Printing Out: Silver Chloride Made and Darkened in the Paper

A salted paper print is made by two solutions applied in sequence to a sheet of ordinary paper, and almost everything odd about the process follows from the fact that the second solution is applied in large excess and most of it never reacts at all. The unreacted part is not a residue to be tolerated. It is what makes the paper fast enough to use, and it is the reason the print has to be washed before it is toned, the reason the fixer takes some of the picture away, and the reason a salt print that was processed carelessly in 1846 is now a stain on a mount.

This page explains the chemistry from the precipitation to the residue. It gives no procedure — the salting lab and the printing lab do that — and it repeats no quantities that live in the formulary.

The precipitation, done in the sheet

You have precipitated silver chloride twice already. In Part IV you did it in a beaker and watched it fall as a curd. In Part V you did it inside warm gelatin, where the binder controlled nucleation, growth and habit, and where the whole craft of emulsion-making was the control of those three. Here you do it a third time, in cellulose, with no control at all.

The reaction is the same double replacement in all three places. Reilly writes it out in exactly those terms — the ions of two compounds change partners, and because silver chloride is nearly insoluble in water it is immediately precipitated:

NaCl + AgNO₃ → AgCl(s) + NaNO₃

Sodium chloride and silver nitrate give sodium nitrate and silver chloride, which comes out of solution

NH₄Cl + AgNO₃ → AgCl(s) + NH₄NO₃

The same with ammonium chloride, which is the other salt the historical formulae use

The fate of the nitrate is the part worth holding on to. Sodium nitrate, or ammonium nitrate, is freely soluble, takes no part whatever in image formation, and is removed in processing — some of it stays behind in the silver bath when the sheet is lifted off, the rest washes out of the print. Reilly says both things: the nitrate “takes no role in forming the image and either dissolves into the silver solution or is washed away in processing”. It is not inert in the bath, though. Because it accumulates there sheet by sheet, an old sensitising bath is a solution of silver nitrate in increasingly strong sodium or ammonium nitrate, and that matters to albumen rather than to plain salted paper — the accumulated nitrate helps coagulate an albumen layer. Reilly records nineteenth-century advice to add sodium or ammonium nitrate deliberately for that purpose, and adds that the advice belonged to weak silver baths of 5 to 8 per cent and does not apply at 10 per cent and above.

The two-step precipitation, drawn at the scale of a fibre

1. 1Cellulose fibres of the rawstock — the support, and also the only binder a plain salted paper has
2. 2Dried chloride from the salting solution — sodium or ammonium chloride, with a little gelatin to slow the sinking-in
3. 3Silver nitrate solution applied over it — in large excess: Talbot found about six times more nitrate than chloride was needed
4. 4Silver chloride, precipitated in place — formed where the two solutions meet, at and just under the fibre surface
5. 5Free silver nitrate remaining — the majority of what was applied; the working part of the sensitiser, not a residue

Drawn to show the arrangement, not to scale, and no source read for this course publishes how deep the sensitised zone goes in a plain salted paper. Reilly's point about depth is comparative rather than dimensional: the further the light-sensitive material penetrates, the lower the maximum density and the more matte the surface, because light reflected from the sheet is scattered by the fibres.

What is different when there is no gelatin

Three things, and each one is a limit on the print.

Nothing controls crystal size or habit. In Part V the gelatin concentration, the addition rate, the temperature and the ripening decided what kind of crystal you got, and you could make an emulsion faster or slower on purpose. Here the crystal size is decided by the strength of the solutions and by nothing else. Reilly notes the one lever that survives: stronger silver solutions produce larger grains of precipitated silver chloride, which are correspondingly less likely to be absorbed down into the paper fibres. That is a coating-quality argument rather than a speed argument, and it is why the papers with the least binder need the strongest silver bath and the shortest contact with it.

Nothing keeps the image on the surface. Reilly’s most consequential paragraph in the whole book is about location rather than chemistry. If the light-sensitive layer penetrates deeply into the fibres, the maximum density obtainable falls and the surface is matte, because light reflected from the sheet is scattered and diffused by the paper. If the image sits in a compact layer on top of the fibres, that scattering is minimised and the maximum density is much greater. Every binder in the variants lesson — gelatin, starch, whey, resin, albumen — exists to fight the sinking-in, and the plain salted paper is the case where the fight is barely joined at all. Reilly’s own plain formula still carries 2 g of gelatin in a litre, and he says exactly why: the sizing already in the paper is usually insufficient, and without some additional organic material the prints would be too flat.

Some organic matter is chemically necessary, not merely physically. This is the subtler point and it is easy to miss. Reilly divides the organic substances into two classes. Active ones — albumen, gelatin, and the organic acids citric, tartaric and oxalic — “facilitate the more complete reduction of silver chloride” and themselves form light-sensitive compounds in contact with silver nitrate, silver albumenate and silver citrate among them. Inactive ones, of which starch is the most useful, do nothing of the kind and only keep the sensitive material near the surface. His verdict on a paper with neither is unambiguous: pure silver chloride paper is unsatisfactory, because it produces grey and flat images. That single sentence is why the arrowroot formula contains citric acid — the starch is inactive, so without the acid the print would be grey and flat, which Reilly calls the hallmark of a pure chloride image.

Why the order cannot be reversed, and why the sheet must be dry

Silver chloride is insoluble, so it cannot be brushed on. That is the whole argument, and Reilly puts it in one line: silver chloride is insoluble in water and in most solvents, so it cannot simply be brushed on to make photographic paper — it must be formed in place. The two steps are not a convention; they are the only way to get an insoluble substance into a sheet of paper using two solutions.

Now reverse them and follow what happens. Silver nitrate goes on first and dries as a deposit of silver nitrate through the sheet. Salt solution goes on second. Silver chloride still forms — but it forms in a bath of chloride, and it forms with chloride in excess rather than silver. Ware’s account of what that does is the important half of this page’s chemistry, and it is set out under photolysis below: a silver halide crystal with adsorbed halide on its surface is the fixed case, not the sensitised case. Its surface carries excess negative charge, which repels photoelectrons towards the interior; the halogen released by photolysis meets no free silver ions to scavenge it; and the print-out silver at the surface cannot survive. Talbot found the same thing empirically a hundred and fifty years before anybody could explain it — his own stabilising treatment for a photogenic drawing was a strong salt solution, and the effect of it was to make the paper stop darkening.

So the order is not arbitrary. Salt then silver gives you a sensitised paper; silver then salt gives you a stabilised one. A reader who tries the reverse on a spare strip will get a sheet that prints out feebly and then stops, which is a good demonstration and a cheap one.

The sheet must be dry between the two steps for a different reason, and it is mechanical. A damp salted sheet floated on a silver bath bleeds chloride into the bath, which precipitates silver chloride in the tray rather than in the paper — the bath goes cloudy and loses strength, and the sheet loses the chloride it was supposed to keep. Reilly’s instruction is that the salted paper is dried, pressed flat and stored, and that it keeps indefinitely in that state in a cool dry place; that is what makes salting a batch operation you do once and sensitising a thing you do on the day. The corollary matters at the bench: once a sheet is salted and dry there is no way to tell which side was coated, so Reilly says to mark the back of every sheet in pencil before salting it.

The excess: what the free silver nitrate is actually for

Talbot established the ratio by experiment. Reilly reports it plainly: when chloride and silver nitrate were present in equal amounts, Talbot’s papers were hardly light-sensitive at all compared with papers in which silver nitrate was present in great excess; he found that about six times more nitrate was necessary, and settled on a salting solution of 2 to 4 per cent with the silver nitrate at 12 per cent. The modern practice has not moved: Reilly’s own rule is that the sensitising solution must be approximately four times as strong as the salting solution, the Getty Atlas describes a salt solution “usually around 4 per cent by weight” and a silver nitrate solution “around 12 per cent”, and Bostick & Sullivan ship a bottle labelled 12 per cent.

Why so much? The explanation Reilly gives is Hermann Vogel’s, and it is the one most people learn: as light dissociates each unit of silver chloride, the liberated chlorine unites with the silver nitrate present to form new silver chloride, light breaks that down in its turn, and the cycle repeats. Where excess silver nitrate is available, more image silver is formed and a greater maximum density is reached.

⚗Vogel's account, and the refinement Ware puts on it

Vogel’s version is right about the outcome and incomplete about the route, and the difference matters because it explains something Vogel’s version does not.

Ware, writing as a chemist about exactly these sensitisers, agrees that the excess silver ions are central and then asks what the chlorine actually does first. Free chlorine and water disproportionate:

Cl₂ + H₂O → Cl⁻ + HOCl + H⁺

Chlorine and water: chloride ion, hypochlorous acid and a proton

In pure water at neutral pH this equilibrium already lies well to the right for chlorine — Ware gives a calculated equilibrium constant of about 500, against 0.01 for bromine and 10⁻⁹ for iodine, which is his explanation of why “fixed” silver iodide is so stable to light. Add free silver ions and the equilibrium is driven right by the extreme insolubility of the silver halide:

Cl₂ + H₂O + Ag⁺ → AgCl(s) + HOCl + H⁺

Ware's version: the halogen is disproportionated by water in the presence of silver(I), and half of it returns as fresh silver chloride

Read the two equations together and the difference from Vogel is precise. Vogel has all the chlorine returning as new silver chloride. Ware has half of it doing so and the other half becoming hypochlorous acid — which is an oxidising agent strong enough to attack silver — plus a proton, which makes the sheet more acidic as the exposure goes on. Both accounts predict that the excess silver extends the photolysis. Only Ware’s predicts the back-reactions, and those are what the section below is about.

Rule 7, stated plainly. Vogel’s mechanism is what Reilly publishes, and Reilly is the standard practical authority. Ware’s is a chemist’s model, published in a conservation monograph, and he marks parts of it as proposals. The course teaches Ware’s because it explains more, and says where it came from.

There is a second consequence of the excess that has nothing to do with sensitivity and everything to do with why this part is the messy one. Reilly is explicit: because of the presence of excess silver nitrate, printing-out papers are far more subject to damage from careless handling and contamination than modern photographic papers are. Every sheet you make carries a corrosive, staining, light-sensitive reagent in unreacted form, spread over its whole surface, ready to react with anything that touches it — a fingerprint, a drop of fixer carried on a glove, a metal clip, the back of the sheet above it on the drying line.

Photolysis without development

Everything in Parts IV to XX rested on the latent image: a cluster of a few silver atoms, invisible, useless by itself, and the point of attack for a developer that reduces the whole crystal around it. Here there is no developer. Light has to build the visible particle itself, out of the crystal it is standing in.

2 AgCl + light → 2 Ag + Cl₂

The net photolysis of silver chloride: metal, and free halogen that has to go somewhere

Ware’s account of the mechanism runs through the same electron and hole that Part IV used. A photon lifts an electron into the conduction band; the electron reduces a silver ion to an atom; the positive hole diffuses to the surface and makes a halogen atom, and then a halogen molecule that leaves the crystal. What is different is the scale and the place.

Scale. To make a visible particle you need not a cluster of four atoms but a colloidal particle of tens of thousands of them, and Ware puts the exposure ratio at “in the order of a million” times that needed to form a latent image.

Place. In a pure silver halide crystal the photolytic silver is trapped inside the lattice and cannot grow past about 10 nm; Ware quotes a measured limiting yield of 1.2 × 10¹⁹ atoms per square metre, which he calculates corresponds to an optical density of order 0.02 — a barely perceptible darkening. A pure crystal cannot make a print. What changes everything is the adsorbed layer of excess silver ions on the crystal surface. That excess positive charge attracts the photoelectrons to the surface, where silver specks can grow free of the constraints of the lattice, and the holes go to the surface too and release their halogen out of the crystal altogether.

Two ways to make a picture out of silver, compared step by step

1. 1Developing out — a photon makes a speck of about four silver atoms, inside a crystal of hundreds of millionsThe speck is invisible. It is a catalyst, not a picture. Part IV puts the amplification that follows at of order 10⁷ to 10⁸ atoms of image silver per atom of latent image
2. 2Developing out — a reducing agent finds the speck and reduces the whole crystal to filamentary silverThe image silver is micron-sized filaments. Coarse, opaque, neutral in colour, and chemically robust because its surface area per unit mass is small
3. 3Printing out — photons build the particle themselves, one absorbed photon at a time, until it is a colloid of tens of thousands of atomsWare: an exposure of the order of a million times that needed for a latent image. Nothing amplifies it afterwards, so the exposure has to pay for every atom
4. 4Printing out — the chlorine released has to be taken up, or the reaction runs backwardsIn a pure crystal it is not taken up, photolysis stalls at a density of about 0.02, and there is no print. The excess silver nitrate is what makes the difference
5. 5Printing out — the product is nanoparticle silver, 10 to 100 nm, in and on the fibresFine, coloured rather than neutral, with a hundredfold surface area, and therefore vulnerable to everything that attacks silver — including the fixer that is about to be used on it

The two families are not fast and slow versions of one process. They differ in what light is asked to do, in the size and shape of the silver that results, and therefore in the colour and the permanence of the print.

What accepts the chlorine, and how much of this is settled

This is the question the design of this part singles out, and the honest answer has three parts.

What is established. That something must take up the halogen is not in doubt, and the evidence is the behaviour of a pure crystal: photolysis stalls at a barely visible density, and Ware names the reason — in the absence of a halogen acceptor the halogen reacts back with the surface silver and re-forms the halide. That is why silver chloride behaves so differently in a sensitised paper and in a fixed one.

What Ware proposes. That in a Talbot-type sensitiser the environment around the crystals consists of excess silver(I) ions, water, the cellulose substrate and probably a sizing agent, and that the combined action of water and silver(I) ions constitutes the major halogen acceptor, contributing the greater part of the image silver by the disproportionation route in the callout above. He marks this as a proposal, in those words.

What Ware rules out, with a reason. The organic binder, as a halogen acceptor. This is the part most likely to surprise a reader who has just come from Part V, where gelatin’s halogen-scavenging is a load-bearing idea. Ware’s statement is specific: gelatin may well be the important halogen acceptor in development emulsions, where it surrounds every crystal and the amount of halogen liberated by latent-image formation is exceedingly minute, but it is known not to be an effective scavenger of halogen at print-out levels of exposure. The sizing agent is therefore not essential to the photochemistry of printing out, which he says can be shown experimentally. What the sizing does do is retain the sensitiser, and therefore the image, in the surface fibres, and possibly protect the colloidal silver by surface adsorption — which is a real effect on colour and stability, and no effect at all on the halogen.

⁇Does the organic matter accept the halogen, or does it not?

The disagreement is real and it is between two sources this course otherwise leans on heavily.

Reilly describes the “active” organic substances — albumen, gelatin, citric, tartaric and oxalic acid — as facilitating “the more complete reduction of silver chloride”, which is the language of halogen acceptance, and reports that a paper with no active organic material gives grey and flat images. Ware says gelatin is known not to be an effective halogen scavenger at print-out exposures and proposes water plus silver(I) instead.

They may not be contradicting each other. Reilly’s claim is that the active substances make the reduction more complete; he does not say by what route, and an organic acid can also work by holding the pH down, by complexing silver, or by being oxidised. Ware’s claim is narrower and about gelatin specifically. The observation both must explain — that citric acid transforms an arrowroot paper from grey and flat to warm purple — is not accounted for in detail by either.

What the course uses, and why. Ware’s account of the halogen, because it is a chemist’s argument with equilibrium constants attached and it explains the acid drift and the back-reactions that Vogel’s does not. Reilly’s account of the active substances as an empirical finding about what those substances do, without adopting his implied mechanism. And it says plainly what neither source settles: why an active organic addition should shift the colour towards red is not explained by any source this course has read, and the process atlas entry already records that gap.

Deeper: the back-reactions, and why a dull sky is not a slow sun

Ware takes the argument one step further than any practical text does, and it explains an observation photographers have made since the 1840s.

The two products of the disproportionation are both hostile to the image. Hypochlorous acid is a strong enough oxidising agent to attack silver:

HOCl + 2 Ag → AgCl(s) + Ag⁺ + OH⁻

The hypochlorous acid made by the halogen acceptor can dissolve the silver the halogen acceptor let you make

And the accumulating acidity lets the nitrate ion do the same thing:

3 Ag + 4 H⁺ + NO₃⁻ → 3 Ag⁺ + 2 H₂O + NO

Nitrate oxidising silver in acid conditions: the other back-reaction available in the sheet

Ware then makes the observation that gives the whole section its point. Add the photolysis equation, the halogen-acceptance equation and the hypochlorous attack together and the overall outcome is zero. The formation of image silver is entirely dependent on the relative rates of these reactions, not on where their equilibria lie, since the equilibria bar any product at all.

That is the explanation for a fact every nineteenth-century manual reports and no practical text explains. Printing under a dull light cannot be compensated by extending the exposure. Ware records it as a case of low-intensity reciprocity failure: inadequate light levels produced weak prints after wet processing, regardless of the duration of the exposure, and a meteorological limitation was therefore set on the days, seasons and even the locations in which good printing was possible. The colour and vulnerability of such an image both suggest a very small particle size, which is the opposite of what simple precipitation chemistry predicts for a slow reaction — slow precipitations usually make larger particles. Ware’s proposal is that the small particles are what survives after the slow, diffusion-controlled back-reactions have redissolved the rest. Schaaf’s alternative explanation, which Ware quotes and questions, is that lower light intensity simply produces smaller particles that the fixer then attacks more easily.

Classify the three statements. That weak light gives weak prints regardless of time is a nineteenth-century observation repeated by modern practitioners: established. That the mechanism is back-reaction is Ware’s hypothesis, and he presents it as one. That it is smaller particles being lost in the fixer is Schaaf’s interpretation. The course teaches the observation as fact and both explanations as explanations.

Why the image is not neutral

A developed silver gelatin print is neutral black because its image silver is micron-sized filaments, which absorb across the whole visible spectrum. A printed-out image is not, and Reilly says why in one sentence of Chapter One: the aggregate particles formed in printing-out papers are of a size chemists call colloidal, they are not large enough to absorb all wavelengths and appear black, so they absorb some wavelengths and not others — depending in part on the refractive index of the material in which the particles are dispersed.

Ware supplies the physics behind that sentence, and it is worth having properly, because it is the same physics that governs what gold toning does in the next lesson.

🔬Why nanoparticle silver has a colour at all

Photolytic silver has particle sizes in the 10 to 100 nm range — smaller than the wavelengths of visible light. At that size a metal particle absorbs light by a mechanism a bulk metal does not have: light excites a collective oscillation of the conduction electrons, which quantum mechanics calls a plasmon. For most metals the resulting surface plasma resonance sits in the ultraviolet, which is why most nanoparticle metals are simply grey or brown. Copper, silver and gold are the exceptions: the way their dielectric function varies with frequency puts sharp absorption bands in the visible, and that is why they are the only three metals useful for making coloured images. Faraday noticed the effect in 1857; Mie gave it a theory in 1908.

The colour depends on four things, and a photographer moves all four without meaning to.

Size. Ware reproduces Wiegel’s calculated and experimentally confirmed table for spherical silver particles in water: 10 to 20 nm transmit yellow, 25 to 35 nm red, 35 to 45 nm purplish-red, 50 to 60 nm violet, 70 to 80 nm dark blue, 90 to 100 nm light blue.

Shape. Departing from a sphere shifts the main band to shorter wavelengths and adds a new long-wavelength band, which dominates the perceived colour.

Aggregation. Particles in a line behave like elongated ones: a yellow 10 to 20 nm sol turns red-brown as it aggregates.

The refractive index of what surrounds them. Raise it and the absorption maximum moves to longer wavelengths. Ware works the case that matters most here: silver nanoparticles about 10 nm across inside a silver chloride crystal, whose refractive index is 2.071, absorb at about 550 nm rather than at the 390 to 400 nm they would show in water — which is why “sunned” silver chloride is violet.

Four practical consequences follow, and each one is something you will see.

The print looks best before you process it. Ware describes freshly printed-out silver from a Talbot-type sensitiser as a rich, dense purplish or brownish black, and adds that the shift to yellowish-brown on wet processing “disappoints everyone who has ever made such a print”. He is candid that the initial black is the hard thing to explain: neutral density in nanoparticle silver requires a wide spread of particle sizes and shapes, and there is no obvious reason why the colloid should become less polydisperse on wet processing. He offers a conjecture — that print-out silver may be structurally untypical, possibly even amorphous — and marks it as entirely conjectural.

The binder changes the colour. Reilly says so directly: different binders or vehicles, such as albumen, gelatin or starch, produce prints of different colours, and one of the reasons is the refractive index of the medium the particles are dispersed in. A plain salted paper, an arrowroot paper and an albumen print made from the same negative on the same day are three different colours.

The fixer changes the colour, dramatically, and the mechanism is physical. That is the next section.

And drying changes it again. Reilly: after drying, prints become darker and colder in tone, because the refractive index of the system and the distances between the particles have both changed. Ware attributes the enrichment towards brown to coalescence ripening as the particles aggregate, and notes that heat promotes the same change — which is exactly why heat toning with a hot iron was Talbot’s own finishing step and worked at all. A print judged wet is a print judged wrong, and that is a chemical statement here rather than a piece of darkroom lore.

📷What Part XX established, and what changes here

Part XX’s account of image colour is the same physics reached from the other end: there, the developed silver of a gelatin print is broken down and rebuilt at a smaller particle size by a sulfide or selenium toner, and the print goes warm. Here you start at the small end of that scale and every subsequent operation moves the particle size, the shape, the packing and the surroundings.

The distinction to keep is between toning and tinting, which Part XX insists on and which this part needs constantly: nothing here is a dye. Every colour on a salt print is metallic silver, or a compound of it, at some particular size in some particular medium.

Self-masking

Here is the property that makes a printing-out paper worth using in spite of everything else on this page.

As the shadow areas of a print darken, the silver already formed there absorbs the light that would otherwise go on exposing them. The shadows slow down. The highlights, where little silver has formed, do not. Reilly describes the density built up in the shadows as behaving as if it were additional density in the negative — a mask that delays the attainment of maximum density and prevents the loss of shadow detail before the highlight detail has had time to print in. The Photographers’ Formulary kit sheet says the same thing in a sentence and adds where the effect is concentrated: the silver metal shields the unreacted silver chloride remaining on the paper, and the effect is greatest in the shadow areas, allowing shadow detail to remain in the final print.

Self-masking: what the shadow areas do to their own exposure

• Highlight: under a dense area of the negative
• Mid-tone
• Deep shadow: under a thin area of the negative

Show the numbers behind this plot

Two falling curves and one flat one against printing time. The flat line at the top is a highlight area under a dense part of the negative: almost no silver forms there, so the light reaching the sensitive material stays at its initial value throughout. The steeply falling curve is a deep shadow area under a thin part of the negative: silver builds quickly, and by the end of the exposure the light reaching the material below has fallen to a small fraction of what it was, so the area is exposing itself far more slowly than at the start. The intermediate curve is a mid-tone, falling less far. The consequence drawn on the plot is that the shadows brake themselves while the highlights do not, so the tonal scale keeps opening out instead of blocking up, and a negative of very long density range can be rendered.

Series	Printing time (arbitrary units)	Relative light reaching the sensitive material
Highlight: under a dense area of the negative	0.00	0.98
Highlight: under a dense area of the negative	2.00	0.97
Highlight: under a dense area of the negative	4.00	0.96
Highlight: under a dense area of the negative	6.00	0.95
Highlight: under a dense area of the negative	8.00	0.94
Highlight: under a dense area of the negative	10.00	0.93
Mid-tone	0.00	1.00
Mid-tone	2.00	0.82
Mid-tone	4.00	0.66
Mid-tone	6.00	0.54
Mid-tone	8.00	0.45
Mid-tone	10.00	0.38
Deep shadow: under a thin area of the negative	0.00	1.00
Deep shadow: under a thin area of the negative	2.00	0.55
Deep shadow: under a thin area of the negative	4.00	0.31
Deep shadow: under a thin area of the negative	6.00	0.18
Deep shadow: under a thin area of the negative	8.00	0.11
Deep shadow: under a thin area of the negative	10.00	0.07

The shape is drawn to teach the mechanism and was not measured from a material. What is sourced is the mechanism and its direction: Reilly describes the silver built up in the shadows as acting like additional density in the negative, delaying maximum density and preserving delicate gradation in the lighter tones, and the Photographers' Formulary kit sheet describes the same shielding with its greatest effect in the shadows. 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.

Two consequences, and the second is the reason this part exists at the point in the course where it does.

Printing-out papers have a very long scale. Reilly reports Hübl’s measurements: the scale length of glossy albumen paper equalled that of platinum paper, and the scale length of salted papers exceeded platinum by a considerable margin. His method for measuring it is the one the printing lab uses — a 21-step grey scale printed to full maximum density, with the number of distinguishable steps counted in the finished print — and the appropriate negative density range for that paper is approximately the difference in density between the highest and lowest steps you can still tell apart.

They therefore require a negative that would be unprintable on an enlarging paper. Reilly is categorical: even the densest and most contrasty negatives that will still print satisfactorily on develop-out papers do not possess sufficient density and contrast to make good prints on albumen and salted papers, and a compromise negative prints well on neither kind. There is no contrast grade and no filter here. The negative is matched to the paper, not the paper to the negative — which is the same rule Part XXI established for the whole cluster, arrived at by a completely different chemistry.

The arithmetic of speed

Both families start from silver halide. One of them is a hundred thousand times slower. Where does the difference come from?

🧮Where the orders of magnitude actually go

The published figures. Reilly, from the practical side: printing out needs more light energy than developing out, “in some cases 100,000 times more” — five orders of magnitude. Ware, from the photochemical side: the exposure to form visible photolytic silver must be increased over that needed to form a latent image by a factor “in the order of a million” — six orders. They are measuring slightly different things, which is why they differ by one order: Reilly is comparing two materials as used, Ware is comparing two photochemical end points in the same material.

Where it comes from. Not from the absorption of light, which is much the same in both cases, but from what happens after the photon is absorbed. Take Part IV’s amplification factor for development — the course’s own calculation, of order 10⁷ to 10⁸ atoms of image silver per atom of latent image — and set it against Ware’s requirement that print-out silver be built photon by photon to a colloidal particle of tens of thousands of atoms.

of order 10⁷ to 10⁸ (Part IV’s amplification, on a latent-image speck of a few atoms)

Atoms of image silver per absorbed photon, developing out

of order 1 (Stark–Einstein: one absorbed photon, one silver ion reduced)

Atoms of image silver per absorbed photon, printing out

The ratio between those two is seven or eight orders of magnitude, which is more than either published speed figure. That is the right way round, and the reason is worth stating: printing out is not as slow as the amplification argument alone would make it, because the excess silver nitrate recycles the halogen and lets the photolysis continue past what a pure crystal would allow, so a printing-out paper gets some of that gap back. The course offers this as an argument rather than a measurement — it is an order-of-magnitude comparison between a published amplification factor and a published exposure ratio, not a quantity anyone has measured on one material.

What it costs you in practice. No enlargement, ever: Reilly says the amount of light energy required to enlarge onto a printing-out paper is enormous and impractical, so all prints are made by contact. Sunlight or an ultraviolet lamp, not an enlarger. And exposures in minutes, judged by eye rather than by a timer, which is the one compensation the family offers — Reilly lists it first among the advantages: because the image appears during exposure, the progress can be checked visually and stopped at the right moment.

What the fixer does, which is unlike anything in Part XI

In Part XI the fixer had one job and did it cleanly: thiosulfate complexes the unexposed silver halide into a soluble anion and leaves the developed silver alone.

AgCl + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Cl⁻

The reaction you want, and the only one Part XI had to think about

Here the same bath has two jobs that pull against each other, because the image is made of the kind of silver a fixer can attack. Ware’s numbers are the argument: a whole-plate salt print carries about 3.3 mg of silver in total, roughly a tenth of a modern print, at a coating weight of about 0.1 g/m², in particles of about 10 nm radius — perhaps a hundredth of the size of a modern print’s filaments, so about a hundred times the surface area for the same mass. There is very little silver, and nearly all of it is surface.

Three things happen in the fixer, and only one of them is the reaction above.

The unexposed silver chloride dissolves. That is the intended job, and Reilly’s inventory of what has to go includes not just silver chloride but silver citrate, silver chromate and whatever else the salting and sensitising solutions put there.

The image loses density and shifts colour, and the mechanism is physical rather than chemical. Reilly’s account is the one to have in your head before you fix a salt print for the first time, because otherwise you will think you have ruined it. In the exposed, unfixed print the image silver is highly dispersed, forming what he calls a kind of solid solution of metallic silver in silver chloride. Remove the silver chloride and two things change at once: the particles pack together into aggregates, and the refractive index of the whole system falls, because the high-index silver chloride has gone. Both reduce the covering power of the silver, and both shift the colour towards yellow. The rich purple or brown of the unwashed print becomes a duller, yellower brown.

And some image silver is genuinely lost, particularly if the bath is acid, if it is too strong, or if the print stays in it too long. Reilly gives the reason an alkaline bath is specified for these papers: an acid fixing bath tends to attack the finely divided metallic silver of the image, causing excessive bleaching of the highlights and mid-tones, and that attack is minimised when the pH is kept on the alkaline side. He is equally emphatic about time — fixation is completed fairly rapidly, and prolonged fixation is much more injurious to prints than is generally believed, because thiosulfate that penetrates the paper fibres becomes almost impossible to remove.

What leaves the sheet in the fixing bath

1. 1Unexposed silver chloride, as the soluble thiosulfato complex — the intended job; most of the mass that leaves
2. 2Free silver compounds carried in from the sensitiser — only if the first wash was skipped: exhausts the bath and stains the print
3. 3Part of the image silver itself — nanoparticle, high surface area, and attacked by an acid or over-strong bath
4. 4And, without leaving at all: the refractive index of the layer — the particles pack, covering power falls, the colour shifts yellow

The three arrows are drawn at different weights to show relative mass, not to any measured proportion. What is sourced is which four things happen, from Reilly's chapters three and nine.

This is why the print is exposed past the point where it looks right. Reilly’s starting figures are one and a half stops of overprinting for albumen and two stops for salted papers, with the caveat that experience is the only way to know precisely how much, since it depends on the binder, the negative and the toning bath. Bostick & Sullivan give the same instruction as an appearance: stop when the print is about one half to two thirds as dark as the desired final image, and expect it to be about three quarters as dark on leaving the fixer, with the rest coming back on drying.

The Photographers’ Formulary sheet turns it into something you can actually calibrate, and this is the version the printing lab uses. Print a step table. Immediately after exposure, mark the lightest step that shows any darkening. That step, and usually four more, are lost upon washing, toning and fixing. Knowing exactly how many steps your own process loses is what lets you compensate by exposure.

🧮Five steps, in the units you actually work in

A 21-step transmission wedge has 0.15 density between steps, so

5 × 0.15 = 0.75 log H

Five steps of a 21-step wedge, in log exposure

0.75 ÷ 0.30 = 2.5 stops

The same, in stops

which sits just above Reilly’s two-stop starting figure for salted papers, from a completely independent source and by a completely different method. Two published figures agreeing to half a stop is a reason to expect two to three stops on your own paper, not a licence to skip the measurement: both sources say the amount depends on the binder, the fixer strength and the toning, and the printing lab has you measure your own.

Why the first wash is not optional

Between the frame and the toner there is a wash in plain water, and it is the step beginners skip because nothing appears to be happening. Four things go wrong if it is skipped, and Reilly names all of them.

Toning is retarded or prevented outright. Free silver nitrate in the sheet reacts with the gold in the toning bath instead of the image doing so. Reilly’s Step 1 is unambiguous: if the excess silver nitrate were not removed at this stage it would retard or completely prevent any toning from taking place. Gold is the most expensive consumable in this cluster and the free silver spends it on nothing.

Black stains appear in the fixer. Reilly again: if the excess silver nitrate were still present when the print is fixed, black stains would be the result. Silver nitrate meeting thiosulfate throws down silver sulfide directly, which is a stain rather than an image.

The fixer is exhausted long before its time. Every milligram of free silver carried in has to be complexed by the bath, and the bath’s capacity is spent on it rather than on the unexposed chloride it is there to remove.

And the silver is thrown away. Reilly records the industrial practice: in large-scale printing operations the first wash water was carefully saved, and from it the majority of the silver used to sensitise the paper was recovered. That is the direct consequence of the fact that only 6 to 8 per cent of the applied silver forms the image.

⚗What the milkiness in the first tray actually is

Everybody who makes a salt print notices it: the first tray of wash water goes white and cloudy, and the print lightens and turns redder while it does. Both effects have specific causes and neither is the image washing away.

The cloud is silver chloride, and it is being made in the tray. Reilly explains it in his chapter on sensitising, where he is warning you to use distilled water for the silver bath: tap water carries chloride and hard-water ions such as carbonates, and silver nitrate meeting them throws down a cloudy white precipitate of silver chloride, silver carbonate and the rest. Then he closes the loop — the clouding of the water in the initial wash is that very reaction, between the excess silver nitrate left in the paper and the ions in your tap water. Bostick & Sullivan describe the same thing from the bench as a white, milky cloud floating off the print and tell you to wash until it stops, which is the right end point because it is the end point of the reaction rather than a time.

The reddening is the layer swelling. Reilly attributes it to a physical change: swelling of the image layer alters the spacing of the particles and therefore the colour, which is the same refractive-index and packing argument as everywhere else on this page. Photographers’ Formulary describe the print lightening and becoming reddish during the intermediate wash as the normal appearance.

And there is a limit in the other direction. Photographers’ Formulary warn in one line: do not over-wash, the image can be lost. Their end point is a trayful of water that does not go cloudy.

What is left in a finished salt print that should not be there

Two classes of residue, and the second is the one the nineteenth century died of.

Silver in forms other than the image. A fixer does not remove all the silver from a paper that has been soaked in silver nitrate. The evidence for this is a chain of nineteenth-century experiments that Reilly sets out and that deserves to be read as a piece of investigative chemistry. In December 1859 Davanne and Girard reported that a 2 per cent solution of potassium cyanide removed all traces of silver from albumen prints while strong solutions of hypo did not — and noted the two reasons cyanide could not be used, that it bleached the image and that it was highly poisonous. In 1866 Matthew Carey Lea looked for a solvent for the retained silver and failed to find one. In January 1868 John Spiller reported that the metal was retained in the whites, and in all parts of the coating, as a colourless argentic organic compound unalterable by light and comparatively insoluble in hyposulphites; he had found it by moistening a white area with ammonium sulfide and watching a brown stain appear. Twenty-five years later Haddon and Grundy measured it: an albumen print that had been sensitised, thoroughly fixed and washed but never exposed still contained nearly 5 per cent of the silver left in it after sensitisation, and they demonstrated the point by converting that residual silver back to silver chloride with chlorine water and printing out an image on it nearly as strong as one made the ordinary way.

That work was on albumen, where the silver is bound to sulfur-containing side groups on the egg protein, and the course must not simply transfer the figure. What carries over to a plain salted paper is the principle, not the 5 per cent. Ware’s account of albumen prints turns on exactly that protein chemistry, which a plain salted paper does not have. What a plain salted paper does have is cellulose and whatever sizing the mill put in, and no source read for this course reports a measured figure for residual silver in an unbinder-ed salt print. Say so rather than borrowing the number.

Thiosulfate, and the complexes it makes. Reilly’s statement of why this matters on these papers in particular is the one to remember, because it is the difference between this part and Part XII. In a modern fibre paper a substratum of baryta and gelatin separates the image layer from the paper base. In an albumen or salted paper it does not: the silver image is in much more intimate contact with the paper fibres, so both proper fixation and effective washing are needed to stop the base paper becoming a reservoir of image-threatening substances. And thiosulfate does not wash out of paper easily — Reilly notes that the rate of removal slows tremendously at low concentrations and that it is impossible in practice to remove every trace by water alone, which is why the sequence includes a sulfite washing aid before the final wash.

!An honest gap in the evidence, stated by the standard source

Reilly writes, in the washing chapter of the standard reference work on these processes: experimental evidence on the washing of albumen and salted paper prints is almost nonexistent. His own recommended times are extrapolated from what is known about gelatin prints, on the stated assumption that the factors affecting the rate of washing a gelatin print are probably valid for these papers too.

That was 1980. This course has not found a later study that closes the gap, and it does not have one of its own. So the wash times on the printing lab are published as what a named source recommends, with the basis of the recommendation stated, and the page asks you to test the result with the residual tests rather than to trust the time.

And the failure mode this produces is documented rather than theoretical. The 1855 committee of the Photographic Society of London, appointed to investigate the fading of positive prints, concluded that the most ordinary cause of fading could be traced to the presence of sulfur, “the source of which may be intrinsic from hyposulphite left in the print, or extrinsic from the atmosphere, and in either case the action is much more rapid in the presence of moisture”. Its two practical recommendations were thorough washing after fixing, and gold toning. Those are still the two controls, and the argument for the second is the next lesson.

Reilly also preserves the single best piece of evidence that this is about workmanship rather than about the process. Nicolaas Henneman, who printed The Pencil of Nature at Talbot’s Reading establishment, told a meeting of the Photographic Society of London in May 1856 that of those prints he had made twenty-five in one batch, that they had only three washings, and that some of them remained perfectly good as if printed yesterday while others had totally failed. Same chemistry, same day, same batch, opposite outcomes.

Why the process was displaced

Not by a better silver chemistry. By a binder.

The dates are close together and Reilly gives them: gelatin was tried as a salting-sizing addition about 1850, albumen in 1850, starch in 1854 and whey in the early 1850s, and during the period 1850 to 1855 fewer and fewer prints were made with a plain salt solution alone. By the second half of the 1850s albumen paper had ascended to a dominance of the photographic paper market that it kept for thirty-five years. The Getty Atlas dates the transition the same way and adds the industrial reason: Blanquart-Evrard’s printing establishment near Lille, running from 1851 to 1855, produced up to several hundred prints a day from a single negative — using chemical development of the positive rather than sun printing, which was the other thing that made mass production possible.

Four things drove it, and three of them are on this page already.

Exposure time. Sun printing is slow, weather-dependent and seasonal. Reilly notes that sensitivity falls off below 5 °C, so printing outdoors in winter is practically impossible.

Maximum density and sharpness. The image sinks into the fibres, so a plain salted paper cannot reach a deep black and cannot resolve fine detail. Reilly describes the early history of printing papers as a steady evolution of techniques for producing papers capable of greater resolution and contrast, driven by the desire to reproduce the detail that waxed-paper and glass negatives could record. Once the wet collodion negative arrived, a printing paper that threw that detail away was a bottleneck.

Surface, and taste. The 1850s and 1860s wanted gloss, and wanted it badly: the carte de visite portrait might put a head half an inch high on the print, so maximum detail on a smooth surface was commercially necessary. Reilly records the reaction against gloss beginning in the 1880s and the modest revival of matte salted and arrowroot papers that followed it — which is how a process that died in 1855 came to have a second life in 1895.

And permanence, or the fear of it. Reilly’s chapter on the question opens on the late 1840s, when the fading and staining of paper prints “threatened to discredit photography on paper altogether, and reduce it to the status of a scientific curiosity”. The daguerreotype held commercial portraiture partly for that reason. Albumen did not solve the problem — it brought its own, and the highlight yellowing peculiar to it is Part XXIII’s subject — but it arrived with gold toning and separate toning and fixing already established, which the salted paper years had worked out the hard way.

What decides contrast, previewed for the labs

There is no paper grade and no contrast filter. Five things move contrast in this process, and the labs work with the first four.

The chloride salt and its concentration. Reilly: the chloride content of the salting solution has an effect on contrast, and lowering it tends to produce prints that lack brilliance and density. His working range for most salted papers is a chloride content around 2 to 2.5 per cent with the silver bath at 10 to 12 per cent, and he traces the historical drift to lower chloride in albumen paper — down to 1 to 1.5 per cent in the period 1880 to 1900 — as an attempt to accommodate the shorter density ranges that gelatin dry plates produced. Lower chloride, thinner negatives printable, weaker image.

Citrate, and the other active additions. Reilly’s plain formula has a citrate variant, and his description of what it does is empirical and exact: the addition of a neutral citrate will cause the prints to be more reddish in colour and slightly more brilliant. The why is the gap this page has already admitted.

The binder. More binder holds the silver nearer the surface, which raises maximum density and therefore contrast, which is the whole argument of the variants lesson and the reason a glossy albumen print needs a shorter-scale negative than a plain salted one.

The light source. Reilly: a very intense source lowers print contrast, while a weaker light raises it slightly — hence the old rule that dense vigorous negatives are printed in sunlight and thin weak ones in the shade, where “shade” means facing open sky rather than in direct sun. The colour of the light matters too: more blue and less yellow gives a softer, flatter print, more yellow and less blue a more contrasty one at the cost of a longer exposure.

And the negative, which is the one that actually decides the picture.

☠The fifth lever, which this course does not use

Reilly is straightforward that large increases in contrast are possible only with chromates, that the technique does allow otherwise unprintable negatives to give good prints, and that the discovery was Ferdinand Hrdliczka’s in the mid-1890s. He is equally clear that it has no place in regular practice, that the stability and predictability of the papers decline as the bichromate increases, and that the finest results are obtained without it — his judgement, from the accumulated experience of many writers, is that the best salted paper prints are always made in sunlight with optimum negatives and no contrast-enhancing addition at all.

Both bought kits ship a dichromate solution for this purpose. This course does not use it and does not print the quantities, because potassium dichromate is a chromium(VI) compound and the chromium ruling forbids chromium(VI) at any level in any quantity. The ruling is not a judgement about salted paper; it is the same ruling that governs gum bichromate and the dichromated colloids of Part XXVI. What the course does instead is the four levers above, and Reilly’s own preference happens to be the same thing.

Recap

The two coatings are forced by solubility. Silver chloride cannot be brushed on, so it is made in place: chloride first and dry, silver nitrate second and in large excess. Reverse the order and you have made a stabilised paper rather than a sensitised one.

The excess silver is the mechanism. Talbot found about six times more nitrate than chloride was needed. Vogel explained it as the liberated chlorine returning as fresh silver chloride; Ware refines that to a disproportionation with water in which half the chlorine returns as silver chloride and the other half becomes hypochlorous acid and a proton — which is why the sheet becomes acid as it prints and why the image can go backwards.

Nothing amplifies. Reilly’s 100,000 times and Ware’s million times bracket the same fact from two directions, and the consequences are absolute: contact printing only, minutes of ultraviolet, and a judgement made by looking.

The image is coloured because it is small. Photolytic silver is 10 to 100 nm, so it has a plasmon resonance in the visible, and its colour moves with particle size, shape, aggregation and the refractive index of what surrounds it — all four of which change during processing.

The fixer is a threat as well as a necessity. It dissolves the unexposed chloride, and in doing so it drops the refractive index and packs the particles, so the print loses density and yellows before any silver has left. Overprint about two stops, or five steps of a 21-step wedge, and measure your own figure.

The first wash pays for itself four times over: it protects the toner, prevents black stains in the fixer, saves the fixer’s capacity, and is where the recoverable silver is.

And the residue is what killed the process’s reputation. Silver in forms other than the image and thiosulfate in the fibres, in a paper that has no baryta layer between the two — which the 1855 committee diagnosed as sulfur, from the print or from the air, and answered with thorough washing and gold toning.

Check your understanding

Question 1. Why is the paper salted first and sensitised second, and what happens if a student does it the other way round?

• Silver nitrate is more expensive, so it is applied last to waste less of it
• Silver chloride is insoluble and cannot be brushed on, so it must be formed in place; and applying silver first leaves the chloride in excess, which gives a stabilised paper rather than a sensitised one
• The order is arbitrary; either sequence gives the same sensitised sheet
• Salting first is a nineteenth-century convention retained for historical fidelity

Show the answer and why

Answer: Silver chloride is insoluble and cannot be brushed on, so it must be formed in place; and applying silver first leaves the chloride in excess, which gives a stabilised paper rather than a sensitised one

Reilly gives the first half: silver chloride is insoluble in water and in most solvents, so it cannot simply be brushed on and must be formed in place by treating the paper with a soluble chloride and then with silver nitrate. Ware gives the second half. A silver halide crystal with excess silver ions adsorbed on its surface attracts photoelectrons to the surface, where silver can grow free of the lattice; one with excess halide adsorbed repels them and its liberated halogen meets no silver ions to scavenge it, so print-out silver cannot survive at the surface. Talbot reached the same conclusion empirically: his stabilising treatment for a photogenic drawing was a strong salt solution.

Question 2. Two sheets are salted with the same amount of chloride. One salting solution contains gelatin and one contains no organic material at all. What differs in the finished prints?

• Nothing measurable: the gelatin is a coating aid and takes no part in the image
• The gelatin sheet gives a higher maximum density and a less matte surface, and the two prints are different colours; the sheet with no organic material tends to grey and flat
• The plain sheet is faster, because there is nothing between the light and the silver chloride
• The gelatin sheet is more contrasty only because gelatin absorbs ultraviolet

Show the answer and why

Answer: The gelatin sheet gives a higher maximum density and a less matte surface, and the two prints are different colours; the sheet with no organic material tends to grey and flat

Two effects, and it is worth keeping them apart. Physically, the gelatin slows the sinking-in, so the image sits nearer the surface: Reilly says that where the sensitive layer penetrates deeply the maximum density falls and the surface is matte because reflected light is scattered by the fibres. Chemically, gelatin is one of the substances Reilly classes as "active", which facilitate more complete reduction of the silver chloride, and he states flatly that pure silver chloride paper gives grey and flat images. The colour differs as well, because the refractive index of the medium the nanoparticles sit in is one of the things that sets a plasmon absorption maximum.

Question 3. Give two reasons a salted paper print needs a negative of much greater density range than an enlarging paper does.

• Self-masking: silver built up in the shadows shields them from further exposure, so the scale keeps opening out instead of blocking up
• The matte, sunken-in image scatters reflected light, so more silver is needed for a given apparent density and exposures run longer, which would print the highlights too dark unless the negative holds them back
• The paper is much slower, and a slower paper always needs a longer-scale negative
• There is no contrast grade and no filter, so the negative is the only variable that can be matched to the paper

Show the answer and why

Answer: Self-masking: silver built up in the shadows shields them from further exposure, so the scale keeps opening out instead of blocking up, The matte, sunken-in image scatters reflected light, so more silver is needed for a given apparent density and exposures run longer, which would print the highlights too dark unless the negative holds them back, There is no contrast grade and no filter, so the negative is the only variable that can be matched to the paper

The first two are the mechanism and the third is the consequence. Reilly describes the shadow density as behaving like extra density in the negative, delaying maximum density and preserving the light tones, and reports Hubl finding that salted paper exceeds even platinum paper in scale length. He gives the second reason directly: a matte surface needs relatively more reduced silver to produce a deep black, which means longer exposures, which means the negative must hold enough density in the highlights to keep them from going dark. Speed by itself is not one of the reasons - a slow material is not automatically a long-scale one.

Question 4. A print comes out of the fixer noticeably lighter and yellower than it went in, although the bath was fresh, neutral and used for the stated time. What has happened?

• Image silver has dissolved in the thiosulfate; the bath was too strong
• The silver chloride has been removed, so the refractive index of the layer has fallen and the silver particles have packed together, and both reduce covering power and shift the colour towards yellow
• The print was under-exposed and the fault is in the printing rather than the fixing
• Residual silver nitrate has been converted to silver sulfide

Show the answer and why

Answer: The silver chloride has been removed, so the refractive index of the layer has fallen and the silver particles have packed together, and both reduce covering power and shift the colour towards yellow

This is the normal, expected change and Reilly explains it as a physical one. In the unfixed print the image silver is dispersed almost as a solid solution in silver chloride; removing the chloride lets the particles pack into aggregates and lowers the refractive index of the whole system, and both effects reduce the covering power of a fixed mass of silver. That is why prints are overprinted - Reilly gives about two stops for salted papers - and why the Photographers Formulary sheet tells you to expect to lose the lightest step that showed darkening plus about four more.

Question 5. From the sensitiser strength and the coverage a supplier publishes, calculate the mass of silver nitrate on one 8 by 10 inch sheet, and say what fraction of it ends up in the image.

• About 0.24 g, of which Reilly puts 6 to 8 per cent in the image and Ware, from analyses of nineteenth-century prints, about 2 per cent
• About 12 g, of which nearly all forms the image
• About 0.24 g, of which about half forms the image and half is fixed away
• It cannot be calculated, because coverage depends on the paper

Show the answer and why

Answer: About 0.24 g, of which Reilly puts 6 to 8 per cent in the image and Ware, from analyses of nineteenth-century prints, about 2 per cent

Bostick and Sullivan state that 100 mL of their 12 per cent silver nitrate makes approximately fifty 8 by 10 inch prints, so one sheet takes about 2 mL, and 2 mL of a 120 g/L solution carries about 0.24 g of silver nitrate. Reilly reports that only about 6 to 8 per cent of the silver initially present in a sensitised sheet is used to form the image, the rest being washed or fixed away; Ware, from chemical analyses of salt prints made in 1855, puts the retained fraction at about 2 per cent, with a coating weight of about 0.1 g per square metre. The two figures are not measuring the same thing and this course reports both. The practical conclusion is the same either way: the wash water and the fixer carry more than nine tenths of the silver, which is why the first wash is saved.

Question 6. Ware and Reilly give different accounts of what accepts the chlorine released by photolysis. What does Ware add, and what does he rule out?

• He adds that cellulose is the acceptor and rules out the excess silver nitrate
• He proposes that water and silver(I) ions together are the major acceptor, half the chlorine returning as fresh silver chloride and half becoming hypochlorous acid and a proton; and he rules out gelatin, which he says is not an effective halogen scavenger at print-out exposures
• He adds that no acceptor is needed if the exposure is long enough
• He proposes that the paper sizing is the acceptor and rules out water

Show the answer and why

Answer: He proposes that water and silver(I) ions together are the major acceptor, half the chlorine returning as fresh silver chloride and half becoming hypochlorous acid and a proton; and he rules out gelatin, which he says is not an effective halogen scavenger at print-out exposures

The refinement matters because it explains things Vogel's simpler account does not: the sheet becoming acid as it prints, and the back-reactions in which hypochlorous acid and nitrate can both oxidise the image silver again. Ware notes that the photolysis, the halogen acceptance and the hypochlorous attack sum to zero overall, so whether you get a picture depends on relative rates rather than on equilibria - which is his explanation for why a dull light cannot be compensated by a longer exposure. He marks the water-plus-silver proposal as a proposal, and this course marks it the same way.

Question 7. Why does a printed-out silver image have a colour at all, when a developed silver gelatin image is neutral black?

• Because printed-out silver is chemically different: it is a silver-organic compound rather than the metal
• Because the particles are 10 to 100 nm, small enough to show a surface plasmon resonance in the visible, whose wavelength depends on particle size, shape, aggregation and the refractive index of the surrounding medium
• Because residual silver chloride is yellow and tints the print
• Because the paper fibres scatter the longer wavelengths preferentially

Show the answer and why

Answer: Because the particles are 10 to 100 nm, small enough to show a surface plasmon resonance in the visible, whose wavelength depends on particle size, shape, aggregation and the refractive index of the surrounding medium

Reilly states the practical version - the aggregate particles are of colloidal size, not large enough to absorb all wavelengths and appear black, and the colour depends in part on the refractive index of the medium they are dispersed in. Ware supplies the physics: nanoparticle copper, silver and gold are the only metals whose plasmon resonances fall in the visible, which is why they are the only three useful for coloured images, and he gives Wiegel's table of transmitted colour against diameter running from yellow at 10 to 20 nm through red and purple to blue above 70 nm. Every wet-processing step moves at least one of the four variables, which is why the print changes colour four times between the frame and the mount.

Check my answersClear