides available is correct, but I'm not sure what that is called....

Alan Greene's book has a chapter on developed-out salt prints. It does use less silver nitrate and the latent image is developed. He uses a more ordinary salt print work flow, salting the paper first and then floating on or brushing on a weaker silver nitrate solution. I don't have any rational reason why that process doesn't appeal to me as much... it's really just that it's so enjoyable seeing the printed out image form that I've never been tempted to try it!

A lot of interesting discussion here...

@nmp: Yes I agree with your line of tought. It is exactly the sort of thoughts that lead me to the original question. Great to see I'm not alone dreaming this up . Also with black I mean image tone not density.
@gerald: I think POP is a bit more stable than the "preview" as you describe it. Simple albumen is a POP process and certainly was used as a "real" permanent process.

So I took to the "Focal encyclopedia of photography" to be able to give a "formal" definition on I meant by physical development and it seems the entry actually seems to strengthen the arguments of our discussion as well I never thought they would go into that much detail there as it covers so much... (Note this is not a negative thing about this book I like it very much and can actually recommend it).

So they basically define physical development as "development with excess silver nitrate with acid as a restrainer (gallic, pyro, ...)". Putting the fact POP needs excess AgNO3 (mentioned by several people here, and the unblinking eye article seems the most authoritative source). And that early physical development systems used AgNO3 + adid. It seems we can but those two together and state that salt printing (and other POP) basically includes the developer "in the paper". The fact that lots of people also add acid to avoid fog in salt prints seems to make this "embedded developer" argument even stronger.

Another interesting bit FEOP mentions:
"One of the characteristics of chemical development was a cool black image, the result of a large silver particle caused by the filamentary growth during development."
So FEOP suggets its not merely the size of the silver grains caused by emulsion ripening but really the fact that you use a chemical developer. Which seems to confirm that (untoned) POP couldn't be "true black".

There is an very interesting article in FEOP about development and the differences between physical and chemical and how the physical development process relates to the iron based processes. (However it doesn't directly concern itself with the POP effect so it's more like background reading for this thread I gues...) The text seems to suggest that Pt/Pd do undergo physical development as part of the exposure process. If I can try to sum it up (it's several pages long)
- Development: Doing something to a metal so it becomes a dark "image forming" speck.
- Physical Dev.: The final image is formed by metals that are "brought in". These added in metals are developed by something which got formed in the paper during exposure. That something is silver in salt print & co but could be iron in the iron based processes (Cyano, Pt/Pd, Vdb). So the developing agent is formed in the paper by exposure and the final developed metal is brought in "later".
- Chemical Dev.: The final image is formed by the silver metal originally present in the image grain. No image forming metals are brought in, only some product that converts the whole grain present in the latent image to silver. Although they note there is always some small physical component to it as silver is brought into solution when developing. Which can then be moved (and thus brought-in to) other parts of the image.

-- Nonscientific Romantic Thoughts below---
In a way it seems it's just a lucky coincidence that the AgNO3 was both image forming and developing. I guess it would have taken a lot longer for people to discover that there is some latent image formed in silver halides that comes out when you throw the stuff in something totally unrelated like hydroquinone. Once people started to realize the double function of AgNO3 they started to innovate on the development function of it and came up with Chemical development. Which turned out to be faster and cheaper and thus took off.

[/QUOTE
-- Nonscientific Romantic Thoughts below---
In a way it seems it's just a lucky coincidence that the AgNO3 was both image forming and developing. I guess it would have taken a lot longer for people to discover that there is some latent image formed in silver halides that comes out when you throw the stuff in something totally unrelated like hydroquinone. Once people started to realize the double function of AgNO3 they started to innovate on the development function of it and came up with Chemical development. Which turned out to be faster and cheaper and thus took off.[/QUOTE]

There is an interesting paper here
http://www.midley.co.uk/articles/ANALOGIE.htm
about the Daguerreotype and invisible writing and the latent image which suggests a more prosaic non romantic story.

And even more so here.
http://www.midley.co.uk/articles/gallic39.htm#_ftn2

Good thoughts about the "double function" of AgNO3. Talbot's original process was a "pre-silver" process, meaning that the paper was first coated with silver iodide, and then "sensitized" with excess AgNO3 in a separate step. Early calotypists even "sunned" the paper after it was first coated with silver iodide!

NedL said:

Good thoughts about the "double function" of AgNO3. Talbot's original process was a "pre-silver" process, meaning that the paper was first coated with silver iodide, and then "sensitized" with excess AgNO3 in a separate step. Early calotypists even "sunned" the paper after it was first coated with silver iodide!

Click to expand...

While AgNo3 may have been used as a part of the developer in the early calotype process, I wonder if it can be truly called a "developer" as it does not possess the capability to reduce the silver salt into silver metal as gallic acid does. The question is whether it can act as a developer by itself in absence of a reducing agent like gallic acid. If not, than it is more like a developer's little helper by pushing the reaction further. Probably just semantics...

C.-F. Hollemeersch said:

So I took to the "Focal encyclopedia of photography" to be able to give a "formal" definition on I meant by physical development

Click to expand...

I've just got OCR working on one of my Linux lapdogs, so I thought I'd take the opportunity to chime in with LFA Mason's definition, from his "Photographic Processing Chemistry" (2nd ed., p31)

L.F.A.Mason said:

PHYSICAL DEVELOPMENT.
In this type of development the silver ions for reduc-
tion at the latent image specks are originally supplied from the developer solution.
This contains a reducing agent (the developing agent), and buffer ingredients to
control the pH to a suitable value. The source of silver ions must be a soluble com-
plex ion, so that the free silver ion concentration is low enough to prevent spon-
taneous reduction of silver from the solution, but high enough for the catalytic
reduction on the latent image specks to proceed. To obtain physical development
in its purest form, a post-fixation process must be employed. This involves dissolv-
ing all the silver halide from the emulsion layer after exposure, but before develop-
ment, leaving only the latent image specks in the gelatin. All the silver deposited on
these specks during subsequent development is thus derived from the developer
solution. Although internal and surface latent image specks are utilised in this
process, from five to ten times the normal exposure is required. In prefixation phy-
sical development the silver halide is not removed before development. The process
is then more rapid, and the speed loss much less, but some chemical development
takes place simultaneously with the physical development.

Click to expand...

That is very interesting! Thanks.

I don't know if this has been shared on apug or not, but I came across this really good book called Chemistry for Photographers by William R. Flint (Professor of Chemistry, I think, at Cal Tech way back.) You can download it at this link:

https://archive.org/details/chemistryforphot00flin_0

Lots of stuff related to this discussion.

Thanks, looks interesting! Also in google books here. Also in books.google.co.uk here.
Don't tell Pdeeh, but I'm planning to try to make a print using pure physical development, including fixing immediately after exposing the paper...almost tried it this past weekend but got busy with something else