Extend your d… – Chromium intensifier for silver gelatin negatives

Despite the relative silence on my blog, I have been printing regularly lately, with carbon transfer absorbing most of my lab hours. My printing currently revolves entirely around silver gelatin negatives. Given the relatively inflexible contrast that carbon transfer allows, this means I’ve had to be a little creative sometimes in making my negatives work for the printing process. One of the tricks I use a lot is chromium intensifier. Slightly nasty – but very, very effective.

As said, the degrees of freedom in varying the contrast of the carbon transfer process itself are pretty limited. Earlier, I sensitized with dichromate, and that seems to be far more flexible in terms of contrast adjustments than DAS, which I’m exclusively using presently. This means that in terms of negatives, there are only two kinds I can accommodate: ones with an insanely long tonal scale, and ones with an excessively extreme tonal scale. And to be honest, I don’t always get that straight of the box.

By means of comparison, see the typical density scale requirements of a negative for a selection of printing processes. Silver gelatin on variable contrast paper requires anything between 0.4 logD (grade 5) to 1.6 logD (grade 00). Classic cyanotype does well with about 1.35 logD, while New Cyanotype requires a far more punchy 2.1 logD negative. Salted paper does well with about 2.5 logD – and carbon DAS, in my experience, works particularly well with a negative tonal scale of around 2.7 logD.

To keep things in perspective: 1.0 logD (around grade 2 silver gelatin) means the negative has a contrast range of around 1:10 and the dense areas block about 90% of the light. 2.7logD means a contrast range of about 1:500 and the dense areas will block 98.8% of the light – that’s 50 times more density!

To get these pretty insane levels of density, there are two things I do. Firstly, I prefer to develop my negatives in a concentrated, fast-working developer. Lately, I’ve been using mostly ID62 for this, at a dilution of around 1+2 based on the concentrated stock. I develop Fomapan 200 for 3-4 minutes at room temperature. Depending on the scene brightness range, this gets me close to the required density.

Sometimes, that’s not enough and I need to work on a negative a little to get it where it needs to be. This is where good old (or rather: nasty old) dichromate comes in.

Take this negative and a DAS carbon print made from it, for example:

Nice enough, you might say. The density of the rushing water in the foreground was around 1.9 or so, according to my makeshift densitometer:

Pretty dense, you’d say – it’s deep into (or rather, beyond) grade 00 silver gelatin territory and just about right for a New Cyanotype print. Even a salted paper print would have worked fairly well, albeit with less than optimal dmax. But the carbon print looks decidedly “meh”. This was a relatively brief exposure (15 minutes or so) and about the shortest I dare to go before highlight problems emerge and dmax becomes unacceptably bland. Yet, the exposure was too much to render the highlights in the fresh and brilliant way I envisioned them. So let’s work on this negative for a bit.

Dichromate intensification is a simple process, but the chemistry is kind of complex and I’m not sure if it’s entirely understood. I tried to get to the bottom of it, and to be perfectly frank – I failed. From a distance, the process is straightforward: you bleach the negative with a rehalogenating dichromate bleach consisting of (ammonium or potassium) dichromate and hydrochloric acid. This bleach converts the silver into silver chloride, and also deposits chromium in the image areas, with the chromium concentration tracking with silver density. The negative is then redeveloped in an ordinary B&W developer (this can be a staining/tanning one if you prefer), which converts the silver halide back into metallic silver, and the chromium adds (a lot!) to the optical density.

Coincidentally, you might wonder why I’d bother with nasty dichromate instead of using a more benign (relatively speaking) approach such as selenium toning (actually, selenium is pretty darn nasty, too), bleach and redevelopment with a pyro developer or sepia toning. The simple answer is: just not darn strong enough. Sepia and selenium toning do add some density, but the process can be done only once and then you’re stuck. If it’s enough, you’re SOL and have no choice but to accept the negative as it is.

Pyro/staining redevelopment can be repeated a couple of times, but has two drawbacks: (1) it tends to add base fog/stain, and especially with DAS carbon and its already long exposure times, I find this objectionable. After all, 0.15logD of additional fog/stain means something like an extra 30 minutes of exposure! And (2) the potential of repeating the process is kind of limited, since staining/tanning developers make the silver image less accessible to the bleach, so after two rounds or so, it becomes very difficult or even impossible to bleach back the silver image for another round of intensification. Add to this the limited density gain of pyro stain to begin with, and the process of pyro redevelopment turns out to have a finite net density gain even if it’s repeated.

Dichromate intensification, however, can be repeated as often as you like and there seems to be no practical limited to the density that’s built with it. Moreover, the net density gain in even a single round of intensification can be much more significant than with the alternatives mentioned above, and the result is very clean – there’s virtually no added base density or fog. On occasion, I’ve run the same strip of film through the process four times and the base density remained unchanged.

The main drawback of the process is that it’s somewhat unpredictable. The net density gain can range from marginal to absolutely massive, and in the latter case it’s really a sledgehammer approach. Well, I can get behind “go big or go home”, so I’m fine with a big hammer.

Chromium intensifier materials. 16% ammonium dichromate, 10% hydrochloric acid, some ~20% sodium carbonate and a developer of your choice; in this case ID62. The developer is green because I’ve used it before to develop the negatives in the first place, and the antihalation dyes of the Fomapan 200 film dissolved into the developer. It’s harmless and in fact looks kind of pretty.

Getting this show on the road is pretty easy. I keep some 16% ammonium dichromate solution on hand and I generally use 1-2ml to 200ml of water. It’s in fact not very critical. I then add 1-5ml of 10% hydrochloric acid. That makes the rehalogenating bleach.

By the way, the mixed bleach is said to have a limited lifetime. This is true; while the hydrochloric acid and dichromate solution last virtually indefinitely by themselves, the bleach ultimately ‘goes off’. However, it may take a week or longer for it to die, so there’s no need to worry about the bleach going bad as you’re working with it. I would still recommend to mix it not too long before using it.

Chromium intensifier bleach. Nasty, but pretty looking.

Then, the negative is placed in a tray, the bleach goes in and it’s a matter of agitating continuously until the entire negative has bleached back to a faint, light-colored image.

I use high-walled storage tubs for trays because they’re easy to obtain, cheap and the high walls help to prevent chemistry from splashing all over the place. That’s a benefit in particular when handling stuff as nasty as dichromate.
Bleaching has started; not much to see yet
Note how the low densities have started to bleach out. This may look a little disconcerting, but fear not – they’ll come back, with a vengeance!

At this point, it sometimes happens that the bleaching slows down to a standstill. As bleaching progresses, the chloride in the bleach combines with silver to form silver chloride – and that process slows down as the chloride runs out. So I often pour the bleach back into a beaker, add another 1-2ml of hydrochloride, and put the bleach back into the tray.

In fact, the more hydrochloride you add, the faster the bleach works – up to a certain point, at least. However, some sources suggest that adding more chloride (so a higher chloride-dichromate ratio) will result in less intensification. I’ve done some experiments and could not quite reproduce that effect, at least not reliably. But since I’m in it for the big gains, I try to keep the bleach a little lean on chloride. The consequence is that I often have to add more (hydro)chloride during the bleach process as it runs out.

The bleach has started to eat into the high densities as well; they’re starting to give way.
Bleaching complete; note how the image density has gone orange/tan. There’s virtually no metallic silver left at this point.

By the end of the bleach process, the entire image consists of silver chloride and chromium compounds. Don’t ask me which ones, exactly. I tried to figure out what’s happening at a chemical level, and to cut a long story short: I couldn’t. I asked help from two people who are chemically better versed than I am, but this was also outside their field of expertise, so the whole thing remains a bit of a black…errr..brown box. One thing is certain: there’s silver chloride in the image, and some kind(s) of chromium compound(s).

There’s also some dichromate bleach lingering in the emulsion that we need to get rid of. I find that if this isn’t washed out adequately, some fog is deposited. To aid the wash process, I mix a spoon of 20% sodium carbonate in half a liter of water and soak the negative for a minute or two in that solution. This proves to be very effective at pulling the bleach out of the emulsion. Rinse thoroughly a couple of times afterward.

The negative now looks like this when held up to the light:

I find that the next step is also necessary – or at least, I have the distinct impression it is. It seems that the silver chloride benefits from thorough exposure to strong UV light so it becomes easily developable. The light source used for my alt. process prints is a convenient tool for this. I give about 10 seconds of strong UV exposure at a short distance.

Baking the negative for a few seconds under strong UV light to ensure all of the silver halide is rendered developable.

In goes the developer! Just pour it on; development should be pretty much instantaneous and doesn’t take more than a minute to complete – at least when using concentrated ID62 as I’m doing here. I find a vigorous, fast developer gives a clean and strong intensification. On occasion, I’ve used a staining developer so that the stain also adds to the UV density, but due to the small amount of fog this also easily creates, I generally use a non-staining developer.

Note how development happens instantly as the developer is poured onto the film.
Development progresses rapidly; this sequence of images was taken in the timeframe of just a few seconds.

Don’t worry about uneven development. It’ll right itself. Just keep developing until the density looks even across the sheet.

That’s all there is to it, really. All that’s left is a thorough wash to remove the traces of developer. A regular wash just like after initial processing is fine. No fixing is necessary, since all silver halide is redeveloped, so there’s nothing left that needs to be fixed out.

The negative after intensification and washing, but still wet. Yup – absolutely guaranteed bulletproof!

There’s one more thing left to do – get rid of the bleach somewhat responsibly. The big problem with the bleach is the dichromate itself, with the nasty bit being the chromium VI. The VI refers to the oxidation state; it basically denotes that the chromium ion is 6 electrons short (i.e. it has a +6 electrical charge). The nastiness comes at least in part from the fact that the chromium so badly wants to get these electrons back that it’ll strip them off of anything it runs into – including your body. And that tends to wreak havoc on lots of things, including DNA. Not good.

One easy way to make the whole situation a lot less troublesome is to just give in and hand over some electrons to make the chromium settle down. Ways to do this include dissolving some sodium metabisulfite or ascorbic acid (vitamin C) into the bleach. Each chromium ion greedily scoops up three electrons right away, turning it into much less troublesome chromium III. This change is easily visible, too, because the vibrant orange/yellow of chromium VI gives way to a more dull, paler green.

The bleach with a few grains of sodium metabisulfite added to it; note them lingering at the bottom of the beaker, and the greenish tint surrounding them – this is actually chromium VI that has already turned into much less harmful chromium III.

See above the progression of the conversion as the solution is gently stirred. In the last image, virtually all chromium VI is converted into chromium III.

We do have some more unfinished business – after all, how did we do? What density gain did all this result in? Let’s have a look with the densitometer:

A little hard to read, but let me help: the white waters now measure around 3.0-3.1 logD – they were previously around 1.9-2.0 logD, so a net gain of about 1.0 logD. That’s 3 stops additional contrast – pretty darn steep! The aim was about 2.7 logD and it turns out I overshot the target a little on this negative. But as long as it’s not too extreme, I can live with that.

How about the print? Here it is, comparing the pre- vs. post version:

Now, don’t say you prefer the old one. Ah heck, if you want, feel free to – but I was aiming for the print to the right, so I’m happy with the change. I did end up preflashing the carbon tissue for about 1.5 seconds to strong UV lights to help the very dense highlights render well without getting the shadows blocking all the way up. The exposure time on this print was 30 minutes, which gives a good, solid dmax while still retaining shadow differentiation (although the phone shot doesn’t really show it too well).

On a final note: dichromate intensifier can also be a savior in situations where something went wrong. Last month I was using up the last bit of a bulk roll of Fomapan 100 film, and I decided to play with development a little (resulting in, among others, a case of extreme edge effects). One of these rolls I developed in some parodinal I had made months ago – and I guess that parodinal didn’t really perform as it should have. I was left with the thinnest negatives I’ve ever seen; the density range was something like 0.2 logD – out of range of a grade 5 print!

Chromium intensifier came to the rescue in this case as well. Since intensification can only work with silver density that’s already there, and there wasn’t that much density to begin with, I had to repeat the process twice or three times in order to obtain printable negatives. But it worked alright! Here’s some prints from that roll – coincidentally, the coarse grain made for very pronounced acutance and the reduced initial development caused massive compression, and the net result was actually pleasing in some of these prints.

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