Lab: Clearing, Toning and Making an Iron-Silver Print Last

Two prints are drying, and neither of them is finished. There is iron(III) in both sheets — the iron that made the picture and was handed back when the silver took its electron — and iron(III) oxidises silver. Ware’s diagnosis of this whole family is one sentence: the inherent problem “lies in the danger of leaving residual ferric iron in the print — to its ultimate undoing”.

This is the session that decides whether the prints of the last two labs are objects or experiments. It has three parts and they are not equally hard. Clearing is chemistry and it works. Proving the clearing worked is a test with real limits, and the limits have to be stated. And toning is a second, independent insurance policy that also happens to be where the cheapest process in this cluster is made to look like the most expensive one.

Purpose

To take the Van Dyke and kallitype prints of the previous two labs through a clearing sequence chosen from three published ones, test each for residual iron with a spot test whose sensitivity is not published and whose limits this page states, tone a print in gold or palladium before fixing it, and finish with a record that says what was done and what the evidence supports.

!What this page can and cannot promise

It can make a large, well-evidenced difference. Ware reports that the three-bath EDTA and sulfite sequence was tested against other clearing procedures by X-ray fluorescence, by Matthew Clarke and Dana Hemmenway, and was “the most effective of all they tested, leaving the iron level lower than any other combination of clearing agents, and of an iron concentration comparable with, or even less than, the original iron content of the uncoated paper”, with accelerated ageing producing no perceptible yellow stain.

It cannot promise a lifetime. No accelerated-ageing study of a cleared and toned kallitype or Van Dyke appears in the corpus this course reads; the conservation literature’s own process entry for these processes is, as this page records below, an empty page. What follows is the best-evidenced procedure available and an honest account of how far the evidence goes.

Learning objectives

By the end of the session you will be able to:

• Say what is left in a printed iron-silver sheet, species by species, and which bath removes each.
• Choose between three published clearing sequences and say what each is trading.
• Explain why the order of the EDTA baths is not interchangeable, in terms of pH and hydrolysis.
• Run a residual-iron spot test and state, without prompting, three things it cannot tell you.
• Tone before fixing, and say why three sources give that instruction.
• State what a palladium-toned kallitype is and is not equivalent to, and name the measurement that would settle it.
• Collect a toning bath as a recoverable metal rather than as an effluent.

Prerequisites

• Lab: printing a kallitype and Lab: printing a Van Dyke Brown — you need their prints, their records, and their masked borders.
• Gold toning and the permanence of printed-out silver, which owns the toning mechanism. This page states where the chemistry differs and does not re-derive it.
• Wash testing and the residual hypo and silver tests SOP, whose two procedures this page asks a hard question about.
• Permanence and image deterioration, for what “archival” can and cannot mean.
• Part XX’s toning chemistry, and in particular selenium toning chemistry, which this page cross-references rather than repeats.

Safety classification

Level B, and the letter is not set by the clearing agents.

The clearing agents themselves are mild at these strengths. Citric acid is signal word Warning with H319 in 84.7 per cent of 4,373 ECHA reports and H335 in 23 per cent, and 359 of those reports say it meets no GHS criteria at all. EDTA disodium dihydrate is reported as not meeting GHS hazard criteria in all six reports of its two notifications. Sodium sulfite is the strongest of the three — signal word Danger, H314 in 51.8 per cent of reports, though 618 of 2,482 say it meets no criteria — and it is used here at 2.5 per cent.

Three things set the level instead.

The noble-metal toners, and one statement decides it. Palladium(II) chloride’s aggregated classification carries H317, may cause an allergic skin reaction, in 77.6 per cent of 299 reports, along with H318 in 63.2 per cent and H302 in 60.2. Gold(III) chloride is signal word Danger with H315, H319 and H335 each in 88.2 per cent of reports. Sensitisation is the hazard that does not scale with concentration the way acute toxicity does: it is an individual, cumulative immune response, and once acquired it does not go away. The working bath is very dilute — Bostick & Sullivan’s palladium toner is 7 to 15 drops of their stock in a litre of 1 per cent citric acid — and the controls are gloves that are changed rather than wiped, no residue allowed to dry anywhere, and the tray washed at once. A reader with a known metal sensitisation should not run the toning half of this page, and should run the clearing half, which is where most of the permanence is.

The hexacyanoferrate spot-test reagent, which must never meet the acidic clearing baths. Potassium ferricyanide decomposes on contact with acids producing toxic gases including hydrogen cyanide, on the international chemical safety card’s own wording. Ware’s qualification of the frequently quoted warning is worth having exactly — the heating has to be very strong and the acid very concentrated, and neither is done in a process like this — and the course keeps the rule absolute anyway, because the cost of being wrong is asymmetric and because a waste bottle is where the two things actually meet. The test is done at a separate station with its own waste container, and nothing from that station goes into the clearing-bath bottle.

And every bath here carries metal. Silver in the clearing baths and the fixer, iron everywhere, gold or palladium in the toner. All of it is collected.

One raised step, declared here. Handling the concentrated palladium or gold stock solution — the few millilitres you dose from — is above the rest of this session and carries its own controls under the raised-step rule: gloves and splash goggles, the bottle opened over a tray, the dropper dedicated, the stock never decanted into an unlabelled vessel, and any spill wiped up wet rather than left to dry. The working bath does not need that standard. If you can buy the toner ready diluted, the step is avoided and the rubric prefers that.

⚠What is not a hazard here, and why the absences are worth naming

No ultraviolet, and this is the one page in the part that declares none. Nothing is exposed. The prints arrive already made, so the whole Part XVI apparatus stays switched off — which also means this is the page a reader without a UV source can still run on somebody else’s prints.

No dichromate, no ammonia and no concentrated acid. The clearing baths here are a weak organic acid and two salts. The traditional kallitype kit’s potassium oxalate route would change that assessment — potassium oxalate is H302 and H312 at 100 per cent of the reports carrying those codes, and its supplier calls it an anticoagulant and a poison — and this page does not use it, for reasons set out under the clearing agents below.

And selenium is not used on this page at all. Part XX owns selenium toning, its hazards and its controls, and the cross-reference below explains what it does to an iron-silver print and why it goes after fixing rather than before.

Hazards

Hazard	Where it arises	Control
Skin sensitisation by a palladium salt	The toning stock, and to a much lesser degree the working bath	Gloves changed rather than wiped; no dried residue anywhere; a reader with a known metal sensitisation does not run this section
Serious eye damage from a gold or palladium concentrate	The moment the stock bottle is open	The raised step above: goggles, a tray under the bottle, a dedicated dropper
Hydrogen cyanide, only if the separation rule is broken	A hexacyanoferrate reagent meeting a concentrated acid, or heated	A separate test station, its own waste container, and nothing acidic on that bench. The safety card states the decomposition; Ware qualifies its likelihood; the course keeps the rule
Eye irritation from sodium sulfite and citric acid solutions	Making up and using the clearing baths	Eye protection, gloves, and the sulfite bath made fresh rather than stored
Alkali burn, if the tetrasodium bath is made from sodium hydroxide	Dissolving 9.6 g of sodium hydroxide before adding the EDTA	Goggles, gloves, alkali added to water and never the reverse, and the alkali spill SOP read first. Buying tetrasodium EDTA avoids this step
Silver, iron and noble metal in every bath	All of them	Collect. Silver-bearing waste SOP

Required PPE

Nitrile gloves throughout, changed rather than wiped, and this page’s reason is different from the last two: a sensitiser is a substance you want off your skin, not merely diluted on it.

Chemical splash goggles for the toning stock and for making the alkaline EDTA bath; safety spectacles for everything else.

Non-metal tongs, which do double duty here — they keep silver away from metal and they keep the toner off your gloves.

And a dedicated place to stand for the spot test, which is a control rather than equipment: the separation of the hexacyanoferrate from the acid baths is done by layout, not by care.

Ventilation

The controls here are separation and containment, not airflow, because nothing in this session produces a vapour at room temperature. The clearing baths are a weak organic acid and two salts in water; the toners are metal salts in dilute citric acid; the fixer is thiosulfate. None of them is volatile and none evolves a gas in normal use.

The one thing that could is the hexacyanoferrate reagent meeting an acid, and the control for that is the separation rule, not extraction — an extracted fume cupboard would not make it acceptable to mix them, and a through draught does not make it safe to try. Work in a room with the ordinary ventilation HSE ask for wet photographic work, on a lipped wipeable surface, and put the test station where nothing acidic reaches it.

Materials

• Your prints from the two previous labs, both processes, with their masked borders intact, plus at least one sacrificial strip of each — the spot test destroys the area it touches.
• One coated, unexposed, fully processed control sheet of each process, which is what the residual tests are run on. Carry it through every bath alongside the prints.
• Six trays, three of them for the clearing sequence, and a seventh small dish for the test station.
• Blotting paper, non-metal tongs, a glass rod, and a pipette for the spot test.
• Distilled water for the test reagent and for the final rinse if your supply is hard.
• Narrow-range pH paper.

Chemicals

Chemical	Quantity	Form and purpose
Citric acid	60 g	3 % w/v, two litres. The clearing bath King and Photographers’ Formulary both use
EDTA disodium dihydrate	95 g	50 g for bath 1 of the three-bath sequence and 45 g for bath 3
Sodium hydroxide	9.6 g	Only if you are making the tetrasodium bath from the disodium salt. Buy tetrasodium EDTA and use 50 g of it instead, and this row disappears
Sodium sulfite	25 g for bath 2, 10 g for the hypo clear	2.5 % and 1 % w/v. Bath 2 is made fresh for the session
Potassium ferricyanide	a few crystals	The spot-test reagent. Nothing else on this bench
Sodium thiosulfate pentahydrate	25 g	5 % fixer, with the alkaline additions
A gold or palladium toner	see below	Bought as a kit or made from the published formulas, both of which are on their formulary pages

The toner formulas are not restated here. The gold thiocyanate toner and the platinum toner for printing-out papers carry the quantities, provenance and ingredient functions. What this page gives is what the toners do and which to choose.

Equipment

A balance to 0.01 g, graduates, a thermometer, non-metal tongs, a timer, and — for the analysis — the reflection head of your densitometer if you built one. Nothing here is specialised except the discipline of a separate test station.

Estimated cost

Band £££, and the toner is the whole of it. The clearing chemistry is cheap: citric acid, EDTA and sulfite are among the least expensive substances in the course. Gold and palladium are not, and they are the reason this page sits at the same band as the kallitype lab.

What makes it bearable is the arithmetic King draws attention to, and it is the economic argument of the whole part. In a platinum-palladium print the metal is in the sensitiser, so a failed print takes the metal with it. In a toned kallitype the metal goes on after development and clearing, when you already know whether the print is worth it — and King puts full toning at about a quarter of the chemistry a Pt/Pd print of the same size consumes. Bostick & Sullivan’s palladium kit is 10 mL of stock, which they say is 255 drops, at 7 to 15 drops per litre of working solution and a refresh of 5 to 10 drops after 10 to 25 prints.

Estimated consumables cost

Every price with a number is the planner’s own dated UK figure and every quantity comes from the Materials and Chemicals sections above. The two most expensive items in the session have no sourced price at all.

Consumed	This session	Sourced price	Cost this session
Nitrile gloves	3 pairs	£6.64 to £14.99 per box of 50 to 100	£0.40 to £0.90
Citric acid	60 g	£10.00 per 250 g of the monohydrate, which is the solid these baths weigh out rather than the bottled stop-bath concentrate	£2.40
EDTA disodium dihydrate	95 g	Not priced. It is one of the clearing-bath solids the file names as a gap, and this page is where that gap came from	—
Sodium sulfite	35 g	£13.68 to £19.98 per kg	£0.48 to £0.70
Sodium thiosulfate pentahydrate	25 g	£14.70 per 1 kg of the raw salt	£0.37
Potassium ferricyanide	a few crystals, well under 1 g	£17.99 per 230 g, or about £0.08 a gram	Rounds to nothing at this quantity
Gold chloride or a palladium salt	a few millilitres of a dilute stock	Not priced. Both are gaps the file names, and between them they are the most expensive consumable on the page and the one it cannot cost	—
Unbuffered cotton paper, for the control sheets	2 sheets	Not priced. A gap the file already names	—

The priced rows come to about £3.65 to £4.37, which is a floor and an unusually misleading one: three of the eight rows have no price and one of those three is a noble metal. Do not read this table as saying the session is cheap. What it says is that the course cannot yet cost the two things that matter, and that a reader planning this work should get a current quotation for gold chloride and for a palladium salt before committing to it.

Equipment is deliberately absent, because trays and tongs are not consumed.

Waste streams

Five streams, and this is the page where one of them is worth money.

The clearing baths carry iron — as citrate, as an EDTA chelate, as iron(II) after the sulfite bath — and silver washed out of the sheet. The rinses carry the same, dilute. The toner carries gold or palladium, plus the silver it displaced; Bostick & Sullivan describe the visible evidence, “a black deposit will appear in the bottom of the tray, which is silver that has been replaced by the noble metal”. The fixer carries silver as thiosulfate complexes. The final wash carries thiosulfate.

The spot-test waste is a sixth stream and it is kept separate from all of them, in its own labelled container, because it contains hexacyanoferrate and the clearing baths are acid.

Label everything under the labelling SOP and follow the silver-bearing waste SOP. Check your local regulations; they govern.

Alternative route

This page needs no ultraviolet source and no darkroom, which makes it the most accessible practical page in the part. What it does need is prints, and a reader who cannot make them can still run the clearing and testing half on somebody else’s iron-silver print, or on a coated and processed sheet that was never exposed — the control sheet is where the residual tests are run anyway.

Without gold or palladium, the clearing half stands on its own and is where most of the permanence is. Ware’s argyrotype notes are the honest counterweight to the “always tone” advice: with residual iron and silver kept very low, “image stability and lightfastness are good”, and he does not prescribe a toning step for his own process. Clear thoroughly, test, and record that the print is untoned, which is a defensible position and a documented one.

Without a hexacyanoferrate reagent, or without a bench where it can be kept apart from acid, the masked-border reading is the primary test and does not need it: every kallitype source defines the clearing end point as the disappearance of stain from the coated-but-unexposed border. What you lose is the ability to detect iron that is present but not yet visible, which is exactly the case the spot test exists for — so record that the chemical test was not run rather than leaving a blank.

One thing has no alternative. There is no version of this page without running water. Every source specifies a final wash of between fifteen and sixty minutes, and a print that has been through a thiosulfate bath and not washed is worse off than one that was never fixed.

Preparation

Make up the baths. The 3 per cent citric acid keeps; the sulfite bath does not and is made fresh for the session, because Ware’s reason is that it “does not keep, being oxidised by air to sulphate”, and a bath that has become sulphate reduces nothing.

Decide which sequence you are running, before you start, and write it in the record. The three choices are set out in the next section and they are genuinely different procedures rather than variations.

Set up the test station, physically apart: its own small dish, its own pipette, its own waste bottle, its own square of blotting paper, and nothing acidic within reach.

Check the water. King’s instruction about the rinse before clearing is the one that matters most: neutral or slightly acidic, because an alkaline rinse forms iron(II) hydroxide compounds that make complete clearing difficult or impossible.

And read the two prints before you start. Note the colour of each masked border and photograph or sketch it. That is your before-and-after.

Procedure

Stage 0 — What is actually in the sheet

Everything that follows is aimed at this list, so it goes first.

What is left in a printed, washed iron-silver sheet, and which bath takes each

1. 1Iron(III), free in the fibresUnused sensitiser and every iron centre that gave its electron to a silver ion. Water moves some of it; an acid or a chelating bath moves most of it
2. 2Iron(III), chemisorbed to the celluloseWare: each glucose unit carries two pairs of adjacent hydroxyls that can coordinate to an iron(III) centre. This is the fraction water will not touch, and even powerful chelators remove only about half of it from cellulose pulp
3. 3Iron(III) hydroxide, colloidal, if the pH went above 4Redissolves in dilute acid at first. If the print dries with it still there, it converts irreversibly to iron(III) oxyhydroxide — goethite — which dilute acids will not touch
4. 4Iron(II)Exposed sensitiser that found no silver. Air oxidises it back to iron(III), so it is a delayed version of the first two rows
5. 5Unreduced silver saltStill light-sensitive. Only the fixer removes it
6. 6Silver-thiosulfate complexes and free thiosulfate, after fixingDecompose over years to silver sulphide. The hypo clear and the final wash remove them

Six species, four baths and one deadline: the iron has to leave before the print dries, because the hydroxide it forms is soluble in dilute acid and the oxyhydroxide it becomes is not.

⚗Why water alone cannot do it, and what each agent does instead

Water fails for two reasons that Ware sets out in his clearing chapter. Some of the iron(III) is chemisorbed — bound to the hydroxyl groups of the cellulose rather than dissolved in the sheet — and above pH 4 the rest hydrolyses to a polymeric colloidal hydroxide that lodges in the fibres. Calcium from hard water or from a chalk-buffered paper makes it worse, precipitating calcium oxalate and promoting the hydrolysis.

Citric acid works by lowering the pH, which suppresses that hydrolysis, and by supplying a ligand that iron(III) demonstrably binds — the Van Dyke’s own sensitiser is an iron(III) citrate complex. It is the clearing agent King and Photographers’ Formulary both use at 3 per cent, and it is cheap enough to replace often, which King says is the point: “Renew citric acid bath frequently, as this chemical is very inexpensive and proper clearing is absolutely vital to print stability.” The course marks the ligand half of that explanation as its own reading: the sources prescribe citric acid and none of them says why it works.

EDTA is the ligand argument made properly, and Ware gives the numbers. It has four carboxylato and two tertiary amino donor groups and can occupy up to six coordination positions on one metal centre. Its four acid dissociations are at pK 2.0, 2.7, 6.2 and 10.3, which is why the salt you choose sets the pH: the disodium salt is mildly acidic at pH 3 to 4, the trisodium about neutral, and the tetrasodium alkaline at pH 9 to 10.

[Fe(H₂O)₆]³⁺ + C₁₀H₁₄N₂O₈²⁻ → [Fe(C₁₀H₁₂N₂O₈)]⁻ + 2 H⁺ + 6 H₂O

Disodium EDTA at pH 3 to 4: the chelate wraps six donor atoms round the iron(III) and holds it in solution

Sodium sulfite does something neither of the others can: it reduces the iron(III) that chelation alone cannot detach.

2 Fe³⁺ + SO₃²⁻ + H₂O → 2 Fe²⁺ + SO₄²⁻ + 2 H⁺

The middle bath: iron(III) to iron(II), which binds cellulose more weakly

Ware’s reason for the bath in one sentence: “The reduction product, iron(II), is less strongly bound to cellulose and less extensively hydrolysed than iron(III), so is more easily and completely removed in the final chelating bath that follows.”

And the tetrasodium EDTA bath finishes it, at pH 9 to 10, which is where the iron(II) EDTA complex is strongest — Ware gives its peak formation constant as 2 × 10¹⁴ at about that pH — and which leaves the sheet alkaline, “which is desirable for its preservation”.

Stage 1 — Choose a sequence, and know what you are choosing (10 minutes)

Three published sequences, and they disagree about the reagent, the number of baths and where the whole business sits relative to fixing.

	Citric acid — King; Formulary New Kallitype	Tetrasodium EDTA — Bostick & Sullivan	Three-bath EDTA and sulfite — Ware
Baths	Two or three of 3 % citric acid, rotated so the freshest is last	One, at two tablespoons per litre	Three: disodium EDTA ~5 %, sodium sulfite ~2.5 %, tetrasodium EDTA ~5 %
Time	Until the border is white; a paper needing over 4 to 5 minutes is unsuitable (King) or over 10 minutes needs sizing (Formulary)	3 to 5 minutes	Ware’s own published times, on the formula page
Position	After development, before toning and fixing	After development, before toning and fixing	Published for platinum-palladium and chrysotype after development
Capacity	Two or three 8 × 10 prints per bath	About twenty 8 × 10 prints	Bath 3 has a long life because little iron reaches it
Evidence for it	Universal practice among kallitype printers; no published residual-iron measurement	The supplier’s own kit	X-ray fluorescence measurement: the most effective of all the procedures Clarke and Hemmenway tested
The catch	Cheap, simple, unmeasured	Ware warns explicitly against tetrasodium EDTA as a first bath: at pH about 10 the iron(III) is hydrolysed rather than complexed, “with the eventual result of yellow or brown staining”	Published for the platinum metals, not for iron-silver. The extension is the course’s, and it is argued on the formulary page

⁇The disagreement that matters most, and it is about pH

Bostick & Sullivan ship tetrasodium EDTA as the clearing agent for their kallitype kit. Ware names that recommendation and rejects it: “It is important not to use tetrasodium EDTA for the first processing bath — In spite of the fact that it has been recommended by Bostick & Sullivan — it has a high pH ~10 at which the iron(III) is not effectively complexed, but rather hydrolysed, with the eventual result of yellow or brown staining.”

Both parties are competent and the disagreement is specific, which makes it a good one to reason about rather than a matter of authority. Ware’s argument is a chemical one with numbers attached: above pH 4 iron(III) hydrolyses, an alkaline bath therefore precipitates it rather than chelating it, and once precipitated and dried it becomes insoluble. Bostick & Sullivan give no reasoning at all; they sell a kit.

What this course does. It follows Ware, for three reasons. His position has a stated mechanism, it is supported by the XRF comparison, and it is consistent with the one instruction every kallitype source gives — that the rinse before clearing must be neutral or slightly acidic, because an alkaline rinse forms iron hydroxides that make clearing difficult or impossible. A first bath at pH 10 is that same mistake, made deliberately.

What it does not do is call the kit wrong on the evidence of one author. If you have the kit, use its EDTA as bath 3 of Ware’s sequence, where an alkaline chelating bath is exactly right, and put a citric acid or disodium EDTA bath in front of it. That is the reading that takes both sources seriously.

And one sequence question the sources leave genuinely open: before or after fixing?

Every kallitype source clears before fixing, and every one of them tones between the clearing bath and the fixer. Neither Van Dyke sheet has a clearing bath at all — Photographers’ Formulary give a one-minute initial wash and a fixer, Bostick & Sullivan give three to five minutes of plain water and a fixer — which is why the previous lab said in terms that it does not finish the print. Wynn White, who prints Van Dykes, inserted a clearing regime of his own before fixing after reading Ware, using acidified water rather than a chelating agent.

The course’s practice, and the reasoning behind it: clear before fixing, because that is where every source that clears at all puts it, because toning has to happen before fixing and toning has to happen after clearing, and because the iron must be gone before the print dries. Then test after fixing and washing as well, because the fixer and the wash can redistribute what is left, and because the test is free.

Stage 2 — Clear (10 to 20 minutes per print)

1. Rinse first, one to two minutes, in water you have checked is neutral or slightly acid.
2. Drain the print before it enters the first clearing bath, which is Photographers’ Formulary’s instruction and extends the life of the bath.
3. Bath 1, then bath 2, then bath 3 if you are running three, moving the baths up in rotation afterwards so the freshest is always last.
4. The end point is the masked border, not the clock. King: “until there is absolutely no stain left in the sensitized but unexposed areas of the print”. Photographers’ Formulary: “until the whites appear free of a yellow or grey fog”.
5. Expect the image to lighten, and do not respond to it. King’s warning: “The image will lighten considerably during clearing, but don’t worry because all the lost density will return during toning and fixing.”
6. Rinse, 30 to 60 seconds.

=What the clearing time tells you about three different things

The number of minutes to a white border is the most informative single measurement in this part, because it responds to three separate faults and you can tell them apart by what else is true.

If the first print of the session clears slowly, it is the paper. King’s limit is four to five minutes; Photographers’ Formulary say a print taking over ten minutes is on paper too absorbent to use without sizing. Neither is a fault you can wash your way out of.

If the times get longer as the session goes on, it is the developer or the bath. A kallitype developer accumulates iron and makes clearing progressively harder — King’s replenishment rule exists for that — and the clearing baths themselves have a capacity of two or three prints each.

If one print clears slowly and its neighbours do not, it is that print’s rinse. An alkaline rinse forms iron hydroxides in the fibres, and once formed they are a different problem from dissolved iron.

Stage 3 — The residual-iron test (5 minutes, on a sacrificial strip)

What the test is. A drop of dilute potassium ferricyanide solution on a highlight area of a sacrificial offcut. Blue means iron is still there.

Why it works, and this part is sourced. Ware’s appendix on Prussian brown records that a mixture of iron(III) and hexacyanoferrate(III) “rapidly turns blue when spotted onto cellulose filter paper or any other oxidisable substrate”, because the pair is powerfully oxidising — a redox potential of about +1.5 V, driven by the insolubility of the product — and that this has been the basis of a qualitative analytical spot test in the inorganic-analysis literature. The cellulose reduces the iron(III), and the iron(II) that results precipitates Prussian blue with the ferricyanide.

Fe²⁺ + [Fe(CN)₆]³⁻ → Fe³⁺ + [Fe(CN)₆]⁴⁻

The electron transfer behind the blue; the iron(III) and hexacyanoferrate(II) then precipitate together

!Four things this test cannot tell you, and they all have to be said

No sensitivity is published. No source read for this course gives a reagent concentration, a detection limit or a false-positive list for this test on a print. The published version of it is a test on a solution of ferric oxalate, printed on both Photographers’ Formulary kit sheets to judge the quality of a bottle. So it tells you whether and never how much, and the course publishes it as its own derivation from the established chemistry rather than as a published method.

It responds to iron from any source. A rusty clip, a contaminated tray, iron in a hard water supply, or iron that was in the paper before you coated it will all give blue. It does not distinguish inadequate clearing from contamination — for which the comparison of the coated-but-masked border with the uncoated margin is the right instrument, exactly as in Part XXI’s diagnosis.

It destroys the area it touches, so it is not a test on a finished print. Cut a strip, or test the trimmed edge.

And it needs an oxidisable substrate, on Ware’s own statement of the chemistry, so it may behave differently on a heavily sized sheet and is not applicable to cloth of unknown fibre.

The primary test remains the masked border, which every kallitype source uses and which costs nothing. The spot test is the fallback for a print without one and the confirmation for a print whose border looks clean.

The separation rule, restated because it governs this stage. The reagent, the dish, the pipette and the waste go to their own station. Nothing acidic comes to that bench. Nothing from that bench goes into the clearing-bath waste. The reason is on the international chemical safety card — the solid decomposes on contact with acids producing toxic gases including hydrogen cyanide — and the reason for keeping it absolute rather than proportionate is that a part-used waste bottle is where two solutions actually meet.

Stage 4 — The residual silver and thiosulfate tests, and whether they apply here

Part XII’s wash-testing lab and the residual hypo and silver tests SOP give two procedures: a silver-nitrate test for residual thiosulfate, and a sodium-sulfide test for residual silver. Run them here, on the control sheet, and read them with three caveats the course states as its own reasoning rather than as published findings.

The reagent spreads differently. Both procedures were developed for gelatin-coated papers, where a drop sits on the surface and the stain has an edge. On a binder-free plain-paper print the reagent soaks into the fibres, so the patch is diffuse and the comparison with a reference is harder. That is a practical limitation, not a chemical one.

Residual iron confounds the thiosulfate test. A positive thiosulfate test is a yellow-brown stain in the treated area, and residual iron produces a yellow-brown stain in exactly the same place and the same colour range. Run this test only on a control sheet that has passed the iron test, or you cannot tell which fault you are looking at.

And the sulfide test darkens the image as well as the residue, because the image is silver. Apply it to an unimaged margin of the control sheet, never to a print.

One thing that is not in doubt. The tests’ chemistry is unaffected by the absence of gelatin, and the substances they detect are present in these prints for the same reasons as in a silver-gelatin print. What the course cannot say is what a given depth of stain means quantitatively on this material, and it says so rather than borrowing a threshold from a different paper.

Stage 5 — Tone, before fixing (5 to 20 minutes)

Why before fixing, and three sources give three different reasons that all point the same way. King: toning before fixing increases shadow depth, minimises bleaching during fixing, and makes the change of tone much more dramatic. Bostick & Sullivan: toning “is one way to keep Kallitype prints from bleaching during the fixing process”, and a sufficiently toned print “will not bleach back”. Ware, for print-out silver generally: “Like all print-out silver images, the gold toning etc., if desired, should be done before the thiosulphate bath.”

The mechanism is Part XXII’s and this page does not re-derive it: a noble-metal ion with a higher reduction potential than silver is reduced by the image silver itself, so the metal deposits where the silver is and some silver goes into solution in its place. The redox potentials from the siderotype lesson say which metals can do it: gold at +1.00 V, platinum at +0.73 V and palladium at +0.62 V are all reduced by silver at +0.80 V — except palladium, which is below silver, and which therefore ought not to work at all on a straight potential comparison.

⁇Does the noble metal coat the silver or replace it?

This matters, because the two answers predict different objects.

King’s position is replacement, and it is strong. He says the toned print’s silver is “replaced with another noble metal that is resistant to oxidation by residual ferrous iron”, that double toning must begin with the more noble metal “because the most noble metal will always replace the least noble”, and that a kallitype toned with platinum or palladium “is in every way an exact equivalent, both visually and in terms of image permanence, of a Pt/Pd print”. Bostick & Sullivan report the visible evidence for displacement: “A black deposit will appear in the bottom of the tray, which is silver that has been replaced by the noble metal.”

Ware reports the claim as a claim. His words are that “a notable practitioner, Sandy King, claims that the process of toning kallitypes does not just coat or encapsulate the silver particles with the more noble metal, but totally replaces them with it” — a careful sentence, and the careful part is “claims”.

No independent measurement appears in the corpus this course reads. What would settle it is elemental analysis: X-ray fluorescence, which Ware names as “the ultimate criterion for identifying a print”, would say directly how much silver is left in a toned kallitype. The course’s position is that the direction is established and the completeness is not, and that the honest description of a palladium-toned kallitype is a silver image bearing palladium rather than a palladium image.

And there is a practical corollary worth having. Ware’s own field test for platinum in an image is a drop of 6 per cent hydrogen peroxide, which effervesces because platinum catalyses its decomposition and which “pure palladium prints will not respond to”. A platinum-toned kallitype would respond; a palladium one would not; and neither test says anything about the silver underneath. The two objects are not interchangeable even where they are indistinguishable by eye, and a print described for sale or exhibition should be described as what it is.

What each toner does, from the sources.

Toner	Colour	Density and contrast	Notes
Gold, citric acid type (King’s Toner 1)	Purple, brown and blue tones	Contrast rises about a step through loss of density in the high values; Dmax changed little if at all	Works first on the highlights, then progressively into the midtones and shadows. Does not keep; mix small and discard
Gold, thiourea type (King’s Toner 2, and Clerc’s)	The same family of tones	As above	Works on shadows, midtones and highlights at about the same rate, which makes it easier to stop. Keeps well
Gold, thiocyanate type (Bostick & Sullivan’s kit)	Cooler and bluer, with maroon or purple possible	The supplier claims preservation of the full tonal range and prevention of loss through fixing	Pull early for warm tones; tone to no further change for cool ones
Palladium	Brownish black, warm	Final density somewhat greater than untoned; contrast identical	Begins in the shadows where it is hardest to see. Bostick & Sullivan: “by the time you see this, the toning is well underway”
Platinum	Very neutral black	As palladium	The most expensive, and the two can be mixed for intermediate colours

Double toning, and the rule that governs it. King: begin with the more noble metal and finish with the less, because the more noble always replaces the less noble — so platinum or palladium first, then gold, giving neutral or warm-black highlights and midtones with cool purple-black shadows. Reverse the order and the print looks as though it had only been toned in platinum.

Practical figures. About 20 mL of solution fully tones a 5 × 7 image, on King’s figure, so tone as a one-shot in a flat tray with no ribs. Bostick & Sullivan’s palladium bath refreshes with 5 to 10 drops after 10 to 25 prints, “heavily dependent on the prints you tone. The more shadow areas the faster the toner will be depleted.”

✦Selenium, and why it is the exception to 'tone before fixing'

Part XX owns selenium toning, its chemistry and its hazards, and this page does not repeat them. Two things about it are specific to iron-silver prints.

It goes after fixing, not before, and both practitioner sources give the same reason. King: “it is extremely difficult to get satisfactory results with selenium when toning is done before fixing, because it reacts with residual silver nitrate in the paper and causes staining.” Wynn White reaches the same conclusion from his own bench.

And it reduces the image while it tones. White’s warning is specific: Kodak’s rapid selenium toner contains ammonium thiocyanate, “which reduces the silver image so it must be used at a very weak dilution” — his own working strength is 2 mL in 500 mL, and even then “great care must be taken not to overreduce”. He finds the reduction happens mainly in the darkest part of the image, which makes it usable as a shadow control on a print that lacks contrast, and that the colour moves from reddish-brown towards chocolate brown and finally to a yellow-brown he does not like.

Because it goes after fixing, it does not protect against fixer bleaching, which is one of the three reasons the other toners are used. It is a colour and permanence treatment, not a substitute for gold or palladium here.

Stage 6 — Fix, wash and test again

Rinse for a minute after toning — Bostick & Sullivan give five minutes after their palladium bath, to get the citric acid out. Fix at 5 per cent for two to five minutes, as in the two previous labs. Hypo clear for two minutes in 1 per cent sodium sulfite. Wash 20 to 40 minutes.

Then run both tests again on the control sheet, because the point of a test is the evidence and not the ritual: the residual-iron test tells you whether the fixing and washing disturbed anything, and the thiosulfate test is only meaningful after the wash it is testing.

Dry flat or on a screen, and read everything the next morning.

Expected observations

In the clearing baths. The masked border goes from yellow, or yellow-grey, to white. The image lightens, sometimes markedly. The first bath discolours visibly after two or three prints, which is the capacity limit arriving.

In the spot test. A blue spot within seconds where iron remains. On a well-cleared sheet, nothing — or a very faint blue that is the paper’s own iron rather than yours, which is why the uncoated margin is tested too.

In the toner. Nothing, for a while. Then the shadows shift, and by the time the midtones are moving the process is well advanced. Bostick & Sullivan’s description is the one to have in mind: watch the toning “work its way up the ‘ladder’ of the print’s tonal scale, and stop when appropriate”. A black deposit collects in the tray.

In the fixer. On an untoned print, bleaching, as before. On a well-toned print, very little — which is the single most convincing demonstration in this part that the toning did something.

On drying. Substantial dry-down. Bostick & Sullivan warn that a toned print “should be a little lighter than desired as your print will ‘dry down’ extensively”, and that on some processes “the toning will be faint until the print is fully dried”.

What is happening chemically

Clearing is three reactions and a deadline. An acid or a chelating ligand takes the free iron(III) into solution; a reductant converts what is bound to the cellulose into iron(II), which binds less strongly; and a second chelating bath at high pH takes that away. The deadline is drying, because freshly formed iron(III) hydroxide redissolves in dilute acid and iron(III) oxyhydroxide does not.

Toning is one displacement reaction. A noble-metal ion with a higher reduction potential is reduced by the image silver, and the noble metal deposits where the silver was.

3 Ag + [AuCl₄]⁻ → 3 Ag⁺ + Au + 4 Cl⁻

Gold toning: three atoms of image silver reduce one gold(III), and the gold deposits on the image

And the permanence argument follows from what the two metals are. Silver at +0.80 V is oxidised by iron(III); gold at +1.00 V is not, and neither is platinum at +0.73 V in the chloro-complex form for which the potential is quoted. Replacing image silver with a metal that residual iron cannot oxidise removes the failure mode rather than slowing it, which is King’s whole argument for toning every print.

⚗Why palladium is the interesting case, and what the course does not know

Look at the potentials again. Gold at +1.00 V and platinum at +0.73 V sit where the argument needs them. Palladium, at +0.62 V for the tetrachloropalladate couple, sits below silver at +0.80 V — which on a straight comparison says that silver should not reduce palladium(II) at all.

Yet the palladium toner works: two suppliers sell one, King publishes a formula, and Bostick & Sullivan describe the displaced silver collecting in the tray.

What could reconcile it. Potentials quoted for complexes depend on the ligand and the concentration, and the citric acid in every one of these toners is a ligand for silver as well as for the metal; Ware’s own note beneath his table is that “kinetic factors may make the rates of such reduction reactions too slow to be useful unless the noble metal complex is sufficiently labile”. A displacement that is marginal at standard conditions can be driven by complexing the product away.

What the course does not have is a source that works this through for palladium toning of a silver image. The observation is well attested and the thermodynamic account of it is not established in anything read for this page, and saying so is more useful than a plausible paragraph. If you know the paper that settles it, the gold toning lesson in Part XXII is where it belongs.

Data to record

Use the lab notebook.

Per print: which process, which developer, the clearing sequence used, the time to a white border in each bath, how many prints that bath had already done, the spot-test result before and after clearing, the toner and its strength, the toning time, the fixing time, and the wash time.

Per control sheet: the residual-iron result, the residual-thiosulfate result, the residual-silver result, and — this is the part people leave out — which of them you could not run and why.

And the archival record, which is the point of the session and is a different document from the session log. For each print you intend to keep: date, process, paper and batch, negative, sensitiser batch, developer, clearing sequence, the spot-test result, toner and time, fixing, washing, and the mount and enclosure it goes into. That is what makes a print evidence about a process in ten years’ time rather than an object with a story attached.

Analysis

First, the pair that answers the question. Put a cleared print beside an uncleared one from the same session, both dry, and read the highlights rather than the image: the difference between a clean white and a faint straw colour is the whole of this page. Then run the spot test on both offcuts. If the uncleared strip goes blue and the cleared one does not, you have made the argument of this part with your own materials.

Second, the toned pair. Untoned and toned from the same negative, dry, under one light. Read four things and write them as measurements where you can: maximum density, exposure scale off the step wedge, image colour in words, and how much each lost in the fixer. King’s claim about the noble-metal toners is specific and testable — final density somewhat greater than untoned, contrast identical — and the gold toners’ claim is the opposite shape, contrast up about a step through loss in the high values. Your two prints can confirm or refute both.

Third, what you may and may not claim about lifetime. This is where the honesty has to be deliberate, because the temptation is large.

⁇The permanence question, answered with what the evidence actually is

The reputation is real and it is documented. Ware’s survey of the survival record is the strongest evidence in the corpus: Dick Stevens, searching many major American collections, found one historic kallitype; Ware’s own enquiry to curators and conservators turned up “only a handful”; and the few century-old specimens that have been identified “do show pronounced image fading and seriously yellowed highlights, suggesting the presence of residual iron(III)”. Ware calls the dearth of surviving specimens “the strongest indication we have that this process, as practised at the time, was unreliable”.

The cause is identified and it is a processing failure. Yellowed highlights are residual iron — Ware’s platinotype identification section names “pale yellow or straw-coloured staining, probably due to residual iron(III)” and adds that it “may also occur in other siderotype processes, such as kallitype”. King’s account is the same from the practitioner’s side: residual iron(II), even in very small quantities, will eventually oxidise the silver.

And the remedy is documented, but not for these processes. The XRF comparison that establishes the three-bath sequence as the most effective clearing procedure tested was made on platinum and palladium prints, and the accelerated ageing that produced no perceptible yellow stain was on those. No accelerated-ageing study of a cleared and toned kallitype or Van Dyke appears in the corpus this course reads, and the conservation literature’s own entry for these processes — the American Institute for Conservation’s Photographic Materials Group page whose title carries all seven of their names — was read on 7 September 2026 and has its headings and no text under any of them.

So what may you claim? That the failure mode is identified; that the clearing sequence you used is the best-evidenced one available and was measured to work on a closely related material; that you tested your own print and it showed no residual iron by a test whose sensitivity is unpublished; and that you toned it with a metal that residual iron cannot oxidise. That is a good deal to be able to say and it is not “archival”. Write down what you did and let the print be the experiment.

Troubleshooting

The border will not clear, however long it sits. In order of cost: the bath is exhausted (mix fresh and repeat); the rinse before it was alkaline; the developer was exhausted, so the sheet arrived loaded; the paper is too absorbent, which the unexposed control sheet would have shown; or the print dried between processing steps and the iron has become goethite, in which case nothing on this page will recover it. See the atlas entry.

The spot test is blue in the uncoated margin as well as the picture. Your wet chemistry is contaminated, or the paper carried iron before you coated it. Test a virgin sheet from the same box.

The toner did nothing. Take these in order: the print was fixed before toning, so there is no free silver and much less surface to work on; the bath is exhausted (Bostick & Sullivan’s refresh figures are above); or you stopped too early, because palladium begins in the shadows where it is hardest to see. See the atlas entry.

The toned print stained. Most likely a toner used before fixing on a print that still carried unreacted silver nitrate — which is the specific failure King names for selenium and which Bostick & Sullivan raise for Van Dykes, whose sheet says that if you get bad staining you should tone after fixing and rinsing instead. It is also what metal additives in the sensitiser do, which is one reason this course does not use them.

The print bleached in the fixer even though it was toned. It was not toned far enough. Bostick & Sullivan’s criterion is that a sufficiently toned print will not bleach back, and King’s is that the print is fully toned when the shadows have taken the colour of the toning metal.

A blue tinge in the highlights of a print you spot-tested. The reagent has spread further than you intended. Test the trimmed edge, not the picture.

Clean-up

The test station first and separately, into its own container, before anything acid is moved.

Then the trays, in the order they were used, into their labelled waste containers. The toning tray is worth attention: the black deposit in it is displaced silver, and it goes into the silver-bearing waste rather than down the drain with a rinse.

Wash everything that touched a noble-metal solution immediately and thoroughly, and change gloves before you touch anything else — a sensitiser is a substance to remove rather than to dilute.

Gloves off last, under the PPE removal SOP, and hands washed.

Storage

The clearing baths. Citric acid at 3 per cent keeps; the sulfite bath does not and is made fresh for each session because it oxidises in air to sulphate. The EDTA baths keep, and bath 3 of the three-bath sequence has a long life because so little iron reaches it — Ware’s instruction is to change it “when discoloration begins to be apparent”.

The toners. The gold thiourea type and the platinum and palladium toners keep well: King says the Pt/Pd toners can be stored fresh in one-litre amounts for several months, Bostick & Sullivan say their palladium bath keeps for several weeks in a cool place away from direct sunlight. King’s own recommendation is nonetheless to tone as a one-shot and store used solution in a separate bottle so the fresh stock is not contaminated. The citric acid gold toner does not keep and is mixed in small quantities as needed.

The spot-test reagent: a small quantity, in a brown bottle, labelled, on a different shelf from every acid you own — see incompatibilities.

And the prints. Unbuffered mounts and enclosures, for the same reason as everything else in this cluster: the alkaline reserve in ordinary archival board is the wrong environment for a plain-paper iron-silver print. Keep the record with the print.

Disposal considerations

The chemistry. Four things in these baths matter and they are not the same kind of problem.

Silver carries H400 and H410 in over 99 per cent of ECHA reports and is recoverable, which is the framing Part XII prefers.

Iron chelates are the bulk of the volume. EDTA itself is reported as meeting no GHS hazard criteria, and an iron-EDTA complex is a stable, soluble species — which is exactly what makes it persistent in water rather than benign there. Collect it.

The hexacyanoferrate waste is kept separate from everything acidic, permanently and by layout. The Environment Agency’s waste classification guidance exempts ferro- and ferricyanides by name from the hazard statements it assigns to cyanide salts, which is a statement about what the substance is rather than about how dilute it is, and it does not make an acid addition acceptable.

And the noble metals. A used gold or palladium toning bath contains a metal worth more per gram than anything else in this course. No source read for this course publishes a domestic recovery route for gold or palladium at these concentrations, and the course will not invent one — precipitating a noble metal from a dilute solution is a chemistry with its own hazards. What is practical is to collect and label it: a refiner will take a labelled bottle of known composition, and the same bottle poured into a drain is both a discharge and a loss.

Check your local regulations; they govern. In England and Wales a domestic worker’s route is the council’s household waste and recycling centre.

Questions

1. Why can water not clear an iron-silver print? Name the two mechanisms Ware gives and say which one has a deadline attached.
2. Why is the order of the three EDTA baths not interchangeable? Answer in terms of pH and what iron(III) does above pH 4.
3. Bostick & Sullivan sell tetrasodium EDTA as a kallitype clearing agent and Ware says not to use it first. State each position and say what evidence would settle it.
4. Name three things the residual-iron spot test cannot tell you, and say what instrument answers each.
5. Your print is toned in palladium and looks exactly like a palladium print. Name the measurement that would show it is not one, and say what you would write on the back.
6. A print gives a positive iron test after two clearing baths. State three possible reasons and the order in which you would test them.
7. Why is selenium the exception to the rule that toning happens before fixing?

Further experiments

The clearing comparison, which nobody has published. One coating, cut into three after exposure and developed together: strip A cleared in 3 per cent citric acid, strip B in the three-bath EDTA and sulfite sequence, strip C in water only. Spot-test all three. This is the measurement the literature is missing for these processes, and you can make it in an evening.

How much iron gets past a good clearing. Take the cleared control sheet, dry it, and spot-test it again a week later. Iron that was still mobile when you tested wet may have hydrolysed since.

Toning to completion, in steps. Five identical strips from one coating, toned for 2, 5, 10, 20 and 40 minutes in one bath, then all fixed together. Read the colour, the maximum density and — the useful one — how much each lost in the fixer, which is the direct measurement of Bostick & Sullivan’s claim that a sufficiently toned print will not bleach back.

Split toning. King’s rule is that double toning must begin with the more noble metal. Test it: two prints, one palladium-then-gold and one gold-then-palladium, and see whether the second really does end up looking as though only the palladium had been used.

And the long one, which costs nothing but a label. Two prints from tonight, identical except that one was cleared and one was not; two more, identical except that one was toned. Date them, put them somewhere dry and dark, and look again in five years. This part’s central claim is a claim about time, and no other experiment in the course tests it.

Recap

Water does not clear an iron-silver print, because some of the iron is chemisorbed to the cellulose and the rest hydrolyses above pH 4 to a colloidal hydroxide that lodges in the fibres.

There is a deadline. Iron(III) hydroxide redissolves in dilute acid at first and converts irreversibly, on drying, to an oxyhydroxide that does not. Clear before the print dries.

The three agents do three different jobs: an acid or a chelating ligand takes the free iron away, a sulfite bath reduces what is bound to the paper into a form that binds less strongly, and a second chelating bath at pH 9 to 10 removes that. The order follows from the pH and is not interchangeable.

Prove it rather than assuming it. The masked border is the primary end point, the hexacyanoferrate spot test is the fallback, and the spot test tells you whether and never how much.

Tone before fixing, because three sources give three reasons that agree: more shadow depth, less bleaching in the fixer, and a larger change of colour.

A palladium-toned kallitype is a silver image bearing palladium. King argues it is an exact equivalent of a palladium print; Ware reports that as a claim; nobody in this corpus has measured it, and the measurement that would is elemental.

And the reputation of this family is a reputation for bad clearing. The evidence for the reputation is real — one historic kallitype in a survey of many American collections — and so is the evidence for what caused it. What no source establishes is how long a well-cleared, toned print lasts, so record what you did and let the print be the experiment.

Check your understanding

Question 1. Why does the three-bath sequence put disodium EDTA first and tetrasodium EDTA last, rather than the other way round?

• Because the disodium salt is cheaper and does most of the work
• Because the disodium salt is acidic at pH 3 to 4, where iron(III) is complexed rather than hydrolysed, while the tetrasodium salt at pH 9 to 10 would precipitate the iron(III) it was meant to remove - and pH 9 to 10 is exactly right for the iron(II) the middle bath has just made
• Because the tetrasodium salt would neutralise the sulfite bath
• Because iron(II) is more strongly bound to cellulose than iron(III) and needs the stronger bath last

Show the answer and why

Answer: Because the disodium salt is acidic at pH 3 to 4, where iron(III) is complexed rather than hydrolysed, while the tetrasodium salt at pH 9 to 10 would precipitate the iron(III) it was meant to remove - and pH 9 to 10 is exactly right for the iron(II) the middle bath has just made

Ware gives both halves. Above pH 4 iron(III) hydrolyses, so an alkaline first bath precipitates it in the fibres - which is why he names and rejects the recommendation to use tetrasodium EDTA first, "with the eventual result of yellow or brown staining". And the iron(II) EDTA complex has its peak formation constant, 2 × 10¹⁴, at about pH 10, which is where the third bath sits. The last option inverts the chemistry: the sulfite bath exists precisely because iron(II) is bound *less* strongly to cellulose than iron(III).

Question 2. A print gives a positive iron test after two clearing baths. What are the three things to check, in order?

• The fixer, the wash, and the paper
• The clearing bath's exhaustion, the pH of the rinse that preceded it, and the state of the developer the print came out of
• The negative, the exposure, and the coating volume
• The toner, the mount, and the storage

Show the answer and why

Answer: The clearing bath's exhaustion, the pH of the rinse that preceded it, and the state of the developer the print came out of

The cheapest test first: mix a fresh bath and repeat one print, because clearing baths have a capacity of two or three 8 × 10 prints. Then the rinse, because King records that an alkaline rinse forms iron(II) hydroxide compounds that make complete clearing difficult or impossible. Then the developer, because an unreplenished kallitype developer accumulates iron and delivers a sheet already loaded with it. Two further candidates sit behind those and are worth knowing: paper too absorbent, and a print that was allowed to dry between steps, after which the hydroxide has become goethite and nothing on the page recovers it.

Question 3. Which of these are limits of the hexacyanoferrate spot test as this course publishes it? (Select all that apply.)

• No sensitivity is published, so it says whether and never how much
• It responds to iron from any source, including a contaminated tray or the paper itself
• It destroys the area it touches
• It cannot detect iron(II), only iron(III)

Show the answer and why

Answer: No sensitivity is published, so it says whether and never how much, It responds to iron from any source, including a contaminated tray or the paper itself, It destroys the area it touches

The first three are stated on the page and the course publishes the test as its own derivation from established chemistry rather than as a published method, because the only version of it in the sources is a test on a solution of ferric oxalate rather than on a print. The fourth is wrong in an instructive way: the test works because cellulose reduces iron(III) to iron(II) and the iron(II) then precipitates Prussian blue with the ferricyanide, so iron(II) already in the sheet gives the same blue more directly. What separates contamination from inadequate clearing is not the reagent but the comparison of the coated-but-masked border with the uncoated margin.

Question 4. Three sources say to tone before fixing. What are the reasons they give?

• Shadow depth is increased
• Bleaching in the fixing bath is minimised, and a sufficiently toned print will not bleach back
• The change of tone is much more dramatic
• The toner will not work on a fixed print because the fixer removes the image silver

Show the answer and why

Answer: Shadow depth is increased, Bleaching in the fixing bath is minimised, and a sufficiently toned print will not bleach back, The change of tone is much more dramatic

King gives all three of the first reasons in one sentence, Bostick & Sullivan give the second independently and add that a sufficiently toned print will not bleach back, and Ware states the general rule for print-out silver images. The fourth is wrong: the fixer removes the unreacted silver salt, not the image, and toning after fixing is exactly what both King and Wynn White do for selenium - because a selenium toner used before fixing reacts with residual silver nitrate and stains.

Question 5. Sandy King writes that a palladium-toned kallitype is "in every way an exact equivalent" of a palladium print. What is the course's position?

• That King is right, since two competent printers cannot tell them apart
• That the direction is established and the completeness is not: the honest description is a silver image bearing palladium, and the measurement that would settle it is elemental analysis such as X-ray fluorescence
• That King is wrong, because palladium sits below silver in the electrochemical series and cannot displace it
• That the question cannot be answered because the two processes use different papers

Show the answer and why

Answer: That the direction is established and the completeness is not: the honest description is a silver image bearing palladium, and the measurement that would settle it is elemental analysis such as X-ray fluorescence

Ware reports King's position carefully as a claim - that toning "does not just coat or encapsulate the silver particles with the more noble metal, but totally replaces them with it" - and no independent measurement appears in the corpus this course reads. Ware names XRF as the ultimate criterion for identifying a print, and it would say directly how much silver remains. The third option raises a real puzzle, since the tetrachloropalladate couple at +0.62 V does sit below silver at +0.80 V, but it cannot be the answer: the toner demonstrably works, and the page says plainly that the thermodynamic account of why is not established in anything it has read.

Question 6. What is the strongest evidence in this course's sources that historical kallitypes were unreliable, and what does it not establish?

• Accelerated ageing tests published by conservators; it does not establish anything about modern practice
• The survival record - Stevens found one historic kallitype in a survey of many major American collections, and the few identified century-old specimens show pronounced fading and seriously yellowed highlights - which establishes the failure and its likely cause but says nothing about how long a well-cleared, toned print lasts
• The American Institute for Conservation's process entry, which describes the deterioration in detail
• Roger Child Bayley's account of a drawer of prints with no front and no back, which is a controlled observation

Show the answer and why

Answer: The survival record - Stevens found one historic kallitype in a survey of many major American collections, and the few identified century-old specimens show pronounced fading and seriously yellowed highlights - which establishes the failure and its likely cause but says nothing about how long a well-cleared, toned print lasts

The survival record is the structural evidence and Ware calls the dearth of specimens "the strongest indication we have that this process, as practised at the time, was unreliable"; the yellowed highlights point at residual iron(III) as the cause. What it does not cover is modern practice with a measured clearing sequence and noble-metal toning, for which no accelerated-ageing study appears in this corpus. The AIC entry for these processes was read on 7 September 2026 and has its headings and no text at all. And Child Bayley's account is a vivid anecdote that Stevens objects to on exactly those grounds.

Check my answersClear