Is contrast in two-bath developers controlled by the developer or the alkali?

I'm interested in two-bath developers like D-23 and Pyrocat HD two-bath for their compensating development.

Is it possible to change the rate at which development takes place (i.e. contrast control), the rate at which silver halides exposed to sufficient light are reduced to elemental silver, by changing the dilutions of one, the other, or both baths?

I would think it is possible.

If so, is it better to reduce or increase the concentration of the reducing agent (bath 1) or better to reduce or increase the concentration of the alkali? (bath 2)

Thanks,

-S

First, note that there is a big difference between the divided D-23 type of "two-bath" and a true two bath (divided Pyrocat would be an example).

In a true two bath system such as divided Pyrocat, Diafine etc, virtually zero development takes place in the first bath. The function of the first bath is simply for the emulsion to absorb the developing agents, which are then activated when the film comes into contact with the second bath (alkali). Contrast is primarily controlled by the amount of developing agent present in the emulsion, so increasing the concentration of the first bath can help. A small amount of contrast control with this process may be had by altering the time in the first bath. Typically the time in the first bath is sufficient for maximum uptake of developing agent, but this time can be shortened for somewhat less absorption, which may or may not have an effect on contrast. Most of these types of developers use a relatively high pH second bath, so that development very quick. In most cases altering the concentration of the second bath is not an effective control.

In the second type of two-bath process (more often referred to as two-solution) includes, development takes place in the first bath, and is then "completed" in the second bath. All the "divided D-23" variants (more correctly called metol-sulfite + alkali) and some other formulas fall into this category. In these cases, you have much more control over contrast by simply altering the development time in the first bath (you can also combine this with increased concentration and/or increased agitation). Much of the information out there is fairly faulty regarding alterations to the concentration of the second bath to control contrast.

In either case, this type of development tends to yield a somewhat straightened characteristic curve (which can be useful for negatives destined for scanning), and relatively high emulsion speed. There are risks, however, associated primarily with true two-bath processes using highly alkaline (carbonate) second baths. Since development takes place very quickly, it can be tricky to avoid uneven development/mottle or streaking. Some experimentation is typically required.

Please have a look at the article on two-bath developers on the PyrocatHD web site, www.pyrocat-hd.com/. Though primarily about the use of Pyrocat as a two-bath developer, there are other general issues discussed that may address some of your questions.

http://www.pyrocat-hd.com/html/TwoBathPyrocat.html

Sandy

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Originally Posted by Sylvester Graham

I'm interested in two-bath developers like D-23 and Pyrocat HD two-bath for their compensating development.

Is it possible to change the rate at which development takes place (i.e. contrast control), the rate at which silver halides exposed to sufficient light are reduced to elemental silver, by changing the dilutions of one, the other, or both baths?

I would think it is possible.

If so, is it better to reduce or increase the concentration of the reducing agent (bath 1) or better to reduce or increase the concentration of the alkali? (bath 2)

Thanks,

-S

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Michael is right on.

The late Barry Thornton devised a variant of Divided D-23. He recommended varying the strength of the accelerator in the B bath to obtain "+" or "-" development using a two bath. Here is his formula - A bath - 750 ml water, 6.5g metol, 80g sodium sulfite, water to make 1L. B bath - 750 ml water, 12g sodium metaborate for N development, water to make 1L (7g/liter for N-, and 20g/liter for N+). I have tried the 7g/liter for N- development and it worked fine for my purposes. I modify his formula, splitting the 80g/liter of sodium sulfite between the two baths. It works. I assume this reduces the pace of the development in the A bath, being a significant reduction from the standard 100g/liter. Thornton reduced the amount of metol in the A bath compared to Adams' version of divided D-23. If I recall, the Adams version called for 7.5g/liter of metol.

Certainly, with a standard D-23 type two bath developer you can extend (reduce) the time in the A bath to increase (decrease) contrast.

Thornton also mentioned an alternative formulation which reduced the sodium sulfite even further, but then used sodium carbonate as the accelerator. I don't have the formula handy and have not tried it. Thornton said the results were similar to those obtained by the Beutler high acutance formula.

All of these options should give you the opportunity increase contrast.

There are many threads on divided developers in the archives. They should provide some useful information. The page Sandy King linked to is also very helpful. This is a link to an article he wrote on two bath developers - http://www.tmax100.com/photo/pdf/devforscan.pdf. It is an excellent article.

Although many photographers do not think two bath developers are as useful for negatives printed in the darkroom as they are for scanning and digital printing, I have been using them for years and printing traditionally. This is after spending many years using typical single bath developers like D-76 or Xtol. I have not seen any reason to go back.

Very interesting responses, thank you.

I'll look through the archives.

Reviewing Sandy's article and the other link yields some good things.

Two-bath development, especially "true" two-bath where reducer and accelerator are separated completely, which is what I originally meant, seem to boil down to being possible because of the porous nature of gelatin. Water and compounds dissolved within water are absorbed into swollen gelatin. They can diffuse in and out of the gelatin if the concentrations of ions on either side of the gelatin (within the gelatin, and the solution surrounding it) are different.

My conception of this process is that, since when the reducing solution is poured away, you remain with gelatin that contains a finite amount of reducing agent, which when placed in the accelerator is therefore self-limiting; unlike conventional developers where fresh reducer and accelerator are always diffusing into the gelatin so the development process can continue and contrast can increase.

So, in your article Sandy, when you say:

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The final contrast of the negative is limited by the amount of reducer the film was able to imbibe in the first bath.

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That makes intuitive sense to me, as I assume there are only so many molecules of reducing agent in the thin gelatin emulsion to reduce the silver halide crystals that have been sufficiently exposed to light to form a latent image. With this I would assume that if "amount of reducer the film was able to imbibe in the first bath" means the quantity of reducing agent molecules that make it into the gelatin, the most straightforward way to do this would be to change the ratio of reducing agent to water. Then you say:

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If dilute solutions are used overdevelopment is impossible, even with negatives made in scenes of very great contrast.

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When weak dilutions are used, can this process yield contrasts as low as ultra low contrast developers like POTA for pictorial development of document films? Can you even compare the two? Which seem to be operating in very different ways ,and I believe the darkroom cookbook explains that the mechanisms behind ultra low contrast developers were and are not well understood.

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Originally Posted by Sylvester Graham

When weak dilutions are used, can this process yield contrasts as low as ultra low contrast developers like POTA for pictorial development of document films? Can you even compare the two? Which seem to be operating in very different ways ,and I believe the darkroom cookbook explains that the mechanisms behind ultra low contrast developers were and are not well understood.

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Generalizations are problematic, but no it doesn't work that way. The mechanisms of action in POTA-type developers are different.