Silver Mirror Toning on Jorden Senior

037 · The Metal Trees

Sat, 01 Aug 2026 00:00:00 +0000

The Metal Trees

Part 4 of 4 in the Silver Mirror Toning series | ← Previous: Part 3

There are eighteen of them at Gardens by the Bay — steel armatures between twenty-five and fifty metres high, planted with bromeliads and ferns and flowering climbers, fitted with photovoltaic cells and rainwater collection. They are called Supertrees. They are not trees. They are infrastructure wearing a hundred and sixty thousand plants, and from a distance the illusion holds completely; up close it collapses into welded steel and irrigation, and then, if you keep looking, reassembles.

Two of the eighteen, against a sky that is no longer behaving like a sky.

I photographed them with the process already in mind. The workshop at Mörk was only weeks before the trip, and I went to Singapore knowing I had a technique that puts a reflective metal surface on top of a photograph and no idea yet what it was for. The Supertrees answered that on sight: reflective structures that spend all day imitating something organic, photographed by someone carrying a process that makes photographs behave like metal. This is the rare case where I knew what the prints would be before I had the negatives.

The Sky Went Metal

Everything here is direct toning — the sequence in 034 that mirrors the highlights rather than the shadows — on Ilford Satin RC, straight down Lexy Xiao's recipe with no modifications. This is the one part of the project where the chemistry was settled and the pictures could be the point.

Direct toning is the obvious choice for this subject, and the consequence is one I went in wanting. Mirroring the highlights of a photograph taken outdoors in Singapore means mirroring the sky. So the sky is metal. The bright faces of the structures are metal. The blown areas where light came through the canopy are metal. The only parts of the print that behave like a normal photograph are the shadows.

What I could not know in advance was how far it would go. Anticipating a metallic sky and standing in front of one are different experiences, and the prints are considerably more disorientating than the plan for them was.

The extreme case. Almost the entire frame above the treeline is deposit, and the streaking is the toning rather than anything that was in the negative.

This inverts something basic about how these pictures work. Photographed conventionally, the Supertrees are dark silhouettes against a bright tropical sky — the structure is the subject and the sky is the ground it sits on. Mirrored, the sky becomes the most physically active part of the print, shifting and flaring as you move, while the trees sit still and dark inside it. The subject stops being the thing the picture is of and becomes the only stable place to rest your eye.

I would not have predicted liking that. It works because it is doing the same thing the gardens do. The Supertrees are a reflective armature performing an impression of vegetation; the prints are a reflective armature performing an impression of a photograph. Both only convince from certain angles.

There is a practical consequence to this that took me a while to name. A conventional print holds still while you look at it, and you can therefore compose it as a fixed arrangement of tones. These do not hold still. Move your head thirty centimetres and the tonal relationships in the upper half of the picture invert — sky that read as bright goes dark, or the reverse, depending on where the light is coming from. The composition is not a property of the print; it is a property of the print, the light, and where you happen to be standing. What you can control at the enlarger is the range of pictures a print will produce, not which one a viewer gets.

The other end of the range. With a trunk, a hotel and a walkway full of people occupying most of the frame, there is far less for the toning to act on — and the print behaves accordingly.

Ilford Satin turned out to matter more than I expected. Satin already sits between matte and gloss, so an unmirrored area has a low, even sheen and a mirrored one reads as a genuine departure from it. On a full gloss paper I suspect the effect would be muddier — the base surface is already doing some of what the toning does, and the two would compete rather than separate.

As a Series

The frames do not all take it equally, and the reason is arithmetic rather than aesthetic. Direct toning acts on the highlights, so the proportion of a print that becomes reflective is roughly the proportion of the frame that was bright — which here means the proportion that was sky. A wide shot of the grove against open sky is mostly mirror. A tighter frame looking up into the planting, where the structure fills the picture and the sky survives only as gaps between the fronds, is mostly a normal photograph with a scatter of bright interruptions.

Sky-dominant: nearly the whole sheet is deposit.

Structure-dominant: the sky is reduced to the spaces between the struts.

Both are worth having, but they are not the same kind of object, and hanging them together needs some thought. The sky-heavy prints are almost entirely surface; they change completely as you cross the room, and they dominate whatever is next to them. The structure-heavy prints hold still and reward standing closer. Sequenced badly, the first kind eats the second.

What I have not resolved is whether the set wants to be consistent or wants that range. There is a version of this series that picks one density of sky and repeats it, which would give a coherent wall of objects that all behave the same way. There is another that runs deliberately from almost-all-mirror to almost-none, and lets the sequence itself be the subject — the same process shown at every dilution. The second is more interesting and much harder to hang.

Showing Them At All

There is a practical problem sitting underneath all of this, and I do not have it solved.

Almost every distribution channel a photographer uses assumes a flat image. Instagram, a website, a PDF portfolio, the JPEG upload field on an open call — all of them want one file that looks the same to everybody. For these prints, you cannot reproduce these on a digital printer, because the whole effect is a metal deposit rather than a colour that could be matched.

None of which is new. The daguerreotype has had exactly this problem since 1839 — a mirrored plate that flips between positive and negative as you tilt it, and whose every reproduction is a compromise somebody made on your behalf. Institutions never solved that either. What they arrived at instead was a convention: photograph the object at several stated angles, present them together as a set, and say in the caption that the object behaves this way. That is a workable answer, and it reads as accurate description rather than apology. It is roughly what I have done in this post.

The day I started writing this I was at Filmverkstaden in Vaasa and saw a set of Lippmann plates made there in a workshop led by Armands Andže, which put my problem in perspective by being a worse version of it.

Lippmann's process, which won him the Nobel Prize in Physics in 1908, records colour as interference. Light reflects off a mercury mirror in contact with an ultra-fine-grain emulsion, forming standing waves; the emulsion records those waves as layers of silver through its depth; and when you illuminate the finished plate, white light reflecting off those layers interferes constructively at exactly the wavelengths that made them. The colour comes back out. There are no dyes in a Lippmann plate at all.

Which makes it the same family as everything else in this series — structural colour in silver, the 015 argument taken to its logical end. And it is far harder to reproduce than what I am doing. A flat photograph of a mirrored print loses the sheen but keeps the picture. A flat photograph of a Lippmann plate loses the colour, because the colour was never a property of the material — it was a property of the light coming back out of it at a particular angle. Photograph it wrong and you get a grey plate.

The more interesting option is one I have not tried yet. Reflectance Transformation Imaging is a documentation method in which the camera stays fixed and the light is moved to many known positions; software then solves a reflectance function for every pixel, and the output is an interactive viewer in which the reader drags a virtual light across the surface and watches it respond. Not a video of me tilting a print — control handed over.

RTI was developed for cultural heritage documentation, which means the community that built the best available tool for recording angle-dependent surfaces is the same community that classifies silver mirroring as deterioration. They needed to document precisely the phenomenon I am inducing on purpose. The toolchain is open — Relight and RelightLab from CNR-ISTI for processing, OpenLIME as a browser viewer — and capture no longer requires a lighting dome; highlight-based acquisition with a handheld flash and a reflective sphere in the frame is the standard low-budget method, and there is published work on doing it with a phone.

It is worth being precise about what RTI would and would not fix, because the Lippmann plates mark the boundary. RTI holds the camera still and moves the light, so it records how a surface responds to illumination direction. That is most of my problem: the mirroring on these prints depends overwhelmingly on where the light is. It is only part of the Lippmann problem, where the viewing angle matters as much as the lighting angle, and capturing that properly means measuring both — a bidirectional reflectance measurement rather than a one-sided one, which is a considerably larger undertaking. So RTI is probably sufficient for what I make and probably not sufficient for what I saw in Vaasa. Knowing which of those you have is the useful first question.

For open calls in the meantime: most now accept moving-image files, and where they do not, submitting stills together with a plain statement that the work is angle-dependent is ordinary practice for object-based work rather than special pleading.

I should be careful not to overclaim here, though. Resisting reproduction is not a distinction — it is true of every glazed ceramic, every piece of iridescent glass, every daguerreotype, Lippmann plate, what have you. What is worth saying is narrower: the resistance is the same physics the work is made of. A print whose appearance is a report on its surroundings cannot be lifted out of its surroundings and still be the thing.

Closing

The thing that convinced me was watching someone else hold one.

I had been looking at these prints alone, in a flat, under a single lamp, moving my own head to find the angle — which means I had been seeing them the way you see something you made. Then I handed one to somebody who had not seen it before, and watched them do what everybody does: tilt it, catch the light, tilt it back, and then keep going, working the print around in their hands to find where the sky came alive.

What that behaviour shows is not that the picture only exists in motion — the print is perfectly real sitting on the table, and the image is right there in the silver. It is that a reflective surface has no appearance of its own. A matte print carries its tones intrinsically; you can put it in any reasonably lit room and it will look like itself. A mirrored one is always reporting on something else. It needs a light source to be somewhere in particular, and a viewer to be somewhere in particular, and it will give you a different answer for every combination of the two.

Perhaps I need to share my prints by video from now on.

That is not a mystical property. It is what shininess is, and it is the same reason the Supertrees look different at midday than they do lit up after dark. But it does mean I have made an object that cannot be fully specified at the enlarger — the print sets the range, and the room decides the rest. The loop above is the closest this page can come to it, and it is not close.

036 · How Far the Recipe Travels: Silver Mirroring Beyond RC

Fri, 31 Jul 2026 00:00:00 +0000

How Far the Recipe Travels: Silver Mirroring Beyond RC

Part 3 of 4 in the Silver Mirror Toning series | ← Previous: Part 2 | Next: Part 4 →

Lexy Xiao developed her silver mirroring recipe mostly on black and white sheet film and resin-coated paper — Ilford Pearl, largely. That is a sensible place to develop anything. RC is cheap, fast, dimensionally stable, and forgiving of the kind of repetition you need when you are working out a process from scratch rather than following one.

It is also not what I print on. My practice runs mostly to fibre, to lith, and increasingly to colour, and so I left the workshop at Mörk with a fairly narrow question: would the recipe work on my papers?

What follows is a working document rather than a set of conclusions — several substrates, one recipe, and two results interesting enough that I still cannot fully account for them. The chemistry and the two sequences are in 034; I will not restate them here.

Fibre: FomaBrom 111

The obvious first test, and the least dramatic to write about, which is exactly why it matters.

I had half expected fibre to behave differently. A fibre print has a thicker emulsion sitting on a baryta layer sitting on paper that actively absorbs the bath, where an RC print has a thin emulsion between two plastic layers that absorb nothing. If the mirroring depends on what the emulsion can hold and how quickly the bath clears out of it, those are not obviously equivalent situations.

Direct toning on FomaBrom 111. The sky carries the deposit; the statue and the foliage stay as ordinary silver.

In practice, both directions worked, and worked well. Direct toning gave clean mirrored highlights against normal blacks. Indirect gave the inversion — shadows that go bright and metallic at a glancing angle while the highlights stay paper white.

The one adjustment fibre needed was time. Where RC mirrors quickly, 111 wanted a few minutes in the bath, which is roughly what you would expect from a thicker emulsion on an absorbent base — more material for the chemistry to work through, and more of the bath disappearing into the paper instead of staying where it is useful.

The RC comparison — Ilford Pearl, the paper the recipe was developed on. Cooler, more even, and the deposit sits visibly on top of the surface rather than in it.

What does differ is the surface underneath. On RC the mirroring sits on a manufactured, uniform plastic sheen and reads as a coating. On fibre it sits on baryta, and reads more like something the paper is doing than something applied to it. That is an aesthetic judgement rather than a measurement, and you may well disagree, but it is why most of what I have made since has been on 111.

Lith: Two Papers, Two Answers

This is the most interesting failure in the set, and the one I would like to understand a bit better.

Lith prints are an obvious target for indirect toning specifically. Direct toning would compete with the print's own highlights, but indirect works on the shadows and leaves the highlights largely alone — and the highlights are where a lith print keeps its colour. In principle you get metallic shadows with all the warmth and grain of the lith intact above them. In principle.

Indirect toning over a lith print on Retrobrom 152. The lith colour survives intact; the mirroring is in the stonework and the shadow mass of the Milan cathedral, and only shows at an angle.

On Retrobrom 152, that is precisely what happened. Shiny in the shadows, and the lith colour stays. I would not claim it as the best print of the project, but the two effects genuinely coexist rather than one flattening the other, which was the open question going in.

On FomaTone 132, nothing. Not a weak effect, not an uneven one. No visible change at all.

Same bath, same temperature, same session, same sequence. I have four candidate explanations and no way to choose between them yet:

Grain morphology. Lith's infectious development produces large, irregular, heavily aggregated silver particles rather than the fine even grain of normal development (see 030 for why). Perhaps the copper chloride bleach simply does not rehalogenate that structure as readily, and on the 132 print there was nothing for the toner to work with.
Emulsion chemistry. FomaTone is a warm-tone paper with a different halide balance. Halide composition affects both bleach behaviour and how readily silver re-precipitates.
Density. The 132 print may have been denser than the bleach could penetrate in the time I gave it. This is the boring explanation and therefore probably deserves testing first.
Surface. The two papers do not have the same surface, and a deposit that is invisible on one might be obvious on another. It is possible something happened and I could not see it.

The honest answer is that I ran a couple of prints on each, and I need to do more.

RA-4: There Is Enough Silver After All

The received wisdom about RA-4 colour paper is that there is not enough silver in it to do silver-based process work. The silver in a chromogenic print is a means to an end — it develops, it triggers dye formation, and then the bleach-fix removes it entirely, leaving an image made of dye. Whatever is left is not supposed to be enough to build anything on.

I have had reason to doubt this before. The RA-4 mordançage work in 012 and 013 ran on the same suspicion — that there is more going on in these materials than the datasheet implies — and it paid off there. So RA-4 went into the direct toning sequence.

It works. Not marginally: it produces the strangest and most specific results of anything I tried.

Because I printed the same black and white negative — a wall of stamped clay tokens near Chinatown in Melbourne — on black and white paper in the same session, the comparison is unusually direct. Three prints, one negative, three chemistries.

The control: Ilford Satin RC, direct toning. Ordinary monochrome with mirrored highlights.

RA-4 paper, developed in black and white chemistry, then direct toned. Pale blue image field, gold border, and a thin white line holding them apart.

RA-4 paper developed in RA-4 chemistry, then direct toned. The dye couplers still fire (ignore my poor filtering), and you still get the pronounced gold border.

I ran two variants. Developed in black and white chemistry first, the print comes out pale blue wherever the image was exposed, with a band of gold along the borders where the paper was not.

The interface between the two is the thing I keep going back to. I had assumed it would be a soft transition, and it is not — the boundary is surprisingly hard, and there is a thin white line separating the gold from the blue, as though a third thing is happening in the millimetre between them. My guess is a gradient: some concentration or potential falling off across that edge, passing through a narrow band of conditions that produce neither deposit. That is a guess made while looking at a print, not a measurement, but it is testable — masking the paper to make deliberate edges and seeing whether the white line tracks them would settle it quickly.

Developed in RA-4 chemistry first, the dye couplers still fire. A colour image forms - ignore my poor filtering. The warm border survives around it exactly like the black and white version.

Two caveats. I do not know how stable any of this is, and given that the whole process is a controlled version of a degradation mechanism, that question is not academic. And I have made very few of these. Both variants want a proper session, and if the gold border turns out to be as controllable as it looks, this becomes its own post rather than a section of this one. Suffice to say, I really like this effect, and need to understand that gradient better.

Stacking It With Mordançage

The last thing to try was combining silver mirroring with mordançage, which is arguably my most used process.

Direct silver mirroring first: poor. The mordançage that followed had no middle register. The veil either refused to lift at all or came away wholesale, taking the image with it, with very little in between. My working explanation is that mordançage depends on a fairly specific amount of developed silver being available for the copper chloride to attack, and silver mirroring has already spent that budget — either locking silver into a deposit the bleach cannot reach, or loosening the emulsion so thoroughly that it releases in one piece. Either way, there was too much silver in play and not enough control over it.

Mordançage first, indirect silver mirror toning later: excellent. The mordançaged shadows came up gold, which on its own would have made the session worthwhile. But the paper base also turned a distinct purple.

Mordançage first, then indirect toning, on Ilford Pearl. Gold in the mordançaged shadows, bleached veiling along the roofline — and a sky that has gone lilac for reasons I cannot account for.

I cannot account for the purple. It has the look of a lumen print, but this print was fixed, and as I recall, fixed correctly. The image was prepared for my Beneath the Surface show a year ago and has been stable since, so something is being deposited in or on the base rather than in the image. The obvious suspects are silver liberated during the mordançage stage and re-precipitated by the toning bath somewhere it has no business being, or halide that survived the earlier processing in a form I am not expecting. Both are guesses. A print that has been through mordançage has had a lot done to it, and the number of things that could be sitting in that paper is not small.

I love the effect and I do not yet know whether it repeats — I will definitely be trying more of this.

What I Haven't Tried

Everything above is tray work: the whole sheet goes in, the whole sheet gets whatever the bath decides to give it. That is one way to use this chemistry and not the obvious one, given that I already work the other way elsewhere.

In 016 I apply chromoskedasic chemistry with a brush — stabiliser across the whole surface, then activator selectively, then developer into chosen areas to trigger silvering exactly where I want it. That is the same family of chemistry as this bath, applied by hand instead of by immersion. There is no reason it would not transfer. Brush-applied silver mirroring would mean choosing which highlights mirror and which stay paper, rather than accepting the distribution the negative happens to provide — and on the evidence of 016 it would also mean streaking, drips and over-silvered patches to control.

Two other things suggest themselves and remain untested. Masking, which came up in the RA-4 section as a way to find out what the white line at the border is doing, would double as a compositional tool if the boundary really is that hard. And partial immersion — dipping rather than floating, or drawing a sheet slowly out of the bath — would produce a gradient of coverage across a print, which if the percolation argument holds should also be a gradient from mirror through mixed to colour on a single sheet.

There is a practical argument for the brush as well as an aesthetic one. Tray work commits a full volume of thiocyanate and silver-bearing solution per session; brush work uses a fraction of it.

What Travelled and What Didn't

The recipe is more portable than expected: it worked on neutral fibre with nothing more than a longer time in the bath, on one lith-compatible paper, on RA-4 in two different developer chains, and stacked on top of mordançage. It is tuned to silver, not to a paper — which in retrospect is what you would expect from a process that is, at bottom, a controlled version of something that happens to every silver print given a few decades and bad storage.

Next: 037 · The Metal Trees — the process settled, and pointed at something.

035 · Why the Silver Comes to the Surface

Thu, 30 Jul 2026 00:00:00 +0000

Why the Silver Comes to the Surface

Part 2 of 4 in the Silver Mirror Toning series | ← Previous: Part 1 | Next: Part 3 →

In 015 I worked through why silver particles of a particular size look like a particular colour — Mie scattering, localised surface plasmon resonance, the whole business of structural colour from a material that contains no dye. That post answers one question well, and I am not going to repeat it here.

It does not answer the question silver mirroring actually raises. A normal print already contains silver particles; it does not shine. The mirroring sits on top of the emulsion rather than in it, and it is reflective rather than coloured. So: why does the silver move, why does it stop where it stops, and what has to happen for a scattering surface to become a reflecting one?

Those are three different physics problems — transport, nucleation, and film continuity — and the process in 034 is a way of driving all three at once.

Two Ways to Make Silver

Almost everything a printer does in a darkroom is chemical development. The developer finds an exposed silver halide crystal, reduces it in situ, and the metallic silver grows where the crystal already was. The developing agent supplies electrons; the silver comes from the grain itself. The result is the familiar filamentary, tangled silver of a black and white print — high surface area, tuned by a century of emulsion engineering to absorb light efficiently. That is precisely why a print looks black rather than shiny: the silver is shaped to trap light, not bounce it.

Physical development is the other route, and it is the one that matters here. The silver is dissolved into solution first, moves as a soluble complex, and is then reduced onto whatever nuclei it encounters. The silver that ends up in the image did not necessarily start in that spot. Because it is deposited from solution onto a growing surface rather than reduced inside a crystal lattice, physically developed silver tends towards compact, rounded, smooth particles instead of filaments.

That single difference — grown in place versus deposited from solution — accounts for most of what makes these prints strange. Compact particles at high coverage can behave like a metal film. Filaments never do.

What the Thiocyanate Is Doing

The toning bath in 034 is three things at once: a solvent, an activator, and a reducer. Take them in that order, because that is the order the silver experiences them.

Sodium thiocyanate is a silver halide solvent. Silver halides are effectively insoluble in water, but thiocyanate binds silver ions into soluble complexes — the same class of chemistry as the thiosulfate in your fixer, which is why fixer removes halide at all. The difference in intent is everything: fixer complexes silver in order to carry it out of the print and down the drain. Here, the complexed silver is meant to stay, become mobile, and be put back down somewhere new. The bath is a fixer that has been told not to finish the job.

Sodium hydroxide is the activator. Developing agents are far more aggressive at high pH, and a strongly alkaline bath drives the reduction fast. Speed matters for morphology, not just for convenience: a slow reduction onto few nuclei grows a small number of large particles, while a fast one throws down many nuclei at once. Fast and coarse is the goal, which is the opposite of what you want from a print developer.

The developer supplies the electrons. Nothing exotic — the same reducing agents doing their usual job, on silver that arrived by a different route.

So: mobilise, transport, reduce.

Why the Surface

Mobile silver in a gelatin layer is not going anywhere in particular. What gives the process direction is where the silver gets reduced, and reduction requires a nucleus — a site where the first few atoms can assemble. Homogeneous nucleation in the bulk of a solution is expensive; heterogeneous nucleation on an existing surface is much cheaper. Silver comes out of solution preferentially at interfaces.

The gelatin–solution boundary is the most available interface in the system, and it also sits at the steep end of every gradient that matters: it is where fresh bath keeps arriving, where the alkalinity is highest, and where a complexed silver ion has the shortest distance to travel. Silver that is reduced deep in the emulsion is buried and optically irrelevant. Silver reduced at the top accumulates on a surface that is already partly covered, which makes further deposition there easier still. The process is self-reinforcing, which is why the deposit is a distinct layer rather than a general fogging.

The Accidental Version

Here is the part that made me want to write this post at all: conservation science has already studied this process in detail, because it is one of the principal ways photographs die.

Natural silver mirroring — the bluish sheen on the dense areas of old prints — proceeds by exactly the three steps above. Oxidants in the air, helped by humidity and by residual thiosulfate left behind by poor rinsing, oxidise image silver to silver ions. Those ions are mobile in the gelatin and migrate. On reaching the surface they are reduced back to metallic silver, depositing as colloidal particles, along with some silver sulfide.¹ Mobilise, transport, reduce.

The mechanism has been characterised directly. Moon and Curran, working with artificially aged silver gelatine prints, used transmission electron microscopy to distinguish two deterioration pathways that look different to the eye: well-processed prints that yellowed via colloidal silver formation, and insufficiently washed prints that went redder and darker through more homogeneous silver sulfide filaments.² Different particle morphology, different appearance — which is the same principle as 015, arrived at from the other direction by people trying to stop it happening.

The toning bath is not analogous to this. It is this, run deliberately. Instead of atmospheric oxidants you use a bleach or the print's own undeveloped halide; instead of residual thiosulfate you use thiocyanate on purpose; instead of ambient reduction over decades you use a developer at 35 °C. The decades collapse into minutes because every accidental reagent has been replaced by a chosen one at working concentration.

I find that genuinely satisfying as a piece of practice. Most alternative processes are described as historical revivals. This one is a degradation mechanism with the sign flipped. Intentional impermanence.

From Colour to Mirror

The last question is the one 015 sets up but does not close: when does a collection of scattering particles stop being coloured and start being a mirror?

The answer is coverage, and the physics is a percolation problem. Thin metal films grown by deposition follow a well-studied sequence: discrete islands nucleate, the islands grow and begin to touch, coalescence produces a connected but ragged network, and eventually the film becomes continuous. Optically, that sequence has a threshold in it. Below it, each island supports its own localised surface plasmon resonance and you get selective scattering and absorption — colour, in other words, the regime 015 describes. Above it, the plasmon modes delocalise across the connected network and the film starts behaving like bulk metal, which means specular reflection.³ It is a dielectric-to-metal transition, and it can be modelled with effective-medium approaches such as Maxwell–Garnett.

For sputtered silver, the percolation threshold falls somewhere around two-thirds to four-fifths surface coverage, depending on deposition method and substrate.⁴ I would not transfer that number to a gelatin emulsion — the geometry, the substrate and the deposition mechanism are all different — but the shape of the behaviour should carry, because it is a connectivity argument rather than a chemistry one.

If that framework is right, then colour and shine are not two different results of this process. They are the same result at different coverage, and everything that pushes coverage up — time in the bath, temperature, how much silver was available to mobilise in the first place — moves a print along one axis from coloured to mirrored. It would also explain why prints that carry both are so common: coverage is not uniform across an image, so different parts of the same print can sit on different sides of the threshold.

One observation from the darkroom is at least consistent with this, though it is not evidence. With a fresh bath I could not see much difference between one minute and two — which would make sense if a fresh bath drives coverage past the threshold quickly and then has nowhere further to go. What did change the result was letting the chemistry age. A tired bath produced patchier, less uniform mirroring, which is exactly what a print sitting at the percolation threshold rather than comfortably above it should look like: connectivity varying across the sheet, some regions percolating and some not.

Melbourne City Skyline. Here, the coverage varies across a single sheet: heavy, broken deposit at the edges, very light in the middle. Whatever sets the local density of the deposit, it is not uniform across a print.

That is a hypothesis fitted to an accident, and I want to be clear about which way round that happened. But it does suggest the cleanest test available to me without a microscope: run one bath from fresh to exhausted, print the same negative through its whole decline, and see whether the sequence moves in an orderly way from mirror to mixed to colour. A coverage ladder made out of bath fatigue. If the prints come out in that order, the framework is doing real work; if they come out in some other order, it is a nice story that happens to be wrong.

Direct, Indirect, and Where the Silver Comes From

In these terms the two sequences in 034 stop being separate techniques and become the same physics applied to two different inventories.

Direct toning uses the silver that is already sitting in the highlights as undeveloped halide. It is soluble, it is exactly what the thiocyanate is designed to mobilise, and it is present in the areas that received least exposure. Mirroring lands in the highlights because that is where the raw material is.

Indirect toning has to manufacture its raw material. The print is finished, so the highlights have been fixed clear and the only silver left is the metallic silver of the image. The copper chloride bleach rehalogenates it — converting metallic silver back to silver halide — which recreates a soluble reservoir precisely where the image is densest. Mirroring lands in the shadows because that is where the bleach has just put something the bath can dissolve.

This also predicts the failure mode I ran into when stacking with mordançage: both processes are competing for the same finite pool of image silver, so whichever runs first sets what the second one has left to work with.

Why Heat

Every step above is rate-limited. Complex formation, diffusion through gelatin, nucleation, and growth all speed up with temperature, but not by the same factor — and it is the ratio between nucleation rate and growth rate that sets particle size. Favour nucleation and you get many small particles; favour growth and you get fewer large ones. Since particle size is what 015 says determines colour, and coverage is what determines shine, temperature is not a convenience setting. It is the main handle on morphology available without changing the chemistry.

Which is a long way of justifying a heating mat under a tray, but it does suggest the experiment worth doing: hold everything else constant and make a temperature ladder rather than a time ladder, and see whether the two behave differently.

That gives me two ladders to build — one in temperature, one in bath fatigue — and between them they should settle whether any of the above is right. Neither needs equipment I do not have. Both need more prints than I have made.

Next: 036 · How Far the Recipe Travels — the same chemistry on fibre, lith, RA-4, and mordançage, and the places it refuses to go.

For an accessible overview of the accepted mechanism, see the American Institute for Conservation's Photographic Materials Group entry on silver mirroring. ↩︎
Moon, J., and K. Curran. “A study of the relationship between the migration of image silver and perceived yellowing of silver gelatine photographs.” Heritage Science 5, no. 45 (2017). Open access. ↩︎
On the transition from localised to delocalised plasmon modes as coverage increases, see the literature on semicontinuous and percolating metal films — e.g. “Plasmonic Percolation: Plasmon-Manifested Dielectric-to-Metal Transition,” ACS Nano 6, no. 10 (2012). ↩︎
Reported percolation thresholds for sputtered silver films fall in roughly the 66–80% coverage range, varying with deposition conditions and substrate. See for instance the determination via Stokes parameters and in-situ conductance in Applied Optics 53, no. 24 (2014). ↩︎

034 · Silver Mirror Toning: Making the Fault the Picture

Wed, 29 Jul 2026 00:00:00 +0000

Silver Mirror Toning: Making the Fault the Picture

Part 1 of 4 in the Silver Mirror Toning series | Next: Part 2 →

Sometimes when glancing at old silver gelatin prints that have been on display for decades, you may have seen it: a bluish metallic sheen sitting on the darkest areas, visible only at a glancing angle, towards the light. Conservators call it silver mirroring, and they treat it as damage. It is the slow migration of silver out of the emulsion and onto its surface, driven by residual thiosulfate, oxidising pollutants, and decades of patience. Whole careers are built on preventing it.

The workshop I took at Mörk in April, during the Helsinki Analog Festival, was about causing it on purpose, in a couple of minutes.

Direct toning on FomaBrom 111. The La Trobe Reading Room at State Library Victoria, Melbourne — the upper walls and windows, the highlights, are where the silvering has occurred.

The workshop was led by Lexy Liangzi Xiao, an artist based in Bergen who has spent several years developing silver mirroring into a controllable printing process rather than an accident of ageing. Her framing stuck with me immediately: the technique takes a form of photographic degradation and makes it intentional. That is a description that would fit half of what I do in the darkroom — mordançage is emulsion failure made deliberate, lith is development running away from you on purpose. In silver mirroring, the print acquires a metallic surface that behaves like a mirror, and the picture changes depending on where you stand, with the colours depending on the exact mixture of toner you use.

This is the first of four posts. Here I cover what the process is, the chemistry I used, and the directions it can run. The second goes into the science of why any of it works — how silver ends up on top of the emulsion instead of inside it. The third documents what happened when I took Lexy's recipe and tried it on my usual papers, and combined with some of my other favourite processes. The fourth is a picture-led post about a series made with it in Singapore.

Where This Sits

Silver mirror toning is a close relative of the chromoskedasic Sabattier process I wrote about in 014–016. The physics specific to mirroring is in the next post; for the scattering and plasmon story underneath both processes, 015 remains the reference. The short version: both processes work by depositing metallic silver as particles rather than as the fine, light-absorbing grain of a normal print. Chromoskedasic printing aims for particles in the size range where they scatter selectively and you perceive colour. Silver mirroring aims for a dense, more or less continuous deposit at the emulsion surface, where the dominant effect is specular reflection instead. Similar activator chemistry, similar non-equilibrium behaviour, but at different extremes — indeed, taking chromoskedasic Sabattier too far can lead into silver mirror toning.

The other thing worth saying up front: this is a process that photographs badly. A flatbed scan of a mirrored print records almost nothing, because the scanner illuminates the print from a fixed angle and the whole point is angular dependence.

The two photographs below make the point better than the paragraph does. Same table, same prints, same afternoon — the only difference is where I was standing.

From above: a table of prints from one session.

From a low angle: the same prints, with the light reflecting differently. Nothing about the prints changed between these two frames.

Direct and Indirect

The process runs in two directions, and the difference between them is simply which silver you feed it.

Direct toning happens before the fixer. At that point the highlights of a conventionally developed print are not empty — they are full of undeveloped silver halide that a normal workflow is about to dissolve and collect in your spent fixer. The toning bath instead complexes that halide and reduces it back out of solution as coarse silver at the surface. The result: mirrored highlights, black shadows. You are toning the highlights with the material you would normally remove.

Indirect toning happens after the print is finished, and starts by undoing it. A copper chloride bleach converts the developed metallic silver of the shadows back into silver halide, at which point the toning bath has something to work on in exactly the places direct toning could not reach. The result inverts: mirrored shadows, untouched highlights.

Both are worth having, and they suit different pictures. Direct toning puts the shine where the light is, which reads as glare, sky, water, chrome — it flatters images that already have specular highlights in them. Indirect toning puts the shine into the mass of the image, and gives the shadows colour and specularity. Indirect also would appear to be the best way to go when combining with other experimental processes, from my experience.

The Mix

Lexy's approach is well described in a write-up on AlternativePhotography.com, but here is the version I've been using for my test prints.

Toning bath, mixed 1:1:1 immediately before use:

Sodium thiocyanate (NaSCN), 5.6% — the silver halide solvent. This is what gets silver into solution so it can be redeposited somewhere other than where it started.
Sodium hydroxide (NaOH), 5% — the activator. Strongly alkaline, and the reason the reduction happens fast and coarsely rather than slowly and finely.
Print developer at normal working strength — the reducing agent. I used some slightly old neutol eco.

Bleach (indirect only): copper chloride, 0.7%. This is the same rehalogenating bleach I use for mordançage, but at a much gentler concentration; it is converting silver back to halide, not attacking gelatin.

Temperature. I ran the toning bath at around 35 °C, on a heating mat under the tray. Heat is not optional here — this is a kinetically controlled process in the same way chromoskedasic work is, and a cold bath gives you a slow, thin version of the effect rather than a different one. I have not yet worked systematically down the temperature range to find where it stops being worth doing.

Sequences:

Step	Direct	Indirect
1	Develop	Bleach (CuCl 0.7%)
2	Tone	Rinse
3	Rinse	Tone
4	Fix	Rinse
5	Rinse	Stabilise
6	Stabilise

Note that indirect never returns to the fixer — the print was fixed the first time around, and the bleach/tone cycle is working on silver that is already committed to the paper.

A word on handling, since this is not a benign mix. Sodium hydroxide at 5% is caustic and will damage eyes on contact; thiocyanate can liberate hydrogen cyanide in an acidic mix. Gloves, goggles, ventilation, and separate tongs. I would avoid an acid stop bath into this sequence.

As the bath aged, the results became less uniform — patchier, less even, with more incident in them. And those are the prints I want more of. A tired bath has less silver-mobilising capacity and less reducing power, so it lays down a thinner, more broken deposit; if the coverage argument in the next post is right, that is a print sitting near the threshold where mirroring becomes patchy rather than continuous, with different parts of the same sheet falling on different sides of it. For some “interesting” results, one can deliberately work a bath through its life and print the whole decline.

Where the Waste Goes

I spent thirteen posts arguing that the environmental question in analogue photography is dissolved silver rather than developer choice, so I can hardly write up a process whose entire purpose is dissolving silver and moving it around, and leave that out.

A used toning bath contains silver in solution, thiocyanate, and sodium hydroxide. The indirect sequence adds a copper chloride bleach on top. None of that goes down a drain. It is the same disposal category as the chromoskedasic chemistry I covered in 025 — collect it, label it, and take it to hazardous waste collection, including the small volumes, because thiocyanate is an aquatic toxin and the silver is worse.

Two things make this less bad than it might be. The bath is a thiocyanate solvent at 5.6% rather than the 15–20% ammonium thiocyanate of a chromoskedasic stabiliser, so the burden per litre is substantially lower. And the volumes are small: a tray of working solution does a session, not a month.

The silver accounting is more interesting, and it runs the opposite way to what you might expect from a process this alarming-sounding. In conventional printing the silver you lose is the undeveloped halide in the highlights, which the fixer dissolves and carries out of the print — that is the bulk of what makes darkroom waste a problem in the first place. Direct toning gets there first. It intercepts that halide before the fixer and reduces it back down onto the sheet as image silver, so by the time the print is fixed there is markedly less left to dissolve. Indirect does the same thing from the other end: the bleach rehalogenates silver that is already in the print, the bath puts it straight back, and the sequence never returns to a fixer at all.

In other words, the silver mostly stays on the paper. That is the whole mechanism — it is what “printing with the material you would normally throw away” means when you follow it through to the drain. On silver alone, this process is plausibly better than straight printing rather than worse.

What it does add is a different hazard set: thiocyanate, sodium hydroxide, and copper chloride in the indirect route, none of which a conventional print run produces. That is a real cost and the reason the waste gets collected. But it is a different argument from the silver one, and worth keeping separate — which is the point the sustainability series keeps arriving at anyway.

Dry Them Slowly

One hard-won piece of practical advice: hang the prints vertically and leave them overnight.

Most of mine dried that way and came out clean. For a few I got impatient and used a print dryer with cold air, and every one of those developed drying marks — streaks and tidelines sitting in the mirrored areas, visible precisely at the angle where the mirroring is visible, which is to say exactly where you do not want them. Asking around afterwards, this seems to be a known hazard of the process rather than anything I did unusually wrong; the surface deposit is fresh, loosely held, and evidently sensitive to how the water leaves it. Similarly, finger marks can accrue.

There is something appropriate about a process that mimics a century of slow chemical migration insisting that you also wait for it.

Next: 035 · Why the Silver Comes to the Surface — physical development, nucleation, and the point where scattered particles become a mirror.