Kodak D-156

The same five chemicals as D-72, in different proportions, and the print comes out a different colour. That is the whole subject of this page, and it is the cleanest demonstration in the formulary that a formula is a set of ratios rather than a list of substances. Kodak Limited heads it “warm-tone developer for Kodura, Bromesko and other chlorobromide materials”, and its product list calls the same thing “a normal contrast developer giving an image of medium warm tone”.

Ingredient	Quantity	Form the source specifies
Metol	1.7 g	Kodak calls it Elon
Sodium sulfite	22.0 g	anhydrous — or 44.0 g crystalline
Hydroquinone	6.8 g
Sodium carbonate	16.0 g	anhydrous — or 44.0 g of the crystals
Potassium bromide	6.3 g	or 63 mL of a 10 per cent solution
Water to make	1000 mL

Purpose

To develop a chlorobromide paper to a warm image tone rather than a neutral one, without giving up normal contrast. It is the first of two steps along that axis in this library: D-166 goes further, and Kodak’s own product list says so — it describes D-166 as “giving warmer tones than D-156”.

Recommended uses

Chlorobromide papers at 1:1, approximately two minutes at 18 °C. That is all the handbook publishes: one dilution, one time, one class of material.

The material matters here more than for any other formula in the library, because a warm-tone developer on a paper that was not designed for warm tones mostly just develops slowly. Chlorobromide emulsions carry both halides, and Part V’s warm-tone project covers what that does and — importantly — what it does not do. It does not, on its own, set the image colour.

When another formula is preferable

• For a warmer image still, D-166 doubles the bromide again and dilutes further.
• For a cold blue-black, D-158 is this formula’s opposite on both levers, and comparing the two ingredient lists is the fastest way to learn what each lever does.
• For neutral general printing, D-72 or, with more contrast and a longer dish life, D-163.
• For low contrast, D-165 removes the hydroquinone entirely.
• If metol is a problem for your skin, D-173.

Mixing

Dissolve in the order printed, for the reason Kodak Limited’s handbook gives once for all its formulas: a developing agent dissolved before the preservative oxidises in air and forms coloured products, and where a formula contains Elon the Elon goes first, since it is readily soluble in warm water but only slightly soluble in a sulfite solution with no alkali in it. Bromide has no action on the agents and may go in at any stage.

1. Dissolve the metol completely, in warm water.
2. Add the sodium sulfite.
3. Add the hydroquinone.
4. Add the sodium carbonate.
5. Add the potassium bromide, or 63 mL of a 10 per cent stock.
6. Make up to 1000 mL, and dilute with an equal volume of water for use.

🧮Weigh 6.3 g, or measure 63 millilitres

The bromide is the ingredient that distinguishes this formula, and it is also the one most often got wrong, because it can be added as a solid or as a solution and the two numbers look nothing alike. 63 mL of a 10 per cent w/v stock contains 63 × 0.10 = 6.3 g, which is the same thing. Keeping a 10 per cent potassium bromide stock is worth the bottle: it is used in three formulas in this library and it is the only practical way to add small, repeatable doses. Label it with its strength and the date, per the labelling SOP, and remember that the water in it counts towards your final volume.

The handbook prints both salts twice, at 44.0 g of crystalline sulfite or 22.0 g anhydrous, and 44.0 g of crystalline carbonate or 16.0 g anhydrous. Use one system throughout: the handbook’s weights-and-measures page warns in capitals that its avoirdupois and metric columns “are not exact equivalents”.

Behaviour

It is slow on purpose, and slow is the point. At the working dilution this bath carries 8 g/L of carbonate against D-158’s 34.5 g/L, and 3.15 g/L of bromide against D-158’s 0.45 g/L. Both changes retard development, and the two minutes Kodak publishes is four times D-158’s thirty to forty seconds.

⚗Five Kodak formulas for paper, at working strength

Comparing stock solutions is misleading, because Kodak dilutes them differently. At the dilution each is published for, per litre of the bath the paper actually sees:

Formula	Dilution	Carbonate	Bromide	Hydroquinone	Metol	Published colour
D-158	1:1	34.5 g	0.45 g	6.65 g	1.60 g	blue-black
D-72	1:2	22.5 g	0.63 g	4.07 g	1.03 g	not stated
D-163	1:3	16.3 g	0.70 g	4.25 g	0.58 g	not stated
D-156	1:1	8.0 g	3.15 g	3.40 g	0.85 g	medium warm
D-166	1:3	6.3 g	3.13 g	2.13 g	0.29 g	extra warm

Read the two middle columns downwards. The carbonate falls by a factor of five and a half from the coldest formula to the warmest; the bromide rises by a factor of seven. Nothing else in the table moves monotonically. This is the course’s own arithmetic on Kodak’s published quantities and dilutions, not a Kodak table, and it is arranged in this order because the colours in the last column are Kodak’s.

Its published capacity is a sentence, not a number, and the sentence is the interesting part. The 1949 useful-life table gives a sheet count for almost every formula in the handbook. For this one it gives instead: exhaustion affects colour of image; life depends on quality required. That is a manufacturer admitting that “exhausted” is not a property of the bath alone. A neutral developer is finished when it will no longer reach maximum black; a warm-tone developer starts drifting in colour long before that, and where you stop is a decision about the print rather than about the chemistry.

Keeping is short. Six hours in a dish — a quarter of D-72’s day — and the tank column is marked not recommended. The stock keeps three months full and one month half filled like the rest.

Image characteristics

Colour. Medium warm, on chlorobromide paper, by Kodak’s own description. The course records that as the maker’s description of the maker’s product, which is what it is: no measurement of image colour is published for this or any other formula in the handbook.

Contrast. Normal, by the product list. The hydroquinone-to-metol ratio is the same four to one as the neutral formulas, which is what keeps the contrast normal while the absolute doses come down.

Tonality. The blacks arrive slowly and late. That is the same slow deposition that produces the colour, and it means a print in this developer looks worryingly weak at forty seconds and finishes properly at two minutes.

Staining. None. Grain, acutance and speed are not meaningful properties of a paper developer.

The mechanism

6.3 g/L ÷ 119.00 g/mol = 0.053 mol/L

Potassium bromide

16.0 g/L ÷ 105.99 g/mol = 0.151 mol/L

Sodium carbonate

6.8 g/L ÷ 110.11 g/mol = 0.062 mol/L

Hydroquinone

22.0 g/L ÷ 126.05 g/mol = 0.175 mol/L

Sodium sulfite

1.7 g/L ÷ 344.38 g/mol = 0.0049 mol/L

Metol, as the hemisulfate

Image colour follows the rate of silver deposition. That is established in Part V’s chlorobromide project from Wall’s controlled series: plates across the whole chloride-to-bromide range, developed in one metol-hydroquinone developer, gave the same coloured image every time, and warm tones appeared only when he weakened the developer and raised its bromide — and then on every plate equally. Chapman Jones supplies the particle side, that image colour depends on the size of the silver particles, which slow deposition makes fine.

This formula is Wall’s two changes, published as a product. Weaken the developer: the carbonate is a quarter of D-72’s and the metol and hydroquinone about half. Raise the bromide: 6.3 g/L against 1.9 g/L. Kodak did in 1949 exactly what Wall described in 1929, and the fact that a manufacturer’s warm-tone formula turns out to be the same two moves is as close to independent confirmation as this part of the literature offers.

Why the bromide does more than merely slow things down. The 1928 primer records that bromide affects hydroquinone strongly and Elon much less, and that the higher an agent’s reduction potential the more bromide is needed to produce a given effect. So raising the bromide seven-fold does not retard the whole bath uniformly: it hobbles the agent that builds density steadily and rapidly, and leaves the agent that starts the image nearly alone. The result is a bath in which development begins everywhere and then proceeds slowly — which is precisely the condition Chapman Jones’s fine particles require.

Why the alkali does the same thing from the other side. The 1928 primer states the general rule that the quantity of alkali governs the energy of a developer. Cutting it to a quarter slows the whole reduction, and it does so without touching the ratio of the agents, which is how the contrast stays normal while the colour moves.

And one thing this formula does not do. It does not change the paper. If the emulsion has no capacity for a warm image — Wall’s series is the evidence that halide ratio alone will not give you one, but the paper’s grain size and its own manufacture do matter — then a warm-tone developer will produce a slow neutral print. That is why Kodak names the papers.

Function of every ingredient

Metol, 1.7 g. The starting agent, at half D-72’s dose. It has the highest reduction potential of the agents Kodak ranked, so it begins development in the lightly exposed regions and brings the image up everywhere at once; it is also barely restrained by the bromide, which is what allows this bath to work at all with 6.3 g/L of restrainer in it. More metol speeds the appearance and pushes the colour back towards neutral; less and the print will not start against the bromide.

Sodium sulfite, 22.0 g anhydrous. The preservative, at half the neutral formulas’ dose. It scavenges dissolved oxygen and intercepts the oxidised agents before they form coloured products. It is small here because this is a bath with a six-hour dish life that is not expected to work through a long session, and because sulfite is also mildly solvent — and a solvent is the last thing a formula wants when it is trying to keep the silver particles fine. More sulfite extends the working life and pushes the tone colder; less and the dish browns within the session.

Hydroquinone, 6.8 g. The contrast agent, at four times the metol by mass — the same ratio as the neutral formulas, at half the dose. It builds density steadily and rapidly and decides how black the blacks are, and it is the agent the bromide is really acting on. It is strongly temperature dependent, so a cold dish gives a flat warm print rather than a warm one. More raises contrast and cools the tone; less gives a print that never finishes.

Sodium carbonate, anhydrous, 16.0 g (or 44.0 g of the crystals). The alkali, cut to a quarter of D-72’s, and one of the two ingredients that make this a warm-tone formula. It still supplies a carbonate–bicarbonate buffer, so the pH holds through the session; it simply holds it lower. More carbonate speeds the bath and cools the image; less and the two-minute time becomes four.

Potassium bromide, 6.3 g. The restrainer, at three and a half times D-72’s dose, and the other half of the mechanism. It suppresses development of grains carrying no latent image — which at this concentration gives conspicuously clean whites — and, by acting far more strongly on hydroquinone than on metol, it slows the density-building half of the bath specifically. More bromide warms the image further, cleans the whites further and eventually costs shadow separation, which is D-166 taken one step on. Less returns the print towards neutral.

Water to 1000 mL. Not inert. Iron is the classic contaminant. A stock that has crystallised in the cold is warmed and redissolved rather than decanted, because the handbook notes that the precipitate often contains the most important constituents of the solution.

Interactions

Alkali against restrainer. The two levers of this page act on the same quantity — the rate of silver deposition — from opposite ends, and they are not independent of the contrast. Lowering the alkali slows everything; raising the bromide slows the hydroquinone selectively. Kodak moved both, which is why the contrast stays normal while the colour moves.

Restrainer and agent. Unequal, and sourced: bromide restrains hydroquinone far more than metol. Part VIII’s restrainer page has the general treatment.

Agent and agent. Superadditivity, discussed in Part VIII. The usual explanation, that hydroquinone regenerates oxidised metol, is not stated by any source this course holds and is not asserted here.

Developer and toner. The 1928 primer records that the colour of a sulfide-toned print is related directly to the colour of the original black-and-white image — blue-black images give cold chocolate tones and olive-green images give warm sepia tones. A warm-tone developer is therefore not only a choice about the untoned print; it is also a choice about where toning will end up. Part XX carries the toning chemistry.

Developer and exhaustion. Because the bath’s colour drifts as it works, and the drift shows in the print rather than in the tray, this is the one developer in the library where the sensible practice is to mix a fresh dish for prints that must match, and to say so in the notebook.

Variants

D-166 is the next step along the same axis and has its own entry: Kodak’s product list describes it as giving warmer tones than this formula, and the handbook lists it for the same two papers.

The packed powder. The handbook marks D-156 as available as a Kodak packed developer powder and does not state its composition. Under Rule 6 the course publishes the formula and makes no claim about the product.

The course’s own variants belong in the formula version record. This is the formula in the library most worth versioning, because both of its distinguishing quantities are single numbers you can move.

Safety

Level B, on the same two substances as every other metol–hydroquinone formula here. Metol is a skin sensitiser and sensitisation is permanent. Hydroquinone carries a Danger classification with suspected carcinogenicity and mutagenicity and causes serious eye damage. The doses are the lowest in the paper library, which lowers nothing about the sensitisation risk: a sensitiser is a sensitiser at any dilution.

Potassium bromide is a larger part of this formula than of any other in the library, and it is worth knowing what that does and does not mean. Its classification is skin and eye irritation with possible respiratory irritation — much lighter than the developing agents’ — and it is an ordinary salt. What 6.3 g/L changes is the weighing operation rather than the tray: a fine, hygroscopic powder that must not be raised as dust. Weigh it with extraction running, or avoid the weighing entirely by keeping a 10 per cent stock solution.

Print tongs, one pair per tray, never interchanged. Print developing is the highest-contact operation in this course. The classification rubric sets what Level B assumes, and the gloves page covers the choice.

Storage

Stock in a tightly closed bottle, filled full: three months full, one month half filled. Working solution: six hours in a dish and not recommended in a tank at all — the shortest dish life of any formula in this library, and a good reason to mix only what a session needs.

Label the bottle with the formula, the dilution and the date, per the labelling SOP. The 10 per cent bromide stock gets the same treatment.

Incompatibilities

Acid, deliberately, in the next tray. A splash of stop bath into a developer with only 16 g/L of carbonate has proportionally more effect than the same splash into D-158, because there is far less alkali to spend. Tongs never travel backwards along the sequence.

Fixer, in either direction. Thiosulfate carried back into the developer fogs and stains; developer carried into the fixer raises its pH, and the 1928 primer records the result — a white sludge, an alkaline bath, and prints likely to stain brown.

Oxidising agents — ferricyanide, permanganate, persulfate — never meet a developing agent in a bottle or a drain. See chemical incompatibilities.

Waste

A dilute alkaline solution carrying two developing agents with aquatic-toxicity classifications, plus more bromide than the other formulas here. It carries essentially no silver and goes into its own labelled container, never into the fixer bottle. The disposal caveat governs and the general chemical waste SOP gives the procedure. Local regulation decides.

Troubleshooting

A neutral print from a warm-tone developer. Most often the paper. This formula is published for chlorobromide materials and it will develop a bromide enlarging paper slowly and coldly. Check the paper before the bath.

Prints that look weak at a minute. Expected. The published time is two minutes and the blacks arrive late; pulling early gives a genuinely under-developed print, not a lighter one.

A colour that drifts across a session. Also expected, and it is what the handbook’s useful-life sentence is about. If the prints must match, mix a fresh dish.

Very slow development, and whites that stay clean past all reason. Over-bromided — check whether you added 6.3 g of the solid and 63 mL of the 10 per cent stock, which is the classic way to double this ingredient.

Flat, cold prints in a cold darkroom. Hydroquinone is the temperature-sensitive agent and it is already at a low dose here. Bring the bath to 18 °C before blaming anything else.

Experiments

The two levers, separated. Mix four working litres: as published; with the carbonate at D-72’s level and the bromide as published; with the bromide at D-72’s level and the carbonate as published; and with both at D-72’s levels. Print the same negative in all four on the same chlorobromide paper, developing each to completion and matching print density by exposure. You are asking which of Kodak’s two changes carries the colour, and whether they are additive. This is Part IX’s one-variable discipline applied to a two-variable claim.

Exhaust a dish on purpose and watch the colour. Put prints through a litre until Kodak’s sentence becomes visible, keeping one print from every fifth sheet. The output is a strip of prints showing what “life depends on quality required” looks like, and a number you can use — your own, for your paper.

Develop the same paper in this and in D-158. The two formulas sit at opposite ends of the working-strength table above. Matching print density by exposure, the only difference left should be colour and the time it took.

Then tone both. The 1928 primer’s claim is that a sulfide toner’s colour follows the colour of the original image. Two prints that differ only in developer, toned identically, are a direct test of it — and a warning about how much of a finished print’s colour was decided two trays earlier.