OHP transparencies prevent Dmax from being reached

[largo]

Hello !

With several brands of OHP transparencies (pictorico, novalith, ...), and with several alternative processes (cyanotype, kallitype, van dyke brown, ...), I never manage to reach the same Dmax after exposure in my contact printing frame as with the same sensitized paper but without a transparency / negative.

You can see this with the 2 attached test strips (one for the cyanotype process, the other for the van dyke brown process) : I cannot reach the maximum Dmax with the OHP transparency, or I guess it would take an infinitely long exposure time...

In other words, the transparency prevents certain UV rays from passing through, or attenuates them in such a way that it would take an exponentially longer time to reach the same Dmax obtained without the transparency.

It's also possible that these transparencies particularly filter out the wavelengths of my UV source (black light phosphorescent tubes), so I'll have to do the same test strips with sunlight (much wider spectrum).

Have you ever encountered the same problem? Should I try other brands of transparencies? I've mainly tested Pictorico OHP (+ premium) and also Novalith TPX25... Is it related to the thickness and/or opacity of the transparency? Or my UV source? Or a combination of both?

Thanks for your replies !

 

[koraks]

All types of film will attenuate UV to an extent. Some are better than others. The 'problem' with some printing processes is that they just keep building density, on and on and on, as you increase exposure. What to do?

Turns out that 'dmax' is a bit of an overrated concept. What you want, is convincing density. What the practical maximum is, turns out to be not so relevant. Enough is enough. If you can get decent blacks (blues/browns) with an exposure through your OHP film, then that's all that matters. That the result is a little different without film, is very nice and all, but of little practical relevance.

Look at it this way: in terms of rendering blacks, alt processes on paper are a mediocre proposition to begin with. There's always considerable reflection. If you want really good blacks, try silver gelatin - it's hard to beat, when it comes to paper.

There are workarounds, of course. With Van Dyke, try gold toning. It makes a world of difference. Another 'trick' is to apply an overcoat on the print; some use wax, but I suppose you could do something like a gelatin overcoat as well. It'll change the surface characteristics, but that's the point - the appearance of contrast and the deepness of the shadows/blacks will increase.

Try not to lose any sleep over it. Focus on getting a nice print with your workflow. In the end, that's all that matters. The human eye+brain tend to adjust to the contrast of the medium anyway.

 

[largo]

All types of film will attenuate UV to an extent. Some are better than others. The 'problem' with some printing processes is that they just keep building density, on and on and on, as you increase exposure. What to do?

I find it hard to imagine a photochemical process that continually increases its density with exposure... After a while, the photo-chemical reaction is complete and blackening/darkening no longer progresses. In practice, we know that solarization can occur with too much exposure, but I don't know the photochemistry of solarization.

Also we know that darkening is not linear, as it's also a question of statistics (photons hitting crystals that have not yet been "transformed") and energy levels being reached.

Turns out that 'dmax' is a bit of an overrated concept.

It is a measurable value. Whether or not this is an overrated concept is subjective.

What you want, is convincing density.

True

What the practical maximum is, turns out to be not so relevant.

I don't quite agree, because then I could stop the exposure after 1 minute and be satisfied with a whisper of image.... Why would I even bother calculating the standard/base exposure time then ?

Enough is enough. If you can get decent blacks (blues/browns) with an exposure through your OHP film, then that's all that matters. That the result is a little different without film, is very nice and all, but of little practical relevance.

Look at it this way: in terms of rendering blacks, alt processes on paper are a mediocre proposition to begin with. There's always considerable reflection. If you want really good blacks, try silver gelatin - it's hard to beat, when it comes to paper.

I choose alt photo processes and papers because their look and feel are close to what an etching for example can offer. The warm, nuanced, matte rendering of the image - it's in the body of the paper - gives it a unique, timeless character.

I don't pick an alt photo process for Dmax, otherwise I'd choose a gelatin sized silver bromide baryta paper.

But I still aim to achieve the Dmax of my alt photo process for my chosen paper.

There are workarounds, of course. With Van Dyke, try gold toning. It makes a world of difference.

At 58€ per gram of gold (and the gold chloride salt being even pricier), gold toning is atm out of question for me.

Another 'trick' is to apply an overcoat on the print; some use wax, but I suppose you could do something like a gelatin overcoat as well. It'll change the surface characteristics, but that's the point - the appearance of contrast and the deepness of the shadows/blacks will increase.

Yes I've tried that, it works.

Try not to lose any sleep over it. Focus on getting a nice print with your workflow. In the end, that's all that matters. The human eye+brain tend to adjust to the contrast of the medium anyway.

Knowing that I can't change my chemistry, paper or brand of OHP transparencies, I can only question my UV source.

My source is good in terms of spectrum, since I reach the DMax without transparency (even under the 2mm glass of my contact printing frame).

I'm going to try the same test strip with sunlight, as we know that the sun is a much more powerful source of UV than our sunlamps. I myself have found, for example with Mike Ware's "Simple Cyanotype" formula, that exposure to sunlight is infinitely faster than in my UV lightbox.

The aim is to find the time when the density under transparency is equal to the density without transparency.

If, after exposure to the sun, I reach the same Dmax with the negative as without it, my UV source is at fault: not that it doesn't have the right spectrum, but it's not powerful enough.

If, after exposure to the sun, I can't reach the same Dmax with the negative as without it, my OHP transparency is to blame: it irremediably blocks part of the spectrum needed to reach Dmax.

 

[koraks]

Good luck!

 

[largo]

Good luck!

Thanks! I'll keep you updated with the results (or frustration

 

[MattKing]

It would be interesting to see if you would achieve different results if you used film negatives.
Your light source may concentrate emissions in a band that just happens to be blocked by the transparency materials you have tested.

 

[Patrick Robert James]

I use Waterproof Film from Ultrafine Online which might be Fixxons. The exposure difference between the base and no base is only about 25% or so. It isn't much. That is using UV LEDs.

 

[PGum]

Largo,

I may have seen this effect before but was always compensating with a longer exposure. Might want to try flipping the transparency around and see if it goes away. The emulsion side of the transparency is highly porous amorphous silica or possibly alumina and holds air and even moisture. Possibly, the oxygen in that air could inhibit the full reduction of the ferric salt -just conjecture but worth an experiment if you have the time.

As far as UV blocking in tests long ago, pictorico was dropping it 30% at 365 nm. Inkjpress was reducing it 20-30% at 385 nm. These reductions of course should be easily compensated with a longer exposure.

 

[largo]

Largo,

I may have seen this effect before but was always compensating with a longer exposure. Might want to try flipping the transparency around and see if it goes away. The emulsion side of the transparency is highly porous amorphous silica or possibly alumina and holds air and even moisture. Possibly, the oxygen in that air could inhibit the full reduction of the ferric salt -just conjecture but worth an experiment if you have the time.

Interesting. I will do the flipping test. I didn't know that the emulsion side of the transparency is highly porous amorphous silica or possibly alumina, which is known to have a very very high surface area.

The thing is, I've always been told to have the inked face of the transparency in contact with the paper in the contact printing frame, which makes sense, not to loose focus... I could maybe use a piece of 0.003 clear Duralar™ between the negative and sensitized paper...

As far as UV blocking in tests long ago, pictorico was dropping it 30% at 365 nm. Inkjpress was reducing it 20-30% at 385 nm. These reductions of course should be easily compensated with a longer exposure.

Longer exposures do not seem to fix the issue. That's also why your first hypothesis sounds interesting.

 

[nmp]

There is also the issue of difference in how much of the moisture and reaction products are trapped in the paper with transparency on top vs none. It would be interesting to do your step exposures with the transparency on top of the glass rather than directly in contact with the paper. Obviously, this is not the practical way to do the exposure but might shed some more light on the issue.

I would also do f-stop like exposures - 2 min steps over 10-20 mins does not do much.

:Niranjan.

 

[largo]

I would also do f-stop like exposures - 2 min steps over 10-20 mins does not do much.

Yes, I know... I'm sometimes a bit lazy to do the proper math :

S = steps count
Emin = Minimal exposition (step #1)
Emax = maximal exposition (step #S)
Q = (Emax/Emin)1/(S-1)
Ei = Exposure received by step #i , 1 <= i <= S

Ei = Emin * Qi-1

 

[fgorga]

Dmax without the transparency is not a useful number... unless, of course, you are only interested in making photograms without using a negative.

My advice jives with what @koraks said above, focus on the "practicals" (i.e. making a nice print) and don't worry so much about "theoreticals".

 

[cliveh]

Is Dmax that important? Try looking at the picture rather than numbers.

 

[largo]

There is also the issue of difference in how much of the moisture and reaction products are trapped in the paper with transparency on top vs none. It would be interesting to do your step exposures with the transparency on top of the glass rather than directly in contact with the paper. Obviously, this is not the practical way to do the exposure but might shed some more light on the issue.

Also found this post : https://www.photrio.com/forum/threa...re-for-palladium-printing.106862/post-1412508

...where you can read :

a too humid paper may compromise the negative, the silica coating is very very hygroscopic, will suck moisture and/or emulsion if it's not dry enough: try to not pre-humidify the paper or putting 1/2 mil impervious and transparent material (mylar for instance) between the paper and the negative.