POP vs DOP/visible vs latent - what's the chemical difference?

w.out · Jan 16, 2017

I've had a nagging question in the back of my mind for a while now, and after experimenting to work it out myself came up with more confusion. Thought I'd put it to APUG as Google has been no help either.

When one is printing with printing-out paper, like salt printing, the image appears immediately. There's no need to develop. When one prints with developing-out paper, the image is latent, and must be developed to be visible.

I've been looking at various emulsion recipes and noticed that they generally have bromide in them. A very simple one was basically bromide, silver and gelatin, and had fairly standard developing instructions with it.

So today I swapped out salt for bromide while contact printing and expected the image to be latent, but it wasn't! It was there as strong as if I'd used regular sodium chloride instead.

So... what exactly is it that causes the image to be latent in DOP? I'm really scratching my head over this one.

signed,
Confused.

Dibbd · Jan 16, 2017

w.out said:
...
So... what exactly is it that causes the image to be latent in DOP?
...

A much shorter exposure.

pdeeh · Jan 16, 2017

w.out said:
So today I swapped out salt for bromide while contact printing and expected the image to be latent, but it wasn't! It was there as strong as if I'd used regular sodium chloride instead.

It may not be just the bromide.
Other substances in the paper, in the size and so on can form light sensitive silver compounds.
It can be fun just to coat a sheet of paper with silver nitrate and see what comes out. You might be surprised!

w.out · Jan 16, 2017

Dibbd said:
A much shorter exposure.

It might be just my experience, but last time I left Ilford multigrade out of the bag it stayed white rather than self developing. On hitting the developer it yielded poorer image quality than a salt print left out for the equivalent time...

w.out · Jan 16, 2017

pdeeh said:
It may not be just the bromide.
Other substances in the paper, in the size and so on can form light sensitive silver complexes.
It can be fun just to coat a sheet of paper with silver nitrate and see what comes out. You might be surprised!

Yeah, fair point. I have no idea how many free chlorides (or whatever else) there might be in the paper. I might do a half/half test and see what happens.

nmp · Jan 16, 2017

w.out said:
I've had a nagging question in the back of my mind for a while now, and after experimenting to work it out myself came up with more confusion. Thought I'd put it to APUG as Google has been no help either.

When one is printing with printing-out paper, like salt printing, the image appears immediately. There's no need to develop. When one prints with developing-out paper, the image is latent, and must be developed to be visible.

I've been looking at various emulsion recipes and noticed that they generally have bromide in them. A very simple one was basically bromide, silver and gelatin, and had fairly standard developing instructions with it.

So today I swapped out salt for bromide while contact printing and expected the image to be latent, but it wasn't! It was there as strong as if I'd used regular sodium chloride instead.

So... what exactly is it that causes the image to be latent in DOP? I'm really scratching my head over this one.

signed,
Confused.

It is primarily the presence of excess AgNo3 in POP vs essentially stoichiometric quantity used in the DOPs. We discussed this at length in this thread.

(there was a url link here which no longer exists)

Without the excess AgNo3, the exposure simply creates the Ag crystal centers that have no means to propagate to higher density needed for the image to form (unless of course you "over-expose" to a great degree.) The result is the latent image. At the develop stage, further reduction of halide takes place (with the help of the developer acting as a "reducing agent") in non-linear proportionality to the Ag metal (acting as a catalyst) concentration, giving rise to the contrast in the final image.

In case of POP, the excess silver nitrate allows a chain reaction (silver halide to silver metal and halide ion to more silver halide to more silver metal, so on) to occur after the first photon has photochemically reduced the silver halide. On continued exposure, this leads to building up of greater density in the higher exposure areas in comparison to the low exposure areas creating the requisite contrast without an external developer.

I think different halides, chloride vs bromide vs iodide etc, have different sensitivities (to the quality and quantity of the exposure) but the mechanisms remain the same.

nmp · Jan 16, 2017

pdeeh said:
It may not be just the bromide.
Other substances in the paper, in the size and so on can form light sensitive silver complexes.
It can be fun just to coat a sheet of paper with silver nitrate and see what comes out. You might be surprised!

There was an article in one of the issues of Post-Factory Photography titled "Salt Printing without Salt" or something similar where the author demonstrated many different papers did not need to be salted to form an image. I can't give you any more details as I am away right now. But, yes you are on the right track.

pdeeh · Jan 16, 2017

nmp said:
But, yes you are on the right track.

well, it is a matter of personal experience ... (there was a url link here which no longer exists)

nmp · Jan 16, 2017

pdeeh said:
well, it is a matter of personal experience ...

Sorry I thought it was a conjecture on your part (would have been a pretty good one) when you had already been there and done that.

locutus · Jan 16, 2017

What is a good currently available POP?

Dibbd · Jan 16, 2017

w.out said:
It might be just my experience, but last time I left Ilford multigrade out of the bag it stayed white rather than self developing. On hitting the developer it yielded poorer image quality than a salt print left out for the equivalent time...

Google "lumen printing"

nmp · Jan 16, 2017

locutus said:
What is a good currently available POP?

There are none currently available, as far as I know. The last manufacturer was Chicago Albumen Works that stopped making about a decade ago what was known as Centennial POP. I happened to buy a box of 25 sheets (which I promptly stored away until recently) probably just before they discontinued it. I am working towards getting a few good prints out of them (colorized digital negatives + Se toned.)

dwross · Jan 16, 2017

It's very easy to make. I'm putting the finishing touches on an instructional page for TLF.

locutus · Jan 16, 2017

Oh thanks! please post here once you publish it.

dwross · Jan 16, 2017

Will do.

pdeeh · Jan 16, 2017

nmp said:
Sorry I thought it was a conjecture on your part (would have been a pretty good one) when you had already been there and done that.

No apology needed. Can you supply a link to the article you mention ( or is it printed matter only)?

Gerald C Koch · Jan 16, 2017

Since the Middle Ages it has been known that certain ores such as argentite and chlorargyrite darken on exposure to light. They remained a curiosity until the discovery of the element silver. Certain silver compounds are sensitive to shorter wavelength light (the ultra violet, violet and blue regions of the spectrum. Such light has sufficient energy to disrupt a silver halide lattice and release metallic silver. In fact these wavelengths are termed actinic light. This is what happens with printing out paper where enough metallic silver is produced to create a visible image.

With regular photographic papers the light exposure is too short to create a visible image but certain activation sites are created in the silver halide lattice. These sites are more readily reduced to metallic silver by various reducing agents like hydroquinone. Unexposed portions of the lattice are less susceptible and are not readily reduced by certain reducing agents. It must be pointed out that not all reducing agents are suitable. Some are so active that they produce silver regardless of whether it has been exposed to light or not. An example would be stannous chloride. Such reducing agents are termed fogging developers to distinguish them from others that are more discriminating in their behavior (metol, hydroquinone, catechol, ...).

To sum things up; POP works by the physical action of light. DOP works with light and certain chemicals.

nmp · Jan 16, 2017

pdeeh said:
Can you supply a link to the article you mention ( or is it printed matter only)?

Print issues only. This article was in the #8. Here is a link where you can see the contents of all the issues and the way to get them if you so desire.

http://www.alternativephotography.com/post-factory-photography/

The particular article I alluded to is by Liam Lawless called "Unsalt Saltprints." I got the last 2 issues before Judy Siegel who published/edited them had to pull the plug. Really good reading. Funny, witty and quirky with lots of hands on write-ups by many of the names you might be familiar with in the alternative universe.

Interestingly, the images that you shared on the other thread looked like any other normal salt prints to me. Then the question is why go thru the trouble of a salting step when you can do without. Just brush AgNo3 and go. No?

pdeeh · Jan 16, 2017

Thanks.
To partially answer your final question-cum-observation: consistency and repeatability.

However, we may be taking the op's thread off at a tangent now.

removed account4 · Jan 16, 2017

w.out

im not sure what the main differences are between DOP and POP
but there are a lot of people here and elsewhere who might be able
to explain what the deal is.
(there was a url link here which no longer exists)
i have some sun prints i made with a neighbor's kid5 ? years ago made with old expired
separating liquid light emulsion that have somehow never faded, never been fixed, been scanned
and have survived sunlight and room light looking the same as the day they were made. the prints
have blues and reds and yellows and greens in them. i have other prints made with commercial papers
( both RC and FB ) and they sometimes stay around, some i made recently on cheap expired polymax rc paper
that are out in room light and still remain with an image on them and look like i just removed the contact negative from on top
of the paper. i think it all has to do with exposure. the paper/negative was exposed in the sun for a long time,
more than 8 hours i think in both direct and open shade and shade and overcast even rainy days inside in a window
they are darkish prints but the images are still there, unlike other brute force exposures i have made whose images vanish
and the paper turns grey.

can't wait to see denise's recipe ! stuff on her website ( and book! ) is is well worth the price of admission !

good luck w.out

dwross · Jan 19, 2017

locutus said:
Oh thanks! please post here once you publish it.

http://thelightfarm.com/cgi-bin/htmleditiongen.py?chapter=POPBasics

Hope it's useful!

removed account4 · Jan 19, 2017

thanks denise this is a beautiful thing ... you have reanimated an emulsion !
it was said centennial was too difficult to make at the end, one of the reasons it was cancelled
it is amazing how simple this recipe is ...

but, do you have to sing "puttin' on the ritz" while you are coating ?

dwross · Jan 19, 2017

Thanks, John!
re Centennial: I think it came down to the market. Niche papers, in general, weren't doing too well at the time. Marvelously, I think the decline in chemical photography has stopped. Fingers crossed. It will be interesting to watch the next couple of years and see how everything goes.

locutus · Jan 20, 2017

Thank you!

This doesn't even look that hard.

w.out · Jan 21, 2017

nmp said:
It is primarily the presence of excess AgNo3 in POP vs essentially stoichiometric quantity used in the DOPs. We discussed this at length in this thread.

(there was a url link here which no longer exists)

Without the excess AgNo3, the exposure simply creates the Ag crystal centers that have no means to propagate to higher density needed for the image to form (unless of course you "over-expose" to a great degree.) The result is the latent image. At the develop stage, further reduction of halide takes place (with the help of the developer acting as a "reducing agent") in non-linear proportionality to the Ag metal (acting as a catalyst) concentration, giving rise to the contrast in the final image.

In case of POP, the excess silver nitrate allows a chain reaction (silver halide to silver metal and halide ion to more silver halide to more silver metal, so on) to occur after the first photon has photochemically reduced the silver halide. On continued exposure, this leads to building up of greater density in the higher exposure areas in comparison to the low exposure areas creating the requisite contrast without an external developer.

I think different halides, chloride vs bromide vs iodide etc, have different sensitivities (to the quality and quantity of the exposure) but the mechanisms remain the same.

This is what I was looking for - thanks very much for the effort. I saw that other thread while I was searching around, but I didn't know enough of the basics to quite follow it.

If I'm understanding this correctly, increasing the speed of DOP paper is not achieved by increasing the silver halide (which explains those 'silver rich is a myth' threads here on APUG), but by keeping the ratio of silver to salt stoichiometric and rather by including ingredients in the emulsion that increase the sensitivity of the halide, such as active gelatin, ammonia, etc. Am I on the right track with that?

POP vs DOP/visible vs latent - what's the chemical difference?

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