The metol is the developing agent that does the actual work of converting the exposed silver halide in the film's emulsion into black silver. It does this in direct proportion to the amount of light that fell on the emulsion during exposure.
But metol on its own would not only be extremely slow working, it would oxidise almost as soon as it was dissolved in water. Hence the second chemical. The sodium sulphite performs two functions in D-23. First, it acts as a preservative to prevent the metol from oxidising. And second, because it is mildly alkaline, it acts as an accelerator to make the metol more active.
The negative above shows full detail into the sky area, while if you adjust your screen gamma, you will see that there is also a clear separation of tones in the land just above the sea. This was an extreme brightness range.
D-23 is a soft-working developer. Metol tends to work quite quickly on under-exposed silver halides, but very slowly on more heavily exposed areas. What this means in practical terms is that shadow detail builds quickly while highlights evolve more slowly. The result is a low contrast negative, which why metol is usually combined with hydroquinone, which is a much faster working developing agent, to produce a so-called normal contrast developer.
This low contrast, however, gives D-23 one of its major advantages; it almost never produces blocked highlights, even when you extend the developing time to produce increased film speed.
Although it was introduced by Kodak in 1945, D-23 was by no means the first developer to use just metol and sodium sulphite. Indeed, the first appeared shortly after Bogisch and Hauff discovered metol in 1891. Perhaps the most famous is the Windisch compensating developer formulated in Germany by Hans Windisch before the second world war. This developer, though, was considerably weaker than D-23, so the negatives it produced were even softer. It was, however, ideal for really contrasty subjects.
Metol 7.5 grams Sodium sulphite, anhydrous 100 grams Water to 1000 mlThere's not a great deal to say about mixing the developer, but here are a couple of tips.
Start with about 500 ml of water at around 40 deg C (104 deg F) and add the metol first. You'll find that it dissolves easily in plain water, but if you add the sodium sulphite first the metol will be very difficult to dissolve completely.
It's a good idea, though, to dissolve just a pinch of the sulphite before you add the metol. This will prevent the metol starting to oxidise as soon as it's in solution, as witnessed by a less yellow appearance to the final developer.
Film Development time (20 deg C) Agfa APX25 9 APX100 9 AP400 10 Fuji Neopan 400 8 Neopan 1600 6-1/2 Ilford 100 Delta 7 400 Delta 7 Pan F 6-1/2 FP4 Plus 6 HP5 Plus 7-1/2 Kodak T-Max 100 8 Plus-X 7 T-Max 400 7 T-Max P3200 11At these times you'll find that D-23 maintains very full film speed. This is probably due to another feature of sodium sulphite, especially when it's used in fairly high concentrations, as it is in D-23. It acts as a silver halide solvent and can actually produce increased film speeds.
What happens is this. The sodium sulphite dissolves part of the outer surface of the silver halide grains, allowing the developing agent - metol - to penetrate more effectively into the grains to develop them fully. A secondary benefit given by this solvent effect is that grain is effectively reduced, because each individual grain is smaller.
A litre of D-23 will process up to ten 36-exposure 35mm films or the equivalent. You'll need to increase the development time by 10% after the first two films and after every two additional films after that. A versatile developer.
As well as being a very simple developer, D-23 is also a very versatile one. I mentioned earlier the Windisch compensating developer. Well, D-23 is more accurately described as a semi-compensating developer, but you can make it into a fully compensating solution by simply diluting it with water.
Diluting one part of D-23 with one part of water produces a developer that not only produces a somewhat softer negative but also a slightly sharper one. But to form a true compensating developer that will handle just about any subject brightness range you care to throw at it, dilute one part of D-23 with three parts of water.
At this strength there's effectively just under two grams of metol and 25 grams of sodium sulphite in a litre of the developer. This is very close to the classic Windisch formula (2.5 grams of metol and 25 grams of sodium sulphite) and gives very similar results.
Naturally, you need to increase the development times when you dilute the developer, and once again the times for D-76 and ID-11 provide the ideal starting point.
Film Development time (20 deg C) 1+1 1+3 Agfa APX25 13-1/2 NR APX100 13-1/2 NR AP400 14-1/2 25 Fuji Neopan 400 14 20 Neopan 1600 10 15 Ilford 100 Delta 10 15-1/2 400 Delta 10-1/2 16-1/2 Pan F 8-1/2 15 FP4 Plus 8 18 HP5 Plus 13 20 Kodak T-Max 100 11 16 Plus-X 8 13 T-Max 400 10 15 T-Max P3200 16 NRWhen using D-23 diluted, treat it as a one-shot developer.
Because D-23 is an inherently low contrast developer you can increase the development time to increase the effective film speed without boosting the contrast of the negative too much. Generally, an increase in development time of about 50% will give you one stop more in film speed, although you'll need to experiment a little to find the best times for your particular requirements. But it's best to stick to undiluted D-23 or a 1+1 dilution if you want to push film speed or development times become very long.
The developer absorbed by the highlight areas of the film becomes exhausted quickly, but that in the shadow areas continues working. As a result you get excellent shadow detail and well controlled highlights that won't burn out when you make your print.
The advantages of two-bath developers are many. You get absolute consistency, because the developer doesn't become progressively less active as you process more films. Contrast is controlled to a much greater extent than with any other type of developer, but without loss of film speed, and exposure latitude is increased.
And finally, you can develop most slow and medium speed films for the same time in the two baths - about 4-1/2 minutes in the first bath and 3 minutes in the second bath. Fast films, though need about 6 minutes in the first bath and 4 minutes in the second.
So you can see that D-23 really is a most versatile developer. More than that, it's almost a universal film developer in that it will solve most photographic problems depending on how you use it. But its main advantage remains the excellent way it allows detail to build up in the shadows while controlling highlights so that they almost never clog. Because of this it produces negatives that are extremely easy to print without a great deal of burning-in of highlights and holding-back of shadows. I love it.
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