What a Toner Actually Does to a Silver Image

A finished silver gelatin print is a piece of paper with a few tenths of a gram of metal on it. Every tone in it, from the faintest highlight to the deepest black, is the same element in different amounts and different physical forms. Toning is the deliberate business of stopping that metal being that metal — turning it into a compound, burying it under another metal, or growing a coloured salt in the space it occupied. Once you hold that as the thing that is happening, every other property a toner changes stops being a surprise. Colour moves because the substance moved. So does maximum density, because a different substance absorbs light differently. So does contrast, because the change does not reach every part of the scale at the same rate. So does the print’s answer to oxidising air fifty years from now, because that is a question about a substance too.

This page is the mechanism for the whole part. It takes no print anywhere near a tray, and the practical pages that follow assume every argument on it.

!Why this page carries no safety level

The classification rubric assigns a level to an operation — a quantity of a substance handled in a particular way for a particular length of time — and this page prescribes none. It gives no bath, no dilution, no time and no sequence. The levels for this part are argued where the operations are: Level B for the sepia lab and the selenium experiment, Level C on the formulary pages that publish a stock-making step, and Level D on the toners we study and do not use. One hazard is nonetheless a piece of mechanism rather than a piece of housekeeping, and it appears below where the chemistry produces it: a ferricyanide bleach and a sulfide bath are an oxidiser and a reducing agent, and an acid arriving in a sulfide bath is the reaction that makes hydrogen sulfide.

The image before you change it

Part XVIII established that a print’s own colour comes from the size and form of the developed silver rather than from any dye, and that argument is not repeated here. What that page did not need, and this one does, is the shape of the deposit, because a toner has to reach it.

Mike Ware draws the distinction as sharply as anyone. Printed-out silver — the kind a salted paper or a printing-out paper carries — has a particle size in the 10 to 100 nanometre region: nanoparticles, smaller than the wavelength of visible light. Chemically developed silver in a gelatin emulsion is not like that at all. It is, in his words, micron-sized bundles of filamentary metallic silver, and a developed paper image “simply appears neutral-toned, owing to the much larger silver filaments that are produced by the development process”.

Two consequences follow immediately, and they run through the rest of the part.

A developed print and a printing-out print are not the same object to a toner. The gold bath that turns a printing-out paper red-violet is working on nanoparticles; the same bath on an enlarging paper is working on filament bundles a hundred times larger. Kodak’s own tables say this in commercial language — Rapid Selenium Toner gives cool chocolate-browns on warm-tone papers, purplish browns on neutral ones and “very little or no change” on cold-tone ones — and the reason underneath the table is that those three papers present three different silver morphologies to the same solution. This is why the alt-process gold baths written for Parts XXI to XXV are not simply borrowed into this part.

Surface area is the currency. A toner is a heterogeneous reaction: it attacks a solid from outside. Ware notes that a print-out image’s small particle size gives it “a very high surface area exposed to attack, proportional to its mass”, which is why print-out silver is both easy to tone and easy to destroy. A filamentary developed deposit has less surface per gram, so it converts more slowly — and the biggest, densest deposits in the shadows, being the most massive, take longest to convert all the way through. Hold that thought: it is half the explanation of why toners move different parts of the tonal scale at different rates.

The conservation literature adds the observation from the other end. AIC’s photographic materials group record that pristine silver gelatin developing-out prints are “generally monochromatic, blue-black and white in appearance”, while printing-out prints run from warm browns to cool purples “because of the nature of the development of the photolytic silver strand” — a difference of morphology, described by conservators looking at surviving objects rather than by chemists looking at a mechanism, and agreeing with the chemists.

Colour without dye, and how much of it is established

Nothing in a toned print is a dye. So where does a colour come from?

⚗Small metal particles and the light they will not let past

Ware’s chapter on the colours of silver images gives the physics, and it is worth having in full because it is the only part of this subject where the course can point at a real theory rather than at an accumulated craft observation.

Light falling on a metal particle drives collective oscillations of the electrons in the conduction band — plasmons. For most metals these surface plasma resonances absorb in the ultraviolet and the particle simply looks grey. For copper, silver and gold, the way the metal’s dielectric function varies with frequency puts sharp absorption bands in the visible instead, which is why those three metals give colours and almost no others do. Faraday was investigating the effect in 1857. Gustav Mie treated spherical particles with Maxwell’s electromagnetic theory in 1908 and Gans extended it to ellipsoids.

The size dependence is not a hand-wave. When the particles are much smaller than the wavelength of light, a dipolar approximation holds and absorption dominates. At larger radii, multipolar terms become important and scattering starts to matter. Shape matters as much as size: elongating a particle into a prolate spheroid moves the main band to shorter wavelengths but introduces a new long-wavelength band that dominates the perceived colour, and as the axial ratio goes 1, 2, 3 the colour goes yellow, red, blue. Aggregation does the same thing for the same reason: a yellow 10 to 20 nanometre silver sol turns red-brown when its particles line up.

Ware reproduces calculated colours for silver hydrosols, due to Wiegel and confirmed experimentally.

Particle diameter	Colour transmitted	Colour scattered
10-20 nm	Yellow	Blue
25-35 nm	Red	Dark green
35-45 nm	Purplish-red	Green
50-60 nm	Violet	Yellow-green
70-80 nm	Dark blue	Yellow ochre
90-100 nm	Light blue	Red-brown
120-130 nm	Grey-green	-

Read the caveats before you read the table. These are hydrosols, silver suspended in water, whose refractive index is 1.0. Ware’s next section is precisely about why that matters: raise the refractive index of the host and the silver absorption band moves to longer wavelengths. His worked case is silver inside a silver chloride crystal, where n is 2.071 and the band that sits near 390 to 400 nm in water is dragged out to about 550 nm — the reason a “sunned” silver chloride looks violet. Gelatin is not water and it is not silver chloride, so this table is a demonstration that size governs colour, not a lookup key for a print.

⁇What the course claims about image colour, and what it does not

Established, and sourced at Tier 1 in more than one process. The colour of a silver image depends on the size and physical form of the silver, not on any dye and not on the halide the emulsion started from. Getty state it for printing-out papers and again for warm-tone developing-out papers. Kodak’s 1928 primer states the same principle for a converted image: silver sulfide runs “from light brown to black, according to its state of subdivision”. Kodak state it again for gold: finely divided gold is red, and more rapid deposition gives blue gold.

Established as physics, for particles smaller than the wavelength. Plasmon resonance, the Mie and Gans treatments, the size and shape dependence, and the refractive-index shift are real optical theory with primary literature behind them, and Ware cites it.

Not established, and this page will not assert it. That the whole of a developed silver-gelatin print’s colour is explained by that theory. Ware’s own framing puts developed silver in a different regime — micron-sized filament bundles rather than nanoparticles — and Berry and Skillman’s result cuts both ways: they obtained a polydisperse nanoparticle colloid by developing a fine-grain emulsion, and found that a wide spread of particle sizes and shapes produces neutral density, because the absorption across the visible is the envelope of a whole family of peaks. So some developed silver is in the nanoparticle regime and some is not, and the course holds no source that resolves where a given modern enlarging paper sits.

Actively unfinished, on the source’s own admission. Ligands adsorbed on a metal particle shift the plasmon band strongly — thiolic ligands move a 390 nm band to 510-550 nm — and a print is full of adsorbed species. Ware quotes Henglein: “a quantitative theoretical model for the effects of chemisorption of molecules on the electronic properties of the metal particles has not yet been developed.”

The course’s position. The link particle size and shape to colour is fact. The detailed optical account is fact for nanoparticle silver and an unfinished extrapolation for a developed print. Any statement that a named toner gives a named colour on a named paper is photographic convention resting on manufacturers’ tables, and this part quotes those tables as tables rather than deriving them.

Three routes, and the fourth one Kodak counted

The 1928 Kodak primer defines toning as “the deposition on the silver image of another substance having a different color… or of the transformation of the silver image into another substance for the same purpose”, and then lists four principal methods. The course teaches three of them, and it is worth saying which three and why.

Kodak's four methods of toning, and what this part does with each

1. 1A - Replacement by another metalRoute two here. A finely divided silver image in a gold or platinum solution: the silver goes into solution and the noble metal is deposited in its place
2. 2B - Deposition of salts of metalsRoute three here. The silver is first turned into silver ferrocyanide, then that silver is substituted by a metal whose ferrocyanide is coloured - iron blue, uranium reddish-brown, copper red
3. 3C - Mordanting a dye to the imageNot taught in this part. The silver is converted to something a basic dye will attach to in insoluble form, historically silver iodide; the colour is then a dye and not a metal compound at all
4. 4D - Conversion to a stable coloured silver saltRoute one here, and the most used of all. The silver becomes silver sulfide or silver selenide, which Kodak call the most popular method of toning developing-out prints and the one to choose when great permanency is wanted

Method C is left out for a reason that is worth stating rather than hiding: a mordanted dye image is not a silver image and does not behave like one. ILFORD say so in one line on their own warm-tone sheet — sulfide, polysulfide and some metal-replacement toners give a protective effect, while dye toners do not give extra protection. A route whose entire product is an organic colourant belongs with the permanence argument, not with the three conversions, and it is the one place where “toning” in the trade sense and tinting come closest to each other.

!One terminology check the course could not complete

This course keeps toning and tinting strictly apart: toning changes the image substance and therefore acts in proportion to density, while tinting colours the base or the binder and lands on the empty highlights as much as on the image. The distinction is useful, it is the one the glossary carries, and the highlights are where you test it on an unknown print.

What the course has not established is that the conservation literature uses the pair in exactly that way. The conservation sources it holds — the Getty atlas and the AIC photographic materials group catalogue — use “toned” as this page does, but use “tinted” for a coloured baryta layer and for hand-colouring, and neither defines the two words against each other. The distinction is therefore stated here as the course’s own convention, adopted because it separates two genuinely different physical situations, and not as a citation of conservation usage. Reading a conservation report, check what its author means by each word rather than assuming this page’s sense.

Route one: conversion, where the silver stays and changes

The silver atom does not go anywhere. It acquires a partner and becomes a compound in situ, with a different colour, a different refractive index and a different resistance to whatever is going to attack it.

⚗Two conversions, and why they run to completion

By sulfide. In the direct route — Kodak’s hypo-alum bath T-1a — the bath makes its own sulfur out of acidified hot hypo and the metal is converted where it stands.

2 Ag + S → Ag₂S

Direct sulfide toning: no halide in between

In the indirect route the image is first bleached back to a halide and the halide is then exchanged.

2 AgBr + S²⁻ → Ag₂S + 2 Br⁻

Indirect sulfide toning: the halide becomes the toned image

That second reaction has nowhere else to go. From the OpenStax table of solubility products at 25 °C, silver bromide sits at 5.0 x 10 to the minus 13 and silver sulfide at 1.6 x 10 to the minus 49 — thirty-six orders of magnitude of driving force, which is why the exchange goes to completion at almost any sulfide concentration and why concentration buys speed rather than completeness. Kodak’s 1928 primer states the same fact without the numbers: silver sulfide is a very insoluble compound of silver, so a silver image or a silver halide treated with sulfur or a sulfide “will at once be transformed into silver sulphide”.

By selenium. Kodak state the product and the course stops there: Rapid Selenium Toner “converts the silver image to silver selenide”. No balanced equation for the toning step appears anywhere in this course, because no source it holds gives one, and the silver selenide page records that it holds no solubility product, solubility figure or formation constant for the compound either. Writing a plausible equation would be exactly the failure Rule 1 exists to prevent. The selenium lesson works out how much can honestly be said about the selenosulfate bath.

The single most useful sentence in the 1928 primer belongs here, because it disposes of the idea that a toner has a colour. Silver sulfide’s colour varies from light brown to black according to its state of subdivision — and the state of subdivision of the toned image depends on that of the untoned image, which depends on the exposure and the development. Kodak draw the practical conclusion themselves: to get good colours in sulfide toning the print should have been fully developed but not over-exposed. And in the hypo-alum bath the relationship is direct enough to predict: blue-black images give cold chocolate tones, olive-green images give warm sepia tones.

So the brown of a sepia print is not the brown of silver sulfide. It is the brown of your silver sulfide, whose particle size you set two hours earlier at the enlarger.

Route two: replacement and plating

Kodak’s method A, in their own 1928 words: when a finely divided silver image is placed in a solution of gold or platinum, the silver replaces the metal in solution, going into solution itself, and the gold or platinum is deposited in the place of the silver. That is a displacement driven by the difference in the metals’ electrode potentials, and it is the same chemistry that plates copper onto an iron nail.

Two things complicate the simple picture, and both are load-bearing.

Gold arithmetic. The gold salt you can buy is gold(III), and silver oxidises only to silver(I), so a plain gold(III) bath spends three silver atoms for every gold atom it deposits — it bleaches the print faster than it tones it. Every useful gold toner is therefore an apparatus for getting to gold(I) first, and the chloroauric acid page holds that argument and its equations for the whole course. It is why Kodak’s three published gold formulas each carry a different sulfur ligand: thiosulfate in T-21, thiocyanate in GP-1, thiourea in T-26.

Replacement or plating — the sources say both, and they are measuring different things. Kodak’s 1928 chapter describes displacement, silver leaving as gold arrives. Kodak’s 2006 toning sheet describes T-21 as working by “plating” the silver image with gold. Getty, who put printing-out papers through the bath and analysed both the print and the bath, report the two happening together: there is “some deposition of gold onto the silver particles”, and the gold bath shows a growing concentration of silver during toning. Their conclusion is that the growth of image particles “is not straightforward”. The course does not referee this: it reports that both processes are measured, that the balance between them is not established for any particular bath and paper, and that XRF of a toned printing-out print shows silver and gold together as the imaging metals.

⚗Why one gold bath gives red and another gives blue

Getty publish two gold-toned developing-out prints: one bright red, one bright blue. XRF confirms gold in both. Many people find that surprising, and Getty say so.

Kodak’s 1928 primer explains it without knowing it will be needed. The rate at which the metal is deposited decides how finely divided it is, and the colour of a finely divided metal depends on the fineness of the division. Slow deposition gives very fine gold, which is red; faster deposition gives the blue. Kodak then give the lever: rapid deposition needs an alkaline bath, which is why borax or sodium acetate is added to gold chloride to make a toning bath, and why weak reducing agents such as thiocyanates or formates are sometimes added too. Platinum baths, by contrast, are used acid.

This is the same particle-size-to-colour argument as the silver one, applied to a different metal, from a Tier 1 source, in 1928 — and it is the cleanest demonstration in the part that colour is a morphology question rather than a composition question.

Gold is also the route with the weakest colour change and, on the manufacturers’ own account, one of the strongest protective effects. Kodak’s GP-1 exists to provide “print protection while changing the image tone only slightly”, and the permanence lesson is where that claim gets weighed rather than repeated.

Route three: deposition, where the colour is a new substance

Kodak’s method B is not a conversion of the silver and not a replacement of it. The silver is used up producing an anion, and a coloured salt of some other metal is precipitated where the silver was.

The primer sets out the two steps plainly, and the first of them is the single reaction that every ferricyanide bath in this course is built on: hexacyanoferrate(III) takes one electron from metallic silver and becomes hexacyanoferrate(II).

Ag + [Fe(CN)₆]³⁻ → Ag⁺ + [Fe(CN)₆]⁴⁻

The oxidation common to a bleach, a reducer and a deposition toner

What happens next depends entirely on what is waiting to catch the two ions, and that single choice separates three quite different processes. In the primer’s own account of this route, the silver ion is caught by the ferrocyanide the reaction has just produced, giving silver ferrocyanide throughout the image; then “the silver in the silver ferrocyanide [is] substituted by another metal of which the ferrocyanide is colored”. An iron salt gives blue iron ferrocyanide; uranium nitrate gives reddish-brown uranium ferrocyanide; copper citrate gives red copper ferrocyanide. The two steps are sometimes run in one bath and sometimes as a bleach followed by a metal salt.

Set the three uses of that one oxidation side by side and the whole family becomes legible:

What is waiting for the silver ion	What is left in the print	The process
Bromide, in large excess	Insoluble silver bromide, in place	The rehalogenating bleach that begins every indirect toner
Thiosulfate	A soluble complex that diffuses out	Farmer’s reducer: the image is removed, not converted
Ferrocyanide, then a coloured metal	An insoluble coloured metal ferrocyanide	Route three: iron-blue, copper and uranium toning

⁇What the copper compound actually is - three sources, three answers

This is a small question with a large lesson in it.

• Kodak, 1928: copper citrate with potassium ferricyanide gives “the red copper ferrocyanide”.
• Wall, 1924: copper toning “is based on the deposition of red cuprous ferrocyanide on the image with the simultaneous formation of white silver ferrocyanide” — cuprous being copper(I), a different oxidation state from Kodak’s unqualified “copper”.
• Getty, 2013: the XRF spectrum of a copper-toned print shows copper and iron “from the precipitate of the copper ferricyanide complex, which is responsible for the red color of the image” — ferri-, not ferro-, and therefore a different anion again.

Three Tier 1 sources, three descriptions of the coloured product, and the course cannot tell you which is right. So it publishes no formula and no equation for copper toning, states the disagreement, and treats the identity of the pigment as an open question rather than picking the version that reads best. What all three agree on is the observable: an insoluble coloured compound is built at the image, and it is red.

Two properties come with the deposition route and neither is optional.

It intensifies. Wall is explicit that copper toning “is actually an intensification process; therefore, the primary image must not be developed too far”. Kodak’s own chapter on intensification gives uranium as an example: a silver image “can be very much intensified by toning it with uranium, the reddish-brown uranium ferrocyanide having very great printing strength”. A pigment with high covering power is being added to the image, so densities rise.

The product can be dissolved. Wall records that uranium ferrocyanide is soluble in alkalis, so long washing in ordinary water reduces the colour by dissolving the uranium salt. The same vulnerability governs Prussian blue, and it is the reason the part warns about mounting an iron-blue print on the buffered, alkaline-reserve board that Part XIX recommends for everything else. ILFORD put the permanence consequence in one sentence on their own datasheet: blue (iron) and red (copper) toners “may not give extra protection and the image might fade”.

✕Kodak's Blue Toner is a gold bath

T-26 is called Blue Toner, produces blue tones, and contains gold chloride, thiourea, tartaric acid and sodium sulfate. There is no iron in it. It is a route-two toner with a route-three name, and the blue comes from finely divided gold rather than from Prussian blue. Meanwhile Getty record that iron blue toning was historically called “cobalt toning”, after the colour, with no cobalt anywhere in the bath. Toner names describe the result and are not evidence about the chemistry.

Direct and indirect: two ways of reaching the particle

Everything above describes what the product is. This section is about how the bath gets to it, and it is where the practical differences between toners actually live.

One bath or two, and what each reaches

1. 1Direct toning: one bath, on the metalThe toner attacks the metallic image in place. It works from the surface of the emulsion inwards and from the outside of each deposit inwards, so it can be stopped part-way and usually is. Selenium and the hypo-alum sepia bath T-1a are direct
2. 2Indirect toning: bleach, then convertThe image is oxidised back to a silver halide, which is then exchanged for the toned compound. Every particle has been through the halide state, so the conversion reaches all of it and the colour change is larger. The sepia toners T-7a and T-52 are indirect
3. 3The gap between them, which is a real objectBetween the two baths the print is pale and its image is a light-sensitive silver halide again. It is not finished and it is not stable, and it must go on to the second bath

Wall’s 1924 division is the same one and older: in the direct method “the image is converted into silver sulphide direct, while in the latter process the silver is first converted into chloride, bromide or iodide, and then into sulphide”.

⁇Do direct toners take the shadows first or the highlights first?

This is the question the manifest asked this page to settle, and the honest answer is that it is a property of each toner and not of directness. The evidence, toner by toner:

• Selenium: the shadows first. Moersch states it flatly — irrespective of dilution or temperature, a selenium toner “always reaches the higher densities first”. A strong short bath (1+5 to 1+20 for 20 to 60 seconds) intensifies the shadows alone; reaching the highlights before the shadows start to lose density needs a dilution between 1+100 and 1+400. Kodak’s sheet says the same thing commercially, recommending 1:20 or 1:40 “to increase shadow contrast and maximum density with a minimum tone change”.
• Blue Toner T-26: the highlights first. Kodak state that “toning will occur in the highlights first and the shadows last” and that partial toning may therefore give blue highlights and untoned shadows.
• Gold Toner T-21: uniformly. Kodak state that it tones the highlights and shadows at a uniform rate, “allowing you to stop the toning when the print reaches the hue you want”.

T-21 and T-26 are both gold baths, both direct, and they behave differently from each other. So directness does not predict which end of the scale moves first, and the surface-area argument earlier on this page — small deposits have more surface per gram and should convert first — predicts T-26 and contradicts selenium. No source read for this course explains selenium’s preference for the high densities, and the T-26 formulary entry records the same gap for gold. The course therefore states the three observations, names the ones its own reasoning fails to explain, and refuses to generalise from any of them.

The bleach, and the print that is not finished

The bath that begins every indirect toner is the rehalogenating ferricyanide-bromide bleach, and its whole chemistry is one step.

Ag + [Fe(CN)₆]³⁻ + Br⁻ → AgBr + [Fe(CN)₆]⁴⁻

Oxidise the silver, and catch it before it can leave

Hexacyanoferrate(III) takes one electron from metallic silver. The silver ion produced would go anywhere, but in a bath loaded with bromide it meets a bromide ion first, and silver bromide is insoluble — so it comes down on the site of the grain it came from. Substitute hypo for the bromide and the same oxidation gives a soluble complex that walks out of the emulsion, which is Farmer’s reducer and takes the picture away. The difference between a bleach and a reducer is which anion is waiting.

The print goes almost white. Nothing has been lost that redevelopment cannot recover: the silver is all still there, in the same places, as a halide. That is why a partly bleached print can be brought back and why a fully bleached one still holds a full tonal scale.

⚠A bleached print is a piece of photographic paper again

The image is now silver bromide in gelatin, which is exactly what unexposed printing paper is. Kodak say so in terms and then measure the consequence honestly: the bleach converts metallic silver “to light-sensitive silver bromide”, you may wish to bleach under safelight, “however, the effect is extremely small, and may not be noticeable”. The course accepts that judgement and gives the reason it holds — a bleached print carries no developer and is about to be given none, so the printed-out silver a few seconds of room light can produce is a negligible fraction of an image that is about to be wholly converted anyway.

The instability that does matter is chemical, not photographic, and Moersch states it. A print whose metallic silver has been bleached to silver bromide and then not converted “is now no longer archival, because the bleaching of the (not yet toned, metallic) silver has produced a (non-stable) silver salt”. Silver bromide will “tone over time (uncontrolled) by gas action”. It must either be dissolved by fixation or converted to silver sulfide by toning again. A print left half-processed between the two baths is not a stylistic choice; it is an unfinished object.

One more piece of bleach chemistry that is really a control: the print must be thoroughly rinsed before bleaching. Moersch’s reason is exact — residual fixer thiosulfate would make the bleach act as a reducer, redevelopment would be impossible, “at least the highlights would vanish irretrievably”. The bleach and the fixer are the same reaction with a different anion, so carrying one into the other converts a bleach into a reducer inside the gelatin.

What toning does to the tonal scale

A toner is not a colour filter laid over a finished print. It rebuilds the substance that produces density, so density changes. Kodak’s sheet gives three statements, and they point in three directions.

Toner	What Kodak state	What Kodak tell you to do about it
Sepia, Sepia II Warm, Brown	“will reduce print densities”	Develop fully, and make the print slightly darker than normal
Rapid Selenium	“tends to intensify the image”	Consider shortening development slightly; a fully developed print toned in it yields an increased tonal scale
Blue Toner T-26	contrast and density “appear to increase”	Reduce the exposure slightly

Kodak add the caution that these modifications also depend on the paper emulsion type and grade, which is the manufacturer’s way of saying the table is a direction and not a number.

⚗Why a sulfide route loses density, with the arithmetic and its limits

Ware gives the mechanism, and it is a property of the two substances rather than of the process. At the absorption maximum, the optical extinction coefficient of nanoparticle silver is about 16,000 dm³ mol⁻¹ cm⁻¹; that of nanoparticle silver sulfide has been experimentally estimated at about 560. Complete sulfiding therefore drops the maximum optical density by a factor of nearly thirty. Ware immediately qualifies it: the visual effect is smaller than that, because the eye is insensitive near 400 nm where the maximum sits and responds more to the tail of the band in the green, where the factor is smaller.

How much of this carries across to a sepia-toned enlargement, stated as the course’s reading and not as Ware’s claim. Ware is writing about nanoparticle silver in salt prints, and his conclusion there is that “the quantities of silver in early salt prints are so small that conversion to silver sulphide greatly weakens the density”. In the same passage he notes that silver sulfide in sufficient concentration is a good, stable pigment, “as the sulphide toning of modern silver-gelatin papers attests”. So the direction transfers — silver sulfide is a weaker absorber per unit than silver, which is why Kodak tell you to print darker — and the factor of thirty does not, because a developing-out print carries far more silver and its deposits are not all in the nanoparticle regime. The course states the direction as established and the magnitude as unquantified for a silver gelatin print.

Selenium moves the other way, and everyone who has measured it agrees about the shape rather than the size. Kodak publish a characteristic curve for POLYMAX Fine-Art Paper developed in DEKTOL 1:2 at 20 °C for two minutes and toned four minutes in Rapid Selenium Toner at 1:40, showing an increase in upper-scale contrast and D-max: the two curves separate in the shadows and converge towards the highlights. Getty describe the same effect from the conservation side — properly done archival selenium toning caused no major colour change, “the only perceivable visual effect was a slight increase in image contrast and brilliance”. Moersch describes it from the bench: a distinctive increase in Dmax, more differentiation in the deep shadows, and a shift towards cooler, less green tints.

!The one published curve, and why it is not on this page

Kodak’s POLYMAX curve is a copyrighted figure in a Kodak publication, and the course has no image pipeline: every diagram here is drawn from data at build time. Kodak publish the curve as a picture and not as numbers, so the course can neither reproduce the figure nor redraw it faithfully. What it can do is state the conditions and the shape, which is what the paragraph above does. The selenium experiment exists so that you produce the equivalent measurement on papers you can still buy — Kodak’s paper is discontinued and the curve is for their developer, not yours.

The direction of each change, drawn to teach

• Model untoned print
• Model selenium-toned: upper scale and Dmax raised
• Model sepia-toned: densities reduced

Show the numbers behind this plot

Three model print curves on the same axes, all leaving a base density near 0.06. The untoned curve rises through 0.55 at log exposure 1.0 and shoulders off at about 1.95. The selenium-toned curve is indistinguishable from it in the toe and lower mid-tones, separates upward above log exposure 1.2, and finishes higher at about 2.12, so the change is concentrated in the upper scale and the maximum density. The sepia-toned curve sits below the untoned one everywhere above the toe, running through about 0.45 at log exposure 1.0 and shouldering at about 1.60, so the whole upper scale is lower and the maximum density is reduced. The directions come from Kodak's statements; the shapes are drawn, not measured.

Series	Relative log exposure	Reflection density
Model untoned print	0.10	0.06
Model untoned print	0.40	0.10
Model untoned print	0.70	0.26
Model untoned print	1.00	0.55
Model untoned print	1.30	1.05
Model untoned print	1.60	1.60
Model untoned print	1.80	1.86
Model untoned print	2.00	1.95
Model selenium-toned: upper scale and Dmax raised	0.10	0.06
Model selenium-toned: upper scale and Dmax raised	0.40	0.10
Model selenium-toned: upper scale and Dmax raised	0.70	0.27
Model selenium-toned: upper scale and Dmax raised	1.00	0.58
Model selenium-toned: upper scale and Dmax raised	1.30	1.14
Model selenium-toned: upper scale and Dmax raised	1.60	1.78
Model selenium-toned: upper scale and Dmax raised	1.80	2.05
Model selenium-toned: upper scale and Dmax raised	2.00	2.12
Model sepia-toned: densities reduced	0.10	0.05
Model sepia-toned: densities reduced	0.40	0.08
Model sepia-toned: densities reduced	0.70	0.21
Model sepia-toned: densities reduced	1.00	0.45
Model sepia-toned: densities reduced	1.30	0.86
Model sepia-toned: densities reduced	1.60	1.33
Model sepia-toned: densities reduced	1.80	1.54
Model sepia-toned: densities reduced	2.00	1.60

Model curves, not measurements. Each direction is a Kodak statement - sepia and brown toners reduce print densities, Rapid Selenium Toner tends to intensify - and the shapes here only make those two sentences visible on one pair of axes. Your own numbers come from the selenium experiment later in this part. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

The practical instruction that falls out of all this is one sentence, and it is the reason this page comes before any tray. Make the print for the toner it is going to receive. A print made to look right untoned, then sepia-toned, comes out flat and light; a print made to look right untoned, then selenium-toned, comes out with shadows heavier than you intended. Kodak’s advice — print darker for sulfide, consider developing slightly less for selenium — only makes sense to someone who decided which toner they were using before they made the print.

Partial, split and sequential toning

Stopping a toner early does not give you a lighter version of the full effect. It gives you a print containing two substances at once, distributed according to which parts of the image the bath reached, and that is a different object from a blend of two colours.

Moersch works this deliberately, and his account is the clearest in the corpus because he varies one thing at a time. The degree of bleaching sets the image tone. With the same toner setting, a print bleached for three minutes and one bleached for two minutes come out different colours. Bleach briefly at a high dilution and only the highlights are converted, leaving warm highlights against untoned shadows — and “the higher the dilution, the smoother the transitions”, because a slow bleach produces a gradient across the scale rather than an edge. He adds a printer’s reason for stopping short that has nothing to do with colour: it is “often an advantage for the image contrast to stop the bleaching process before it wipes out the deepest shadows”.

⚗What is actually in a partly toned print

After a partial bleach and a sulfide bath, Moersch describes the contents exactly: there is silver sulfide from the toning, beside a small amount of silver salt that was bleached and not converted, beside the metallic silver the bleach never reached. Three substances, three permanence behaviours, one sheet of paper.

That is why the order of operations matters and why the halfway state is not a finished print. The untoned metallic silver is stable and will stay as it is. The silver sulfide is stable. The residual silver bromide is neither: it is the unstable component described in the bleach section above, and it has to be either fixed out or converted by a second pass through the toner. A split-toned print that has been left with unconverted halide in it is not a split-toned print. It is an unfinished one.

Sequential toning — two toners in succession — changes the result according to the order, because each toner alters what the next one finds.

• Pre-tone to protect, then bleach. Moersch’s method for reaching the highlights without destroying the shadows: if you want the toner to reach the shadows fully you would have to bleach as far into the shadows, “which would mean a loss of contrast and density”. Pre-toning in selenium or sodium sulfide protects the deep tones from the later bleach instead. The two protectants are not interchangeable: selenium protects the shadows and then the mid-tones according to how long you tone, because it works from the high densities down, while sodium sulfide affects the complete range of tone values. One minute of sulfide pre-toning “leaves only little silver for bleaching”, so there is little density lost in the bleach.
• Sepia first, then blue, gives red. Kodak’s own multiple-toning procedure: treat the print in Sepia or Brown Toner, wash it thoroughly, then tone in Blue Toner T-26, and red or orange appears after roughly 15 to 30 minutes at 32 °C. Cold-tone papers yield a truer red and warm-tone papers an orange. The colour belongs to neither bath on its own — it is a gold deposit growing among silver sulfide, and it is a genuinely new substance mixture rather than a mixed pigment.
• Expect to lose the shadows. Kodak note that multiple toning “usually produces a density loss in the shadow areas of prints”, and their remedy is to start from a print of higher-than-normal contrast. That is the same instruction as “make the print for the toner”, applied twice over.

Selective toning is a fourth variation and mechanically the simplest: Kodak describe masking areas with a liquid or sheet frisket so the toner never reaches them, choosing scenes with a distinct boundary because the edge of the frisket will be visible in the result. Nothing about the chemistry changes; only its geography does.

The variables that decide any toning result

Kodak list four factors on the front page of their sheet and then spend eight pages on the fifth. Assembled, and in the order in which they are decided:

Variable	Where it is fixed	What it governs
Paper emulsion, surface and stock tint	at the coating plant, when you buy the box	The silver morphology the toner meets, and therefore most of the colour. Kodak’s own tables give three different results for one bottle on three papers
Developer and degree of development	at the printing session	The size and form of the developed silver, which the toner inherits. Kodak: for good sulfide colours the print should be fully developed but not over-exposed
Exposure	at the printing session	Whether the print is dark enough to survive a sepia toner, or light enough to survive selenium
Fixing	at the printing session	Whether the print can be toned at all, or only stained. Two-bath, non-hardening
Washing	at the printing session	Whether residual thiosulfate turns the bleach into a reducer, and whether residual silver returns as yellow stain
Toner and dilution	at the toning session	Which route, and how far into the scale it reaches before the shadows start to give way
Time, temperature and agitation	at the toning session	How far the conversion goes, which is not the same as how far it looks as though it has gone

The last row carries the trap. Toning continues in the wash, so a print pulled when it looks right is a print that has gone too far. HARMAN build the countermeasure into their instructions for a first attempt: make two identical prints, leave one in a holding tray of water, and compare against it throughout, because you cannot learn where “before” is without an untoned sheet in your other hand. Moersch adds the diagnostic for the invisible case — how far a selenium toning has really progressed “is only visible if you bleach”, and a test print that bleaches to a red-brown image with tonality still in the highlights was toned far enough.

!What toning is not for

Kodak put it in five words and the course adopts them without softening: toning cannot disguise poor print quality. Their specification for a candidate print is a properly exposed and processed print from a high-quality negative, with “a full tonal scale with good detail in the highlights and shadows”.

The corollary is the part’s most expensive lesson. Almost every defect that appears in a toned print was already present, invisibly, in an untoned one: mottle from an overconcentrated stop bath or from poor agitation in the first seconds, round purple stains from air bubbles trapped in the fixer, yellow stain in borders and highlights from an exhausted fixing bath. None of them show until the toner arrives, and all of them will be blamed on the toner. A badly fixed print cannot be toned. It can only be stained.

Recap

• The image is metallic silver, and a toner stops it being metallic silver. Colour, density, contrast and permanence all move because the substance moved.
• Developed silver-gelatin silver is micron-scale filament bundles; print-out silver is 10 to 100 nanometre particles. They are different objects to a toner, which is why alt-process toning is a different subject.
• Colour comes from particle size and shape, not from a dye. That link is Tier 1 in several processes and for several metals. The full optical account — plasmon resonance, Mie and Gans, the refractive index of the host — is established for nanoparticle silver and is an unfinished extrapolation for a developed print, on its own author’s admission.
• Kodak counted four methods; this part teaches three. Conversion to a silver compound, replacement or plating by a nobler metal, and deposition of a coloured insoluble salt. Dye mordanting is the fourth and gives no protective effect.
• Conversion: silver to silver sulfide runs to completion on a solubility difference of thirty-six orders of magnitude. Silver to silver selenide is stated by Kodak and written as no equation anywhere in this course, because none is sourced.
• Replacement: gold(III) costs three silvers per gold, so every gold toner is an apparatus for reaching gold(I). Slow deposition gives red gold, fast gives blue — which is the particle-size argument again, in Kodak’s 1928 words.
• Deposition: silver becomes silver ferrocyanide, then a coloured metal ferrocyanide. It intensifies, and the pigment can be dissolved by alkali. Three sources give three different identities for the copper compound, so the course publishes none.
• Direct toners work in place, indirect toners bleach first and reach every particle. Which end of the tonal scale moves first is a property of the individual toner: selenium takes the shadows, T-26 takes the highlights, T-21 is uniform. Do not generalise.
• A bleached, unredeveloped print is silver bromide in gelatin. The light sensitivity is small on Kodak’s own measurement; the chemical instability is not, and the print must be finished.
• Print for the toner. Sulfide routes lose density, selenium raises maximum density and upper-scale contrast, T-26 appears to raise both.
• Toning cannot disguise poor print quality. Most toning faults are fixing faults with a delayed fuse.

Check your understanding

Question 1. A print is bleached in a ferricyanide-bromide bath until the image has almost vanished, and the session is then abandoned. The print is washed and dried. What is wrong with it?

• Nothing: the silver is still there, so it is simply a very pale print
• The image silver has been dissolved away and cannot be recovered
• The image is now silver bromide, which is chemically unstable and must be fixed out or converted
• The gelatin has been hardened by the ferricyanide and will not accept a toner later

Show the answer and why

Answer: The image is now silver bromide, which is chemically unstable and must be fixed out or converted

The bleach oxidises silver and the bromide catches it as insoluble silver bromide in place, so nothing has left the print. But Moersch is explicit that such a print is no longer archival: the silver bromide would tone uncontrolled over time by gas action, and it must either be dissolved by fixation or converted to silver sulfide by toning again.

Question 2. Kodak instruct that prints intended for their Sepia or Brown toners should be made slightly darker than normal. Which statement best explains why?

• Sulfide toners physically remove some of the image silver during the bleach step
• Silver sulfide absorbs light less strongly than metallic silver, so converting the image lowers its densities
• The bleach dissolves the baryta layer and lowers the reflectance of the base
• Sepia toners are always used at high dilution, which limits how much conversion occurs

Show the answer and why

Answer: Silver sulfide absorbs light less strongly than metallic silver, so converting the image lowers its densities

Kodak state the effect - Sepia, Sepia II Warm and Brown Toner reduce print densities - and Ware gives the mechanism: the optical extinction coefficient of nanoparticle silver is about 16,000 dm3 per mol per cm at the absorption maximum against about 560 for silver sulfide. The direction transfers to a developed print; the exact factor does not, because a print carries much more silver and not all of it is in the nanoparticle regime.

Question 3. Which of these are supported by sources this course holds? Select all that apply.

• Selenium toner reaches the higher densities first, irrespective of dilution or temperature
• Kodak Blue Toner T-26 tones the highlights first and the shadows last
• Direct toners as a class attack the largest silver deposits before the smallest
• Kodak Gold Toner T-21 tones highlights and shadows at a uniform rate

Show the answer and why

Answer: Selenium toner reaches the higher densities first, irrespective of dilution or temperature, Kodak Blue Toner T-26 tones the highlights first and the shadows last, Kodak Gold Toner T-21 tones highlights and shadows at a uniform rate

The first is Moersch, the second and fourth are Kodak G-23. The third is the generalisation the page refuses: T-21 and T-26 are both direct gold baths and behave differently from each other, and selenium contradicts the surface-area argument that would predict highlights first. Three observations, no rule.

Question 4. Kodak T-26 is called Blue Toner. What produces its colour?

• Prussian blue, formed from an iron salt and the ferrocyanide left by the bleach
• Finely divided gold, deposited from a gold chloride and thiourea bath
• Cobalt ferrocyanide, which is what gives the process its alternative name
• A basic dye mordanted to a converted silver iodide image

Show the answer and why

Answer: Finely divided gold, deposited from a gold chloride and thiourea bath

T-26 is made from gold chloride, thiourea, tartaric acid and sodium sulfate: there is no iron in it. It is a metal-replacement toner with a deposition-toner name. Getty separately record that iron blue toning was called cobalt toning after the colour, with no cobalt in the bath either - toner names describe results, not chemistry.

Question 5. Two prints are made from one negative on the same warm-tone paper in the same session. One is sepia-toned by bleach and redevelopment; the other is left untoned. A conservator later finds them equally brown. Which statement is true?

• They are the same object, since the colour and the paper are the same
• The toned print is silver sulfide and the untoned one is metallic silver, so they are different substances with different permanence
• The untoned print must have been sulfided accidentally by hydrogen sulfide in the air
• Neither print can be identified, because toning leaves no analytical signature

Show the answer and why

Answer: The toned print is silver sulfide and the untoned one is metallic silver, so they are different substances with different permanence

A warm paper is brown because its developed silver is finely divided; a sepia print is brown because its silver has been converted to silver sulfide. Part XVIII made that distinction and this page gives the chemistry. It is also analytically awkward rather than impossible: Getty warn that the sulfur seen by XRF in a sulfur-toned print comes mainly from the barium sulfate of the baryta layer, which makes separating the signal extremely difficult.

Check my answersClear