Part XX Overview: Toning Chemistry

The sentence to unlearn is that a toner makes a print warmer. It is not false so much as it names the smallest thing that happened and hides the mechanism that produced it. A toner takes the metallic silver that is your picture and turns it into something that is not metallic silver — a silver compound, a plating of a nobler metal, or an insoluble coloured salt built where the silver stood. Colour is one consequence of that substitution. So is maximum density. So is contrast in the upper part of the scale, how the image answers to oxidising air, whether it survives contact with the buffered board you were going to mount it on, and which container the spent bath goes into.

Kodak’s own toning sheet orders the same list in the same way. Colour comes first because it is what a customer notices, and then comes the claim that matters more: toning “converts the black-and-white silver image to an inert compound, which reduces the harmful effects of intense light, ultraviolet radiation, oxidizing gases, extremes of temperature and humidity, and fumes”, and “all Kodak toners will protect the image whether or not they produce a color shift”. That is a manufacturer claim in a document selling toner, and this part takes it seriously enough to test it rather than repeat it — the permanence page sets it against what conservation science has actually measured, and against ILFORD’s own sheet, which names blue and red toners as ones that “may not give extra protection” and warns that the image might fade. Two Tier 1 manufacturers, one saying every toner protects and the other naming toners that may not: that disagreement is the shape of the whole permanence argument, and it is why this part treats protective toning as an evidence question rather than a selling point.

Three routes, and why the part is built on them

Every toner in this part does one of three things to a developed silver particle. The routes are not degrees of the same effect; they are different chemistry with different products, different failure modes and different consequences for permanence.

What can be done to a silver particle, and what is left when it is done

1. 1Convert itThe silver stays where it is and becomes a silver compound — silver sulfide by a sepia or polysulfide route, silver selenide in a selenium bath. Kodak states both conversions in terms: its selenium toner "converts the silver image to silver selenide", its Brown and Sepia toners to silver sulfide
2. 2Replace or plate itA noble-metal ion is reduced by the image silver and deposited on or in place of the particle. Kodak describes its gold toner T-21 as working by "plating" the silver image with gold; platinum and palladium do the same thing at a price this course does not ask you to pay
3. 3Build something beside itThe silver is oxidised by ferricyanide and an added metal ion picks up the ferrocyanide produced, so an insoluble coloured compound is precipitated exactly where the silver was — Prussian blue with an iron salt, a copper compound with a copper salt. The colour is a new substance, not a converted one

Route one is the one most people mean by toning, and it comes in two mechanically different forms. Indirect toning bleaches the image back to a silver halide with ferricyanide and a bromide and then converts that halide, which reaches every particle and gives the largest colour change; the rehalogenating bleach that begins it is a formulary entry in its own right, because the same bath begins every sepia toner and every redevelopment intensifier in the course. Direct toning attacks the image in place with a single bath, which is what selenium toning and the hypo-alum sepia bath T-1a do. The distinction is not cosmetic: a bleached print that has not yet been redeveloped is a print whose image is a light-sensitive halide again, and a direct bath reaches the largest deposits before the smallest, which is why some toners move the shadows first.

Route two is gold, and the fourteen entries the formulary has already written for this part lean heavily towards it — for a reason the course did not choose. Gold survives as a formula a reader can mix, with published quantities: GP-1, T-21, T-26 — Kodak’s blue toner is a gold bath and not an iron one — and the three alt-process gold baths. Selenium survives only as a bought bottle whose full composition its maker does not publish. That is not an oversight of the formulary’s: elemental selenium is classified Level D in this course, so Kodak Limited’s own 1949 selenium formulas T-55 and T-56 are published as study entries and not as procedures, and a reader who wants to tone with selenium buys a bottle and is taught its behaviour rather than its recipe. Route three has no silver-gelatin formula in the course at all yet, and the iron-blue toned print is described from the process side instead.

The map

Nine pages, and the order is the argument. The mechanism first, because a reader who does not know what a bleach does cannot judge a bleaching time. Then the two routes the course actually asks you to run, each as a lesson followed immediately by the practical page that spends it. Then the metals that are interesting and expensive, then the ones that are interesting and forbidden, then permanence, then a picture.

Page	What it settles	Level
What a toner actually does to a silver image	The three routes, where image colour comes from, and what each route does to the tonal scale	—
Sulfide and sepia toning	Bleach, redevelop, convert; the hypo-alum alternative; and the rule that governs every sulfide bath	—
Lab: bleaching and redeveloping a sepia print	Two prints toned by the indirect route, one fully bleached and one partly, measured against a control	B
Selenium toning	Selenosulfate, silver selenide, why the shadows go first, and what the waste actually requires	—
Experiment: selenium toning and the print curve	How much maximum density and upper-scale contrast selenium adds, on three papers, measured	B
Gold, iron-blue and the other metal toners	Plating against deposition, the alkali problem Prussian blue brings with it, and what gold costs	—
The toners we study and do not use	Uranium, cyanide, mercury and chromium(VI) on a silver image: chemistry, history, no procedure	D
Toning, permanence and the evidence	What has been measured, what is claimed, and how to write a permanence statement you can defend	—
Assignment: the toned print	A matched pair, an argued colour decision and an argued permanence claim	B

What was decided before you chose a toner

By the time a print reaches a toning tray, most of what will happen to it is already fixed. This is the part of the subject that is easiest to skip and most expensive to skip, so it is stated here rather than left to the labs.

Five decisions that are made before the toner is poured

1. 1The paperA toner acts on a particular emulsion. Kodak’s own tables give cool chocolate-brown on warm-tone papers, purplish brown on neutral, and little or no change on cold-tone; the same bottle at the same dilution is three different toners on three papers
2. 2The developer and the developmentImage colour before toning is a property of the developed silver, so the developer and the degree of development set the starting point the toner works from
3. 3The exposureSepia, Sepia II Warm and Brown toners reduce print densities, so Kodak instructs that prints be developed fully and made slightly darker than normal; Rapid Selenium Toner tends to intensify, so a print made for it may be developed slightly less
4. 4The fixingTwo-bath, non-hardening. Kodak does not recommend a hardening fixer for prints to be toned, because it makes the emulsion less receptive; and improper fixing is "probably the major cause of stains in toned prints"
5. 5The washingResidual silver compounds and residual thiosulfate do not stay invisible once a toner arrives. They come back as yellow stain in borders and highlights

The blunt version, and it is Kodak’s rather than the course’s: toning cannot disguise poor print quality, and a good candidate for toning is a properly exposed, fully processed print with a full tonal scale and detail in both highlights and shadows. A badly fixed print cannot be toned. It can only be stained, and the stain will be read afterwards as a toning fault when it was a fixing fault all along. Kodak’s own list of toning defects is almost entirely a list of processing defects with a delayed fuse: mottle from an overconcentrated stop bath or from poor agitation in the first few seconds, round purple stains from air bubbles trapped in the fixer, yellow stain from an exhausted fixing bath — none of which show until the print is toned.

That is why this part sits where it does. Part XVIII gave you the material and the developer; Part XIX gave you a print you can make twice and a wash you can prove worked; Part XII owns the residual chemistry and the permanence fundamentals, and hands the toning half over here rather than half-teaching it. None of those are re-taught below. The archival processing sequence and the residual-hypo and silver tests are the entry condition for every practical page in this part.

Safety, stated before the chemistry rather than after it

This is the part where a household darkroom’s ventilation and waste practice stops being adequate by default. That is not a figure of speech; it is what the manufacturers’ own documents say about their own products.

The part’s course level, 3, is a statement about difficulty and not about hazard: the two senses of “level” in this course are different things, and the safety letters below are assigned page by page from the rubric.

⚠The three rules that govern every practical page here

Ventilation is a control, not a comfort. HARMAN’s technical sheet for its selenium toner calls the product “a toxic chemical” and strongly recommends working “in a well-ventilated area, preferably with some form of air extraction system”. Moersch’s safety data sheet for its own selenium toner, version 3 of 16 February 2025, classifies the mixture Acute Tox. 2 (H300), STOT RE 2 (H373), Eye Dam. 1 (H318) and Skin Sens. 1 (H317). An openable window is the floor, not the specification; the ventilation check is run before the session, not during it.

Sulfide and selenium baths never meet an acid. Kodak states the photographic form of the rule: sulfide-type toners are not discarded with stop baths or fixing baths, because the combination generates hydrogen sulfide gas, which additionally fogs unexposed paper and film and oxidises unprotected silver images elsewhere in the room — so the rule is a contamination control as well as a health one. The Moersch sheet states the selenium form: violent reaction with strong acids, release of toxic materials with acids. HSE’s EH40 gives hydrogen sulfide — its own table spells it sulphide — a long-term workplace exposure limit of 5 ppm (7 mg/m³) and a short-term limit of 10 ppm (14 mg/m³), and selenium and its compounds, hydrogen selenide excepted, 0.1 mg/m³ as Se. A darkroom containing an acid stop bath and a sulfide toner contains an incompatible pair, and the separation is in space and in time as well as in the tray order.

Waste is collected, not diluted. HARMAN’s sheet says it in one line — do not empty into drains, dispose of the material and its container at a hazardous or special waste collection point — and the Moersch sheet agrees: hazardous waste, do not empty into drains, and toxic to aquatic organisms with long-lasting effects. Kodak’s environmental guidance for amateurs is more discriminating than either: its table allows its packaged sepia toner every route but the household bin, sewer discharge to a public treatment works included, and allows used Rapid Selenium Toner one route only — a household hazardous waste collection facility or a licensed hauler — on the ground that the used toner is regulated as a hazardous waste for commercial users under the American RCRA regulations. The course does not carry Kodak’s sewer column across to a British reader — the disposal ruling reads a home darkroom’s separately collected photochemistry as an absolute hazardous entry that applies however dilute the liquid is — and the general chemical waste and silver-bearing waste procedures govern the containers. The chemistry above does not change; what may lawfully be done with the waste depends on where you are, differs between authorities within one country, and changes. Check your local regulations.

Why the levels came out where they did

The classification rubric assigns a level from the operation, not from the substance’s reputation, and on 5 September 2026 the course owner ruled explicitly that the operation sets the level. That ruling is what makes the sepia lab possible at Level B. Weighing anhydrous sodium sulfide is a Level C operation and stays one; it is done once, outside the session, or avoided by buying the stock ready made, and the formulary entries for T-7a and T-52 are published at Level C for exactly that reason, because a formula page publishes the whole formula including the stock-making. A tray of dilute working toner is a Level B operation, and that is what the lab runs.

The selenium experiment is Level B on the rubric’s own words rather than on custom. Level A’s waste criterion is silver-bearing fixer, spent developer or dilute rinse water — “nothing that requires specialist neutralisation” — and a selenium bath whose own maker routes it to hazardous waste collection fails that criterion before anyone has considered the toxicology. Level B’s controls, local extraction or an open window with a fan, eyewash within reach, splash protection, are the ones the manufacturers’ sheets ask for.

The toners we study and do not use is Level D, and Level D is the level at which the course gives no procedure to anyone. Uranium, cyanide and mercury appear in the rubric’s Level D criteria by name, and the Level D policy is where the argument lives rather than being restated on each page that reaches one. Chromium is governed by a single course-wide policy that separates the two oxidation states, published at safety/chromium and linked rather than paraphrased. What that page gives is the chemistry, the history and the reason a surviving uranium-toned print is usually in poor condition — and, deliberately, no quantities, no sequence and no conditions.

!One honest note about the sepia route

Kodak’s hypo-alum bath T-1a reaches a silver sulfide image without a sulfide reagent ever being handled, by acidifying hot hypo until it precipitates its own sulfur. It is Level B outright, with no Level C weighing anywhere in it. It is not therefore free of hazard: a half-strength fixing bath deliberately acidified and held at 49 °C is exactly the condition under which thiosulfate releases sulfur dioxide, and Kodak’s own instruction is not to exceed 49 °C or twenty minutes, because prints blister or stain above it. A Level C reagent traded for a gas with a workplace exposure limit is a reduction, not an elimination, and the page says so in those terms.

What you produce

Three objects, and a fourth thing that is not an object. A sepia print by bleach and redevelopment, made in a pair so that a fully bleached and a partly bleached print can be compared against an untoned control from the same printing session. A measured set of selenium-toned step-wedge prints on three papers of different image colour, from which you get your own figures for how much maximum black and upper-scale contrast selenium adds to the papers you actually use — not a number quoted from a sheet, because the sheet’s curve is for a discontinued Kodak paper in Kodak’s developer. And one finished toned print with a written permanence claim that says what was done, what the manufacturer asserts, what you measured and what remains inference.

The fourth thing is the habit of stopping a toner by eye against an untoned reference rather than by the clock. HARMAN’s sheet builds it into the instructions: make two identical prints, keep one in a holding tray of water, and compare. Toning continues in the wash, so the print is pulled before it looks right, and there is no way to learn where “before” is except by having the untoned sheet in your hand.

What this part sets up, and what it does not own

Toning does not end here. Most of Parts XXI to XXV tone their own materials, and the chemistry is genuinely different rather than the same chemistry on a different paper. A cyanotype carries a pigment rather than a metal image, and the tannic acid toner written for it works by bleaching the Prussian blue away with ammonia and putting something else in its place — which is a fourth route, not one of the three above, and it sits in the formulary with half its chemistry openly marked as unestablished. Gold on a salted-paper or albumen print is doing something a gold bath on a silver gelatin enlargement is not, and the three alt-process gold baths are written for those parts rather than this one. Where a page here reaches one of them it links and stops.

It also does not own print finishing. Washing, drying, flattening, spotting and mounting are Part XIX’s. What this part adds is a compatibility question those materials did not previously have to answer: alkali destroys Prussian blue, so the buffered, alkaline-reserve board that Part XIX recommends for a silver print is the wrong neighbour for an iron-blue toned one. The question is raised here and answered with Part XIX’s materials, and it is the clearest single case of a toner changing an object’s requirements rather than only its colour.

What you need first

A properly fixed, properly washed print, and the ability to prove it. That is not a formality in this part: it is the single condition under which anything below works, and the test for it is Part XII’s, not this part’s.

The permanence chemistry of Part XII — what actually attacks a silver image, why particle size decides how much of it is surface, and what residual thiosulfate does over years — is assumed throughout and never re-derived. So is the laboratory discipline of Part II: the incompatibilities page is the one that makes the acid rule above make sense rather than sound like superstition, and reading an SDS is what lets you check a toner bottle for yourself instead of taking this page’s word for it. The waste containers are already in your darkroom; this part adds two labels to them.

Trays that are not metal, and are not the trays you print in. Kodak is explicit that toning solutions do not go in metal trays or tanks — unchipped enamel, hard rubber or plastic only — and the separation of toning from printing is a contamination control as much as a hazard control, because hydrogen sulfide fogs unexposed paper.

Cost

The band for this part is ££, and the reason is that the chemistry is cheap and the paper is not. A toning session consumes prints, and the prints were the expensive part before the toner arrived.

Against one British retailer on 6 September 2026, a selenium concentrate was £21.95 for 946 mL of Kodak Rapid Selenium Toner, reduced from £26.90, and £51.52 for a litre of Ilford Selenium Toner, reduced from £57.24 — a wide band for two products of the same description, and worth checking against the dilution each maker specifies before treating either figure as the cheaper. A sepia product was £11.87 for 150 mL of Fotospeed ST20 making 1.5 L, reduced from £16.99, or £23.99 for Foma’s Fomatoner Sepia supplied as two 250 mL bottles. Raw chemicals for a bleach were £10.99 for 113 g of potassium ferricyanide, £17.99 for 230 g and £31.99 for 450 g — sold for cyanotype, but the same substance — against £23.00 for 250 g of potassium bromide recorded by the course on 5 September 2026; a ready-made ferricyanide-bromide bleach concentrate was £7.99 for 100 mL. Several of those were sale prices on the day, and they are ranges to plan against rather than quotations.

!What the price file cannot yet tell you

Three of the figures above are now in src/data/prices.json, read again on 7 September 2026: the Kodak selenium at £21.95, the Fotospeed sepia at £11.87, and the 230 g pack of potassium ferricyanide at £17.99. So the planner can cost part of this part, and each practical page still reports its consumables as a floor rather than a total. What is missing is a dated UK listing for a product sold as a sepia bleach-and-redeveloper pair rather than as a single bottle — and, mattering more than any of the prices, a maker’s own working dilution beside a bottle price. Not one of the three toner listings the file holds states a dilution or a capacity, so a cost per toned 8 × 10 in print still has to be estimated rather than computed. Gold is a different order of expense altogether — the gold working solution at £32.99 for 250 mL is in the file too, and a made-up toner of undisclosed strength is not a price for the salt — which is why the gold lesson carries a cost level of £££, and why it puts most of gold’s real use in Parts XXI to XXV, where the printing-out processes need it, rather than on a silver gelatin enlargement.

Alternative route

Toning itself needs no darkness. Every bath in this part is worked in room light on a print that is already fixed, and the one exception is small and Kodak says so: the sepia bleach converts image silver to light-sensitive silver bromide, so you may wish to bleach under safelight, “however, the effect is extremely small, and may not be noticeable”. What this part needs is a ventilated wet area that is not where paper is stored, and a darkroom only upstream, to make the prints.

A reader without a darkroom can therefore do more of this part than of Part XIX. Bring prints made elsewhere — a workshop, a shared darkroom, a friend — and the whole of the sepia lab, the selenium experiment and the assignment can be run in any ventilated wet space that is not shared with food preparation, or on an outdoor bench, with the print-making done in a session that need not be yours. What cannot be delegated is the matched-control discipline: prints for these pages must come in identical sets from one printing session, which is a constraint on whoever makes them rather than on where they are toned.

A reader who cannot achieve the ventilation is a different case, and this page will not pretend otherwise: there is no substitute control for a sulfide or a selenium bath in a room without air movement. Working outdoors is the one arrangement that supplies the control without altering a building, and it is the route the practical pages describe. Where even that is not available, six of the nine pages remain untouched — the mechanism, sulfide, selenium, metals, Level D and permanence lessons are reading, chemistry and arithmetic from end to end — and the sepia lab publishes the course’s own recorded data set so that its measurement and analysis can be worked through without running the baths. The permanence page and the Level D page need no facility of any kind, and the permanence page is arguably the most useful thing in the part.

✦The habit to start before the first bath

Write down what you want the toner to do to the picture, in a sentence, before you dilute anything. Not “sepia” and not “warmer” — what should happen to the shadows, what should happen to the highlights, and what you are willing to lose. Most of what a toner does cannot be undone on the sheet in the tray — an iron-blue print can be partly bleached back, a sulfide-converted one cannot — and the reader who cannot finish that sentence usually tones until the print looks different, which is not the same as better.

The pages of this part

Part20 of 28Level3 — AdvancedPages9Estimated time13.3 hoursHighest safety levelLevel D

✱3 of 9 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 9 lessons in this part completed

• LessonWhat a Toner Actually Does to a Silver Image60 min
• LessonSulfide and Sepia Toning: Bleach, Redevelop, Convert50 min
• LabLab: Bleaching and Redeveloping a Sepia PrintB✱ Darkroom150 min
• LessonSelenium Toning: Selenosulfate, Silver Selenide and the Protection Claim50 min
• ExperimentExperiment: Selenium Toning and the Print CurveB✱ Darkroom150 min
• LessonGold, Iron-Blue and the Other Metal Toners60 min
• LessonThe Toners We Study and Do Not UseD45 min
• LessonToning, Permanence and What the Evidence Actually Supports50 min
• AssignmentAssignment: The Toned PrintB✱ Darkroom180 min