Kodak DK-20

Set this beside D-25 and you have the best evidence in the formulary that “fine grain” names a result rather than a method. Both are Kodak’s extra-fine-grain developers. D-25 gets there by lowering the pH until development crawls, and Kodak prints the price in the title. This one gets there by adding a second silver solvent at one gram a litre, keeps a real alkali, adds a restrainer to cover the risk, and finishes in fifteen minutes at ordinary room temperature.

Ingredient	Quantity	Form the source specifies
Metol	5.0 g	Kodak calls it Elon
Sodium sulfite	100.0 g	anhydrous — or 200.0 g of the crystalline salt
Sodium metaborate	2.0 g	Kodak sold it as Kodalk
Potassium thiocyanate	1.0 g	the handbook writes “sulphocyanide”
Potassium bromide	0.5 g
Water to make	1000 mL

Purpose

Extra-fine grain at normal contrast, in a working time a darkroom can live with. Kodak’s formula header calls it an “extra fine grain developer for films or plates, in dish or tank”; the packed-developer list a few pages earlier calls the same product a “normal contrast dish or tank developer for extra fine grain images”. Between them those two descriptions state the whole design brief: fine grain, without giving up the contrast.

Recommended uses

Small-format negatives destined for a big enlargement, where grain is the limiting factor and the subject has enough light for a fifteen-minute development to be practical.

Tank work with replenishment. Kodak’s own note under the formula is that the useful life of this developer can be increased greatly by DK-20R, and the keeping table’s footnote repeats it. This is a formula designed for a tank in regular use rather than for one-shot work.

Where a carbonate developer causes trouble in the fixer. The metaborate is the alkali Kodak chose partly for what it does downstream: it does not evolve carbon dioxide when it meets an acid fixer, and it is less prone to throw aluminium sulfite out of an alum hardening bath. The metaborate entry gives Kodak’s statement of both.

When another formula is preferable

• D-23 does fine grain with two ingredients and no thiocyanate. If you want to understand a developer rather than get the last increment of grain, start there.
• D-25 claims minimum grain and states its cost. Compare the two rather than choosing between them blind: they are the same manufacturer answering the same question twice.
• D-76 is the developer to use if the negative has to be sharp. Two silver solvents in one bath is not the way to an acutance negative.
• DK-50 shares the metaborate and drops the thiocyanate for hydroquinone and five times the alkali, which is the same family of chemistry aimed at speed of working rather than at grain.

Mixing

Kodak’s instruction is the usual one — dissolve in the order given — and the order in the handbook is Elon, sulfite, Kodalk, thiocyanate, bromide. Two of the general rules from the same handbook apply and one of them lets you relax:

• The Elon goes first, in warm water, because it is only slightly soluble in a sulfite solution that has no alkali in it.
• The alkali goes after the agent and the preservative, so that no dissolved developing agent sits in an alkaline solution unprotected.
• The bromide can go in whenever you like. The handbook says so explicitly: potassium bromide has no action on the developing agents, so the stage at which it is added is immaterial. It is the one ingredient in this reference whose position in the list does not matter.

The metaborate is the easiest alkali here to get into solution — its solubility at room temperature is roughly eight times borax’s — so unlike D-76 there is no need for hot water at the alkali step.

⚠Never let this bath meet an acid outside the film

A thiocyanate is safe to handle as a salt and is not safe to acidify. Strong acid liberates hydrogen cyanide from thiocyanates, and the encyclopaedia entry sets out the conditions. Nothing in normal use goes near them — a stop bath is dilute and the developer is a litre of alkaline solution — but the dry jar must be stored away from every acid in the darkroom, and spent DK-20 must never be acidified, whatever a disposal instruction for an ordinary developer might say. See chemical incompatibilities.

Behaviour

Fifteen minutes at 18 °C, undiluted, as Kodak’s average figure. No dilution is offered and no second time is printed, which is the pattern for the whole DK family in this handbook.

Its keeping and capacity are D-76’s. 24 hours in a dish, a month in a tank, six months in a full bottle, two months half-filled; 24 sheets of 8 × 10 in a dish and 36 in a deep tank per 160 fluid ounces, with the same footnote about replenishment. That is the signature of the 100 g of sulfite, which is doing the same protective job here as it does everywhere else in this section.

Kodak publishes no exposure penalty, in pointed contrast to D-25, where the warning is in the title. This page does not read that silence as a claim of full speed. A bath containing two silver solvents dissolves image silver as well as fog silver, and a formula that also carries half a gram of restrainer has two independent reasons to lose a little shadow density. Kodak simply does not say, and neither does the course; it is the first thing to measure.

Image characteristics

Grain: extra fine, on Kodak’s own description and by a mechanism the manufacturer names — two silver solvents rather than one. Contrast: normal, which is what distinguishes it from D-23 and D-25, both of which are soft-working. Sharpness: low. Solvent action and edge effects pull against each other, and this formula has chosen. Speed: unpublished, as above. Tonality: nothing is published, and the honest thing to say is that a normal-contrast developer with a restrainer in it is likely to give a slightly raised toe, which is a prediction rather than a fact.

The mechanism

Two solvents, working on different scales. Sulfite at 0.79 mol/L is the bulk silver solvent that every Kodak fine-grain developer of this era relies on. Thiocyanate at 1 g/L is a far more powerful complexing agent per ion — the encyclopaedia entry gives the formation constants that put it about a thousand times ahead of ammonia and just short of thiosulfate — present at a hundredth of the sulfite’s concentration.

100 g/L ÷ 126.05 g/mol = 0.79 mol/L

Sulfite

1 g/L ÷ 97.18 g/mol = 0.0103 mol/L

Thiocyanate

2 g/L ÷ 137.86 g/mol = 0.0145 mol/L

Metaborate

5 g/L ÷ 344.39 g/mol = 0.0145 mol/L

Metol, as the hemisulfate

0.5 g/L ÷ 119.00 g/mol = 0.0042 mol/L

Bromide

Metol is weighed as the hemisulfate, and Part VIII’s convention is that each formula unit supplies two developing molecules, so the agent concentration is 0.029 mol/L. Two comparisons come out of the list. The alkali is at half the agent concentration, 0.0145 against 0.029 — a very small quantity of alkali for a bath expected to develop in a quarter of an hour, which is the design. And the thiocyanate outnumbers the bromide two and a half to one, which is the balance the formula turns on.

Why a strong solvent needs a restrainer. Dissolving silver halide during development means silver is in solution, and silver in solution can be reduced anywhere, including on grains that were never exposed. That is the classic route to fog in a solvent developer. Half a gram of bromide per litre raises the threshold for development and is the counterweight. Part VIII’s restrainer lesson puts this formula at the low end of its published range — 0.5 g/L here and in DK-50, against 4.0 g/L in D-19b — which tells you the fog risk being covered is modest.

Why the alkali is metaborate and why there is so little of it. Kodak’s own description of Kodalk is that it is intermediate in activity between carbonate and borax and permits greater control of the energy of the developer. At 2 g/L it is the smallest published dose in the DK family — a tenth of DK-50’s — and it is the reason this bath takes fifteen minutes rather than ten. A slow development gives both solvents longer to work before the grains are gone, which is the same relationship that makes a low-pH developer fine-grained, arrived at with an alkali you can still adjust.

What no source in this course explains. Kodak does not say why DK-20R carries five times the thiocyanate of the bath it replenishes, and the course does not supply a reason. The thiocyanate page notes the stoichiometry — four thiocyanate ions per silver — as a fact about the complex, and marks the connection to the replenisher as unexplained.

Function of every ingredient

Metol, 5.0 g. The only developing agent, at two and a half times D-76’s dose because there is no hydroquinone behind it. Kodak’s reduction-potential ranking is what makes a single-agent metol developer workable at a modest pH at all. More metol shortens the time and raises activity without touching the solvent balance; less lengthens an already long development. Because there is no second agent in reserve, the exhaustion of this bath is felt as a straightforward loss of activity, which is what the replenisher addresses. Metol is also the ingredient that sets the safety level of the page as a skin sensitiser.

Sodium sulfite, 100.0 g anhydrous. Preservative, buffer reserve and the bulk silver solvent, at the concentration Kodak used in every fine-grain developer of this family. Its solvent action is the foundation the thiocyanate builds on rather than an alternative to it. More would finish the grain and cost more speed; less would remove most of the fine-grain effect and leave the thiocyanate working alone, which is not what the formula was designed around.

Sodium metaborate tetrahydrate, 2.0 g. The alkali. It sets the rate at which the metol works and therefore the fifteen minutes, and it does so with a salt that will not fizz carbon dioxide into a swollen emulsion when the film reaches an acid fixer. Kodak’s own note about the adjustability of Kodalk is printed under DK-50 and applies here: more of it shortens the time or raises the contrast depending on which you hold fixed; less does the reverse. At 2 g/L there is very little buffer reserve, which is why the tank needs replenishing rather than topping up.

Potassium thiocyanate, 1.0 g. The ingredient the formula exists for. It complexes silver ion strongly, dissolving the finest grains and etching the rest, which is what “extra fine grain” means here. It is a mild enough solvent to be used during development rather than after it — thiosulfate at this concentration would attack the image. More thiocyanate means finer grain, more dissolved silver, more fog risk and more lost density; less moves the formula back towards a plain metol-sulfite developer. There is no published latitude on this figure, and one gram in a litre is a small enough quantity that a careless weighing is a large proportional error.

Potassium bromide, 0.5 g. The restrainer. It raises the exposure threshold at which development begins, which suppresses the fog a silver-solvent bath is liable to and costs a little shadow speed doing it. More bromide means cleaner highlights, a raised toe and a slower bath; less invites fog from the dissolved silver the thiocyanate has put into solution. It is also the one ingredient whose position in the mixing order is genuinely free, because it does not react with the agents.

Water to 1000 mL. Warm for the metol, cold to make up, and clean: iron in the supply is the classic developer contaminant, and this bath already has a silver-complexing agent in it without help.

Interactions

Thiocyanate against sulfite. Both dissolve silver halide and their effects are not simply additive, because they compete for the same silver. The course has no source that quantifies the combination and does not model it.

Thiocyanate against bromide. The interesting pair. One puts silver into solution, the other holds development back. Kodak’s balance is 2.5 : 1 in moles, and moving either without the other changes both grain and fog. This is the single best one-variable experiment on the page.

Alkali against everything. The metaborate sets the rate, and the rate sets how long the solvents have. Raising it makes a faster, coarser-grained, more contrasty negative; lowering it makes a slower, finer, flatter one. That is DK-50 and this formula described as two points on one line.

Agent and agent: none. Single-agent developer, so no superadditivity to account for.

Variants

DK-20R is the replenisher Kodak published for it, and it is not a scaled-up DK-20: half again the metol, ten times the metaborate, five times the thiocyanate, twice the bromide, and the same sulfite.

DK-15, the tropical developer of the same family, is a different formula and has no entry here yet; the metaborate page records its Kodalk dose and its sodium sulfate load from the same handbook, and notes that the handbook gives the sulfate quantity without explaining it.

Thiocyanate-free “fine grain” substitutions circulate and are not published here. Under Rule 6 the course prints the formula it can cite, and a substitution nobody has published a result for is a proposal, not a formula — which is what the formula version record is for.

Safety

Level B, on two ingredients rather than one. Metol is a skin sensitiser. Potassium thiocyanate carries a Danger classification — harmful by all three routes, and serious eye damage on the harsher of the two views the notifiers take — and brings the cyanide-on-acid hazard the callout above describes. Sodium metaborate tetrahydrate is classified for reproductive toxicity. Sodium sulfite and potassium bromide are Level A materials.

The classification rubric sets what Level B assumes. For this page in particular: three of the five powders need weighing with extraction or in an enclosure, the thiocyanate jar is stored with the alkalis and never with the acids, and gloves and eye protection stay on for mixing and for the tank.

What is not a hazard here. The mixed developer at 2 g/L of a weak alkali is not corrosive, and it does not evolve a gas in use — the cyanide hazard belongs to the dry salt meeting a strong acid, not to a litre of alkaline developer. Nothing is heated beyond the warm water for the metol. The ventilation requirement is for powder while weighing.

Storage

Tightly corked, full, small bottles rather than one large one, six months and two months. Label the bottle DK-20 — contains thiocyanate, because the waste route and the incompatibility list for this developer differ from every other one in the darkroom and the difference is invisible.

The dry thiocyanate is hygroscopic enough to want a tightly closed jar, and the metaborate melts at 53.5 °C, so neither belongs on a shelf above a radiator. See the labelling SOP.

Incompatibilities

Acids — the strongest incompatibility on any developer page in this section. A thiocyanate and a strong acid give hydrogen cyanide. This governs storage of the dry salt, and it governs disposal: do not neutralise spent DK-20, and do not put it down a drain that carries anything acidic.

Fixer, in either direction, and here with an extra reason: the bath already contains a silver solvent, and thiosulfate carried back into it is the shortest route to dichroic fog that this formulary contains.

Oxidising agents — ferricyanide, dichromate, permanganate, persulfate — never with a developing agent, and never with a thiocyanate.

See chemical incompatibilities for the reactions.

Waste

This one is not an ordinary developer waste. Spent DK-20 carries metol, a large sulfite load, borate, bromide, thiocyanate — and dissolved silver, because that is what the thiocyanate has been doing for fifteen minutes. Two consequences follow. It must never be acidified, and it is a silver-bearing stream in a way that ordinary spent developer is not, so the silver-bearing waste SOP is the right procedure rather than the general chemical waste one.

The disposal caveat governs and local regulation decides. Where you are uncertain which stream something belongs to, the SOP’s rule is to treat it as the more restricted one.

Troubleshooting

Grain no finer than D-23 gave. Check the thiocyanate. One gram in a litre is a small weighing and an easy one to get badly wrong, and it is the entire difference between this formula and a plain metol-sulfite developer with a little alkali.

Overall fog and a dichroic sheen — pink by transmitted light, green by reflection. Silver reduced out of solution rather than in the grain. Either the bromide was left out or short-weighed, or the bath has picked up thiosulfate from a fixer-contaminated tank or graduate.

Thin shadows. Expected to some degree, and unpublished in extent. Establish your own exposure index for this developer before you blame the bath.

Activity falling away in a tank faster than the sheet count suggests. The alkali reserve at 2 g/L is very small. This is the formula the replenisher note was written for.

A smell of ammonia or anything sharp over the tank. Not normal. Stop, ventilate and find the contamination; a thiocyanate bath is not one to investigate by leaning over it.

Experiments

Move the thiocyanate and hold everything else. Three litres at 0.5, 1.0 and 2.0 g/L, matched strips from one exposure batch, same time and temperature. You are testing the one ingredient the formula is named for, and the prediction is finer grain and lower density as the dose rises.

Move the bromide instead. 0.25, 0.5 and 1.0 g/L. The prediction is a rising toe and cleaner highlights. Run it alongside the thiocyanate series and you have mapped the balance the formula sits on. This is the activity series applied to a formula that has both levers in it.

Measure the speed Kodak did not publish. Bracket a film, develop in DK-20 at fifteen minutes, and find the exposure that matches your D-76 shadow density. Whatever number comes out is a real measurement of a real gap in the published record.

Compare the two routes to fine grain directly. DK-20 at fifteen minutes at 18 °C against D-25 at eighteen minutes at 25 °C, both at Kodak’s published figures, on the same film from the same exposure batch. Two manufacturers’ answers to one question, and the comparison belongs in the laboratory report.