Kodak D-166

Twelve and a half grams of potassium bromide in a litre is the largest restrainer dose anywhere in this formulary, and it is six and a half times what D-72 carries. Kodak’s product list places this formula by comparison rather than by absolute description: it is the developer “giving warmer tones than D-156”. Two other things make it worth a page of its own — it publishes an emergence time you can use to judge exposure, and it offers dilution as a tone control with the development time held fixed.

Ingredient	Quantity	Form the source specifies
Metol	1.15 g	Kodak calls it Elon
Sodium sulfite	25.0 g	anhydrous — or 50.0 g crystalline
Hydroquinone	8.5 g
Sodium carbonate	25.0 g	anhydrous — or 68.0 g of the crystals
Potassium bromide	12.5 g	or 125 mL of a 10 per cent solution
Water to make	1000 mL

Purpose

To develop a chlorobromide paper to the warmest image tone Kodak Limited published a formula for, at a contrast the printer chooses by dilution. Kodak lists it for Bromesko and Kodura papers and for L-S Warm Tone Lantern Plates, and describes it in the same breath as an alternative for bromide papers to D-163 — which is the handbook putting the two extremes of its paper library side by side.

Recommended uses

Bromesko and Kodura papers at 1:3, two minutes at 18 °C. Kodak adds the sentence that makes this formula unusually teachable: with that development time, a correct exposure will give a first appearance of the image after about 50 seconds.

!An emergence time is an exposure meter

Fifty seconds out of a hundred and twenty is a specific, checkable claim, and it converts a judgement most printers make by feel into a measurement. If the image appears at 30 seconds the print is over-exposed; if it has not appeared by 70 the print is under-exposed; in both cases the fault is in the enlarger, not in the tray, and no amount of extra development time will fix it.

The reason it works is the mechanism of this particular formula. With this much bromide the development is slow and heavily restrained, so the time to first appearance is dominated by how much latent image there is — which is the exposure. In a fast, lightly restrained bath like D-158, where the whole print finishes in thirty seconds, the same measurement is not available: everything appears at once.

The number is Kodak’s, for Kodak papers of the period, at 18 °C. Its value will not transfer to a modern paper. Its method will: develop a correctly exposed print, time the emergence, write the number in the lab notebook, and you own a criterion for the rest of that box of paper.

Tone is varied by dilution, not by time. The handbook is explicit: the warmth of the image tone may be varied by increasing or decreasing the dilution of the developer, with the development time unchanged, and with a corresponding decrease or increase in exposure. Read that carefully — three things move together. More water means a warmer, slower bath, so the exposure must come down to reach the same density in the same two minutes. Less water means a colder, faster bath and the exposure goes up.

When another formula is preferable

• For a warm tone with normal contrast and a shorter time, D-156 is the same idea one step back, at half the bromide.
• For a neutral print, D-72 at 1:2, or D-163 at 1:3 for more contrast and a longer dish life — the handbook itself names D-166 and D-163 as alternatives for bromide papers.
• For a cold blue-black, D-158, which is this formula’s opposite on both levers.
• For low contrast without the warmth, D-165 reaches it by removing the hydroquinone instead of by restraining it.

Mixing

Dissolve in the order printed, for the reason the handbook gives once for all its formulas: a developing agent dissolved before the preservative oxidises in air and forms coloured products, and where the 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 125 mL of a 10 per cent stock.
6. Make up to 1000 mL, and dilute 1 part to 3 parts of water for use.

🧮Where the bromide comes from, and why 125 mL is not free

12.5 g of solid, or 125 mL of a 10 per cent w/v solution — the same quantity of bromide either way. But 125 mL is an eighth of the finished litre, and it is water you must not add twice. If you use the solution, make the formula up to 1000 mL including it; if you weigh the solid, you have 125 mL more water to play with. This is the commonest arithmetic error in a formula that offers both forms, and it is worth ten per cent of every other ingredient.

Keeping a 10 per cent potassium bromide stock is nonetheless the right practice: it is used in most of the paper formulas here, it removes a hygroscopic powder from the weighing routine, and it is accurate at doses where a darkroom balance is not. Label it with its strength and date, per the labelling SOP.

The handbook prints both salts twice — 50.0 g crystalline sulfite or 25.0 g anhydrous, 68.0 g crystalline carbonate or 25.0 g anhydrous — and warns in capitals that its avoirdupois and metric columns are not exact equivalents. Use one system throughout.

Behaviour

This is the slowest and most heavily restrained bath in the library. At the published 1:3 dilution the working solution holds about 6.3 g/L of carbonate and 3.1 g/L of bromide; the working-strength comparison on the D-156 page sets all five Kodak paper formulas side by side at the dilutions each is published for, and this one sits at the bottom of the alkali column and the top of the bromide column.

No capacity is published at all. The 1949 useful-life table, which gives a sheet count for almost every formula in the handbook, leaves both columns blank for this one. D-156 at least gets a sentence — exhaustion affects colour of image; life depends on quality required — and the reasonable reading is that the same applies here with more force. The course does not supply a number Kodak declined to give.

Keeping is short: eight hours in a dish, not recommended in a tank, three months in a full stock bottle and one month in a half-filled one.

Contrast is lower than the neutral formulas’, and it moves with the dilution. Because Kodak’s tone control is the dilution, and because dilution also changes contrast, the two are not independent here. That is a real limitation rather than a criticism: warm-tone printing is a compromise between colour and scale, and this formula makes the compromise explicit by handing you one knob that turns both.

Image characteristics

Colour. Warmer than D-156, which is Kodak’s comparative claim about its own two products and not a measurement. No image-colour measurement of any kind is published in the handbook.

Contrast. Lower than the neutral formulas at 1:3, and adjustable by dilution in the direction the warmth moves.

Tonality. Long and gentle, with the blacks arriving very late — the emergence at 50 seconds and the completion at 120 tell you that most of the density is built in the second half of the development.

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

The mechanism

12.5 g/L ÷ 119.00 g/mol = 0.105 mol/L

Potassium bromide

25.0 g/L ÷ 105.99 g/mol = 0.236 mol/L

Sodium carbonate

8.5 g/L ÷ 110.11 g/mol = 0.077 mol/L

Hydroquinone

1.15 g/L ÷ 344.38 g/mol = 0.0033 mol/L

Metol, as the hemisulfate

0.105 ÷ 0.077 = 1.4 to 1

Bromide per hydroquinone, by moles

More bromide than hydroquinone, by moles. That last ratio is what makes this formula extreme. D-72 carries one bromide for every seven hydroquinones; this carries three for every two. The 1928 primer supplies the reason the ratio is the right one to look at: bromide affects hydroquinone strongly and Elon much less, and the higher an agent’s reduction potential the more bromide it takes to produce a given effect. Hydroquinone has the lowest reduction potential Kodak ranked, so it is the agent bromide can actually stop.

Slow deposition is the whole design. Image colour follows the rate at which silver is deposited — established in Part V’s chlorobromide project from Wall’s controlled series and Chapman Jones’s finding that image colour depends on the size of the silver particles, which slow deposition makes fine. This formula reaches slow deposition by three routes at once: little metol to start with, little alkali, and a great deal of bromide holding the density-building agent back. Two minutes for a print that has already emerged at fifty seconds is what that looks like from the outside.

Why dilution moves the tone and time does not. Diluting changes the concentrations of alkali and agents, which changes the rate. Extending the time at a fixed dilution does not change the rate; it only lets the same rate run longer, so it changes density rather than colour. Kodak’s instruction — vary the dilution, hold the time, compensate with exposure — is therefore not an arbitrary convention but the only way to move colour without moving density. It is also a small lesson in experimental design: the maker is telling you which variable is confounded with which. Part IX has the general treatment.

The sulfite is not doing what it does in a film developer. At 25 g/L it is a preservative and an acid reserve, nothing more. There is no silver-solvent role at this concentration, which is as it should be: dissolving silver would coarsen nothing and would only cost density in a formula already working slowly.

Function of every ingredient

Metol, 1.15 g. The starting agent, at the smallest dose anywhere in this formulary. It has the highest reduction potential of the agents Kodak ranked and is the one bromide barely touches, which is the only reason a bath carrying 12.5 g/L of restrainer develops at all. Its job here is to get the image started everywhere at once and hand over. More metol speeds the emergence and cools the tone, which would break the 50-second criterion; less and the print will not start.

Sodium sulfite, 25.0 g anhydrous. The preservative. It scavenges dissolved oxygen and intercepts the oxidised agents before they can form coloured products, and it carries most of the bath’s acid reserve into the stop bath. At a quarter-strength working dilution there is not much of it in the dish, which is part of why the dish life is eight hours. More sulfite keeps longer and cools the tone slightly; less and the tray browns.

Hydroquinone, 8.5 g. The contrast agent, at seven times the metol by mass, and the agent the whole formula is built around restraining. It builds density steadily and rapidly when it is allowed to; here it is not, and the slow late accumulation of density is exactly its restrained behaviour. It is strongly temperature dependent, which in a formula this slow makes a cold dish very obvious. More hydroquinone raises contrast and cools the image; less and two minutes will not produce a black.

Sodium carbonate, anhydrous, 25.0 g (or 68.0 g of the crystals). The alkali, and after a fourfold dilution the lowest effective alkalinity in the library. It sets the pH region where the hydroquinone functions at all and supplies a carbonate–bicarbonate buffer that holds it there. More carbonate speeds the bath and cools the tone; less and development becomes impractically slow. It is the ingredient Kodak’s dilution instruction actually moves.

Potassium bromide, 12.5 g. The restrainer, the largest dose in the formulary, and the reason this page exists. It suppresses development of grains carrying no latent image — giving very clean whites — and, acting far more strongly on hydroquinone than on metol, it slows the density-building half of the bath specifically, which is what produces fine silver particles and a warm image. More bromide warms further and eventually stops the print from developing at all; less returns it towards D-156 and then towards neutral. It is also the ingredient that makes the 125 mL solution alternative worth using, because 12.5 g of a hygroscopic powder is a real weighing.

Water to 1000 mL. Not inert, and here it is also a control: the dilution is Kodak’s tone adjustment. Iron is the classic contaminant, and a stock that has crystallised in the cold is warmed and redissolved rather than decanted.

Interactions

Restrainer and agent, taken to the limit. With more bromide than hydroquinone by moles, this formula is the clearest case in the library of the asymmetry the 1928 primer describes. It is also where that asymmetry stops being a curiosity and becomes the design: a restrainer that acted equally on both agents could not produce this developer, because it would simply switch the bath off.

Dilution, contrast and colour. Confounded by construction, as discussed above. If you need to move colour and contrast independently you need a second variable — the paper, or the exposure, or a different formula.

Agent and agent. The pair develops faster than either agent alone, which is what Part VIII calls superadditivity — and with only 1.15 g of metol to 8.5 g of hydroquinone, this formula depends on it more than most. Why it happens is not something this course asserts, because none of its sources explains it.

Developer and toner. The 1928 primer records that a sulfide-toned print’s colour follows the colour of the original black-and-white image — blue-black images give cold chocolate tones and olive-green images give warm sepia tones. Choosing this developer is therefore also a decision about where toning will finish, taken two trays early.

Bath and bath. Bromide carried out of this tray on a wet print is a contaminant for anything it lands in. At 3.1 g/L in the working bath there is a great deal of it, and a stop bath tray shared with a neutral developer session will carry some of it back. One tray per developer, or a thorough wash between.

Variants

The liquid product, which the handbook says is double the strength of the stock made up to this formula and must therefore be diluted 1 part in 8 rather than 1 part in 4. This is the only place in the handbook where Kodak states a quantitative relationship between one of its formulas and one of its packaged products, and it is a statement about concentration, not composition. The course prints it as that and infers nothing further under Rule 6.

D-156 is the same idea at half the bromide and has its own entry.

The course’s own variants belong in the formula version record. The obvious one is a bromide series: this formula is D-156 with the bromide doubled and the alkali raised slightly, so the space between them is a straight line you can walk.

Safety

Level B. Metol is a skin sensitiser and sensitisation is permanent. Hydroquinone carries a Danger classification with suspected carcinogenicity and mutagenicity and causes serious eye damage. Both are at the low end of this library’s doses, which changes the splash hazard and not the sensitisation risk.

The bromide weighing is the operation to plan. Potassium bromide is classified for skin and eye irritation with possible respiratory irritation, and 12.5 g of a fine hygroscopic powder is enough to raise dust if handled carelessly. Weigh with extraction running or inside an enclosure, or keep a 10 per cent stock and measure 125 mL instead.

Print tongs, one pair per tray. Print developing is the highest-contact operation in this course, and a two-minute development is two minutes of a print sitting in a tray inviting a hand. The classification rubric sets what Level B assumes and the gloves page covers the choice.

Storage

Stock in a tightly closed bottle filled as full as it will go: three months full, one month half filled. Working solution: eight hours in a dish, not recommended in a tank. Mix what the session needs and no more, and label it with the formula, the dilution and the date per the labelling SOP — a warm-tone bath and a neutral one look identical in a tray.

A cold stock bottle that has thrown a deposit is warmed and redissolved rather than decanted; the handbook notes that the precipitate often contains the most important constituents of the solution.

Incompatibilities

Acid, deliberately, in the next tray. With only 6.3 g/L of carbonate in the working bath, a splash of stop bath has proportionally more effect here than on any other developer in the library. Tongs never travel backwards along the sequence.

Fixer, in either direction. Thiosulfate carried back into a developer fogs and stains; developer carried forward raises the fixer’s pH, and the 1928 primer records the consequence — 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 and more bromide than anything else in this formulary. It carries essentially no silver and belongs in its own labelled container, never in the fixer bottle. The disposal caveat governs and the general chemical waste SOP gives the procedure. Local regulation decides.

Troubleshooting

The image appears in 25 seconds instead of 50. Over-exposure, on Kodak’s own criterion. Reduce the enlarger exposure rather than shortening the development.

The image has not appeared at 80 seconds. Under-exposure, or a cold bath, or a bath diluted further than 1:3. Check the thermometer, then the dilution, then the enlarger.

A print that will not reach black in two minutes. Expected if the bath is cold or old; this is the formula with the least reserve of everything. If it is neither, the paper may not be a warm-tone chlorobromide.

A print warmer than you wanted. Dilute less, and raise the exposure to compensate, exactly as the handbook says. Do not shorten the development — that changes density, not colour.

Whites that stay clean while the blacks never arrive. Over-bromided, which usually means the solid was weighed and the solution added. Check against the two forms in the ingredient table.

Experiments

Calibrate the emergence time for your own paper. Make a correctly exposed print by test strip, then time the first appearance of the image. Repeat on three papers. You are converting Kodak’s 50 seconds from a number into a method, and the result belongs in the lab notebook as a standing criterion for that box of paper.

The dilution ladder. Mix 1:2, 1:3, 1:5 and 1:7, hold the development at two minutes, and match print density on each by adjusting the exposure. That is Kodak’s own tone control, run as a series. Four prints of the same density and four different colours is the result the handbook predicts, and if the contrast moves as well — it should — you have measured the confounding the maker did not mention.

Bridge the gap to D-156. The two formulas differ mainly in bromide and alkali. Mix this one with the bromide halved and the alkali left alone, and compare it with D-156 at its own dilution. You are testing whether the difference between two published warm-tone developers is really one ingredient.

Then tone. Two prints, same negative, one from this and one from D-158, sulfide-toned identically. The 1928 primer’s claim is that the toned colour follows the original image’s colour; this is the cleanest test of it the formulary can set up.