Kallitype sensitiser

Two bottles, two pipettes, and a shot glass in which you make exactly as much sensitiser as one sheet of paper is about to absorb. The kallitype is the iron-silver process that behaves like a platinum print: the same ferric oxalate, the same ultraviolet exposure, the same developer tray, the same clearing baths — and silver nitrate in place of a platinum-group metal.

Two things have to be understood before the rest of this page is usable. The silver is not the light-sensitive part; light acts on the iron, exactly as in a Van Dyke Brown. And almost nothing appears during the exposure, which is the difference from a Van Dyke and the reason this formula needs a developer at all. Both facts come from one change of ligand: an oxalate on the iron instead of a citrate. That single substitution decides whether your picture arrives in the printing frame or in a tray, how deep the shadows go, and which of the two sensitisers could ever have made a platinum print instead.

Solution A — the silver

Ingredient	Quantity	Form the source specifies
Silver nitrate	10 g
Water	to make 100 mL	Distilled water, and a make-up volume rather than an added one — King's instruction is to mix the 10 g into 70 mL, allow it to dissolve, and then add water to make a total of 100 mL, which is what makes the bottle a stated 10 per cent w/v.

Solution B — the light-sensitive half

Ingredient	Quantity	Form the source specifies
Ferric oxalate	20 g	the powder. King names no hydrate, and the encyclopaedia records formula weights from 375.76 for the anhydrous salt to 483.84 for the hexahydrate, so 20 g is a weight of an ill-defined solid rather than a known quantity of iron
Water	to make 100 mL	Distilled water: 20 g into 75 mL, allowed to dissolve, then water to make 100 mL. It goes into solution slowly and should be mixed about 24 hours before it is wanted.

Mixed in the ratio — the sensitiser, mixed in the coating vessel at the moment of use

1 part Solution A + 1 part Solution B

The sensitizer is prepared as two separate stock solutions, solution A and solution B, which are mixed in equal parts just before use. About 2ml of combined solution is adequate for an 8X10 print, or the equivalent.

No make-up volume is stated for the mixture and none is needed, because it is made in the volume the sheet is about to take. Every strength quoted on this page for the mixed sensitiser is therefore exactly half the strength of the stock it came from, which is the course's arithmetic on a stated ratio rather than a figure any source prints. Two suppliers give the same instruction in the same words for the same two strengths, and the Formulary supplies a separate pipette for each bottle.

Purpose

To coat plain paper with a solution that will hold a latent iron image under ultraviolet light, and then give up a silver picture in a developer whose choice sets the colour. The sensitiser is only the first half of the chemistry. Unlike every printing-out formula in this formulary, it produces almost no image on its own: what it produces is a distribution of iron(II) that a developer converts into a distribution of metallic silver.

That division of labour is what the whole process is for. King’s summary of the family relationship is worth having whole, because it names all three advantages at once: in the kallitype “the light-sensitive element is ferric oxalate; in Vandyke and argyrotype it is ferric ammonium citrate. The ferric oxalate makes a superior process in several important ways: it permits darker shadows, i.e. more Dmax… Another advantage of kallitype is greater control of contrast… Another advantage is that kallitype is a developing-out process, which generally translates into greater depth in the shadows than POP processes such as Vandyke or Argyrotype.”

Recommended uses

A platinum-like print at a silver price. This is the reason the process exists and the reason both surviving kits are sold. Photographers’ Formulary opens both its kallitype sheets with the same sentence: kallitype printing “is similar to platinum and palladium printing in theory and technique. However, the Kallitype printing process uses the less expensive silver salt in place of platinum or palladium salts. Good Kallitypes have a platinum-like quality.” King goes further and says that a well-made kallitype toned with platinum or palladium is “for all practical purposes identical in tonal range and color to a true platinum or palladium print”, and that full toning of a kallitype needs about a quarter as much noble metal as printing in it directly. Whether an expert could really not tell them apart is King’s claim and not a measurement; the cost arithmetic is arithmetic.

Learning the platinotype without spending platinum. Coating, drying, ultraviolet exposure, an oxalate or citrate developer, an acid clearing sequence and a wash are the same operations in the same order for both. King states the overlap directly: “the developers and clearing agents used for platinum can be used for kallitype”. A student who can make a good kallitype can make a palladiotype the following week and will waste far less metal learning to.

Work that is going to be toned. Both of Bostick & Sullivan’s toner kits name kallitype prints on their own title pages — the gold kit “for POP, Vandyke, Kallitype, Albumen and Salt Prints”, whose bath the course publishes as the gold and thiocyanate toner, and the “Palladium Toner Kit for POP, Vandyke, and Kallitype”. King publishes gold, platinum, palladium and selenium toners of his own, and the course’s nearest published entry for the noble-metal route is the platinum toner for printing-out papers, which is the same chemistry a century earlier. Toning here is not decoration: it is the permanence step, it is done before fixing, and it answers two separate faults at once; see Behaviour.

Learning what a developer actually does. The kallitype is the only formula in this course where the same exposed sheet will give you a different colour depending on which tray you put it in. Wall’s 1912 dictionary lists four developers for one sensitiser, “and it will be noticed that on the salt used the tone of the finished print will depend”: black, sepia, warm maroon and purple. The course publishes three of the modern descendants as separate entries — sodium citrate, Rochelle salt and borax — precisely because they are three different pictures and not three ways of doing the same thing.

When another formula is preferable

• For the simplest possible plain-paper silver print, the Van Dyke Brown sensitiser needs no developer, no clearing bath and no ultraviolet box worth the name, and its iron salt is a fraction of the price of ferric oxalate. You give up shadow depth, contrast control and the ability to see the exposure through a developer.
• For a print that will keep without toning, Ware’s argyrotype is the formula designed against exactly this one’s weaknesses. Ware’s criticism of the kallitype is quoted in full under Interactions; his answer replaces silver nitrate with silver sulfamate and runs the whole process acid, so that nothing has to be alkaline and no iron(III) is hydrolysed on the way.
• For permanence beyond argument, print in platinum or palladium. The print-out platino-palladiotype uses the same iron chemistry with a noble metal that does not oxidise, and the redox arithmetic under The mechanism says why the oxalate sensitiser can reduce either metal where a citrate one cannot.
• For the same photochemistry with no silver at all, the classic cyanotype sensitiser needs no fixer and washes in water. If your objection to this page is the silver nitrate rather than the colour, that is the answer.
• For a silver image made from a silver halide, the salted paper sensitiser reaches a similar brown by precipitating silver chloride in the sheet and printing it out. It is the older process, it needs no iron and no developer, and it needs several times as much silver.
• For a historical reconstruction rather than a working print, Nicol’s kallitype is the original arrangement, in which the paper carries only the iron salt and the silver nitrate is in the developer. It is the ancestor of this formula and it is not this formula.

Mixing

Two bottles in the light, one shot glass in the dark. Neither stock is mixed under a safelight and neither is the sensitiser in any meaningful sense until they meet; but the Formulary’s instruction is to mix “under a red safelight or under indirect and low level incandescent lighting”, and since the silver goes into the ferric oxalate at the last moment anyway, the whole of the coating operation belongs in the same subdued light.

1. Solution A. 10 g of silver nitrate into 70 mL of distilled water. Allow it to dissolve, then add water to make 100 mL. That is a 10 per cent w/v silver nitrate solution and the make-up volume is what makes it one.
2. Solution B. 20 g of ferric oxalate powder into 75 mL of distilled water. Allow it to dissolve — which takes far longer than you will expect — then water to make 100 mL. King’s instruction is to mix it about 24 hours before it is wanted.
3. At the bench, for one sheet. Equal parts of A and B in a small vessel, swirled. About 2 mL of the combined solution for an 8 by 10 print; coat and use it at once.

A pipette per bottle, and they never swap. The Formulary supplies two and says so. A pipette that has been in the silver and goes back into the ferric oxalate seeds the ferric oxalate bottle with a silver salt, and the ferric oxalate bottle is the one that has to keep for months.

⚗Why ferric oxalate takes a day to dissolve, and why heating it is the wrong fix

This is the single most frustrating step in the process and it is not a defect in your technique. Ware’s account of the substance is that it “is ill-characterised, evidently polymorphic, apparently uncrystallisable”, and that the one relatively well-characterised form studied by powder diffraction turned out to be a polymer: chains of octahedral iron(III) centres bridged by oxalate, cross-linked into sheets. A polymer does not dissolve like a salt. It dissolves slowly, to a solution of unknown molecularity that is probably a mixture of oligomers and slow to reach equilibrium.

Hence King’s 24 hours, and hence Ware’s own standard preparation, which stirs a 25 per cent solution in the dark at room temperature for about twenty hours and never warms it. Photographers’ Formulary’s limit is the same instruction from the other end: “do not heat the solution (or the sensitized paper when it is being dried) over 50 °C/122 °F”, because “tri-hydrogen ferric oxalate is probably heat-sensitive, but the exact extent is not known.” Warming the beaker to hurry it is trading a known slowness for an unknown loss of photo-activity.

Distilled water throughout, and the sheets say so in capitals. Two of the three reasons appear later in the same documents: a supply that is alkaline will not let the print clear, and hard water carries calcium, which precipitates calcium oxalate in the sheet and promotes exactly the iron(III) hydrolysis the whole clearing sequence exists to prevent. The third reason is that hard water often carries dissolved iron of its own.

🧮What the mixed sensitiser actually contains

The stocks are stated strengths and the ratio is stated, so the mixture can be computed exactly. It is still the course’s arithmetic and not a figure any source prints.

5 % w/v silver nitrate · 10 % w/v ferric oxalate

Nominal strengths in the equal-parts sensitiser

Silver nitrate has a molar mass of 169.87 and silver 107.87, so 10 g of the salt carries 6.35 g of silver per 100 mL of Solution A, and the mixed sensitiser carries 3.17 g of silver per 100 mL. At King’s own coating rate of 2 mL for an 8 by 10 sheet, that is about 63 mg of silver applied to one 8 by 10 print — against about 97 mg for a Van Dyke coated from the Formulary’s kit at its own stated rate. The kallitype is the cheaper of the two in silver per print and the one with the deeper shadows, which is worth knowing before anyone recommends the Van Dyke on grounds of economy.

Do not make up more sensitiser than the sheet in front of you. This is the one instruction all three sources agree on without qualification, and it has three separate justifications, none of which is stated in any of them: the mixture will throw silver oxalate given time (see Safety), the silver nitrate is being asked to sit in an acidic oxalate solution, and any of it left in the glass is wasted silver. The two-bottle arrangement exists so that neither bottle contains what the other attacks.

Weigh the silver nitrate where nothing else is being coated, following the SOP for handling it and the one for weighing a solid. Weigh the ferric oxalate without raising dust, and never over an open bottle of the silver.

Behaviour

It barely prints out, and that is the design working. Bostick & Sullivan state it plainly — “There will be little print out image so timing will have to be done by trial and error or test strip.” Wall’s 1912 description of what you should see is more useful than a modern one: after exposure the paper “presents somewhat the same appearance” as a platinotype, “a faint brown image on a yellow ground”, and “printing should be carried on until the detail is just faintly visible in the densest parts.” The Getty Conservation Institute’s atlas describes the same faint brownish image in the platinotype and identifies what it is made of: photochemically generated ferrous oxalate. A kallitype exposure is judged the way a platinum exposure is judged, by a test strip and by experience, not by looking at a picture.

The image appears in the developer in seconds and the tray time is not for the image. King’s figures are the clearest statement of this in any source read: development is “visually complete in about 15-30 seconds, but a development time of 5-10 minutes is important for archival purposes: much of the residual ferric iron, which if left in the print could cause loss of permanence, is removed at this stage.” Bostick & Sullivan give standard development times of 8 to 10 minutes and offer no reason at all.

The instruction is a century old. Wall’s 1912 dictionary gives the same reasoning in the same order, for a borax and Rochelle salt developer rather than a citrate one: the print “is developed in a dish in exactly the same manner as a platinotype, except that instead of taking the print out as soon as the image attains its full vigour, it must be left in the developer for about ten or fifteen minutes. This is necessary in order to ensure that the iron salts shall be all dissolved. The presence of the iron salts is a most prolific cause of failure.” He adds the reassurance a modern printer needs to hear — “the prolonged immersion in the developer will not produce over-development, providing the exposure has been correctly timed” — and the practical note that a large sheet goes in face down and is turned over at once, with any air bubbles broken by a fingertip.

The two sheets that give a sodium citrate developer disagree by a factor of three about how long to use it, and the disagreement is instructive. King says 5 to 10 minutes and gives permanence as the reason. Photographers’ Formulary says two to three minutes and gives a different reason: “develop for the full two minutes to allow the developer to react with all available iron and silver in the sensitizer.” The course reports both. The reading it offers, which no source states, is that they are optimising different things — the Formulary is optimising the density of the print in front of you, King is optimising how much iron leaves the paper before the print dries, and the two are not the same objective.

The developer must arrive all at once. Both sheets warn about it and the Formulary gives the technique: pour the developer into the tray, tilt it so the solution pools at one end, hold the paper face up clear of the liquid and lower the tray so that the developer runs across the sheet in one movement. “The image will appear almost immediately so this must be done quickly, or else watermarks may appear.” A print that develops in patches has been developed in patches.

A black deposit forms in the developer and must not be allowed to settle on the print. The Formulary describes it as “a residual black substance forming from and next to dark tones”, says to agitate so that it does not stick to the highlights, and notes that it accumulates in the developer and can be filtered out through a coffee filter. No source read identifies it. Given where it forms and what colour it is, finely divided silver reduced in the solution rather than in the paper is the obvious candidate, and that is the course’s guess and not a finding.

The print loses density in the clearing bath and gets it back later. King: “The image will lighten considerably during clearing, but don’t worry because all the lost density will return during toning and fixing.” A printer who has not been told this will over-develop the next one.

It bleaches in the fixer, and the two answers are both listed. Bostick & Sullivan: “Kallitype prints will bleach in the fix, so either tone before fixing or overprint by a stop or two. These are the only effective solutions to the bleaching problem.” The same sheet says why warm water makes it worse, and the Formulary’s New Kallitype sheet adds that acidic water accelerates it. The mechanism is under The mechanism; the practical consequence is that a kallitype’s exposure is set with the fixer in mind, exactly as a print-out process’s is set with the dry-down in mind.

The developer is replenished rather than replaced, and there is a reason to bother. King’s rate is about 200 mL of fresh developer per 500 square inches of print developed, decanting from the top of the bottle and discarding what has settled. His warning is the useful half: “if the developer is not replenished, the accumulation of ferrous iron will make it increasingly difficult to clear the print during processing”, which costs not only a stain in the masked areas but permanence, “because the stain consists in large part of residual ferrous iron.”

Coated paper does not keep. The Formulary is explicit: “Coated Kallitype paper should be used within an hour or two after drying so prepare just as much paper as you will need for a print session.” That is a much shorter life than a Van Dyke coating and it is one of the practical costs of the process.

Drying is a variable with a stated failure mode. King: dry with a fan if you like, “but DO NOT force dry with heat, which may cause fogging.” The Formulary permits a hair dryer on a low setting and repeats its 50 °C limit. Bostick & Sullivan want the coated sheet to rest one to two minutes before drying so that the surfactant pulls the solution into the paper.

Image characteristics

Colour is a decision you make twice. First in the developer: the tartrate bath gives sepia, the borate bath gives black, and the ratio between the two Formulary stocks moves the print along that line, while the 20 per cent sodium citrate developer that King and the Formulary’s New Kallitype kit both use gives what King describes, without enthusiasm, as “a rather unpleasant brown color” — and he adds at once that the subsequent processing “will change final image color quite dramatically”.

Then in the toner: gold gives, on King’s account, “a very attractive purple/brown/blue tone” with contrast raised by about a step through loss of density in the high values; platinum a very neutral black; palladium a brownish black; and double toning, started with the more noble metal and finished with the less, gives split tones — warm highlights and midtones with cool shadows — because “the most noble metal will always replace the least noble” and so gold cannot displace platinum where platinum has already gone.

The reason a silver image has a colour at all is particle size. Ware’s measurement on plain-paper silver prints is that the image consists of spheres of about 10 nm radius, that pure nanoparticle silver of that size appears yellow-orange, and that a thin surface layer of silver sulfide picked up in the thiosulfate bath “deepens the colour of nanoparticle silver to a more satisfying brown”. The kallitype’s browns and blacks sit on that same physics, and the developer changes the colour because it changes how fast the silver is reduced and therefore how large the particles grow.

Maximum density is the kallitype’s advantage over its cousins, and it is worth being careful about how much of one. King reports that “well-made comparison prints side by side show more richness in the shadows of a kallitype than in a Vandyke or argyrotype”, and immediately adds that “the difference is not huge”. The redox arithmetic under The mechanism says which direction the difference should run and why; the size of it is King’s observation.

Tonal scale. Long, and its shape is set after the exposure rather than during it. That is the structural difference from every printing-out process in this formulary: a Van Dyke is self-masking, so its shadows protect themselves as they darken, while a kallitype’s shadows are built in the developer from iron the light put there and can go on getting darker. King’s shadow-depth argument follows directly: in a print-out process the shadows “often appear murky because they are fully exposed before the highlights have a chance to print in.”

Solarisation in the deepest shadows, and a cure for it. King records that “in heavily exposed areas we frequently see tone reversal in untoned kallitypes, that is, with increasing exposure the shadow areas actually get lighter”, calls the effect unpleasant, and says that toning with gold, platinum or palladium “counteracts tone reversal and restores normal tonal values to the heavily exposed shadow areas.” No source read explains the mechanism, and the course does not supply one.

Contrast, and the honest position on it. King says the best negative for kallitype has a density range of about log 1.8 — “a very contrasty negative that will not print well even on a grade #0 or #1 paper” — reachable in-camera by developing sheet film about 50 per cent longer than normal for a grade 2 silver gelatin paper. Every published means of moving the contrast away from that figure, in every source read for this page, is a chromium(VI) salt, which this course does not use at any level; see Variants. What is left, and it is not nothing, is the negative.

Surface. Silver among the paper fibres with no binder over it, matte, with the paper’s texture and colour part of the picture. King’s papers are Crane’s AS 8111, Platine, Bristol 2-ply Rising, Stonehenge Rising and Fabriano Artistico, with the practical test being clearing rather than appearance: “Papers that will not clear completely in about 4-5 minutes should not be used.”

🧮How much of the silver you coat is in the picture

Ware’s analysis is of salt prints rather than kallitypes and the extrapolation is the course’s, but the order of magnitude explains the whole permanence question and the whole waste question.

Of the silver originally taken up by a sensitised salt-print paper, only about 2 per cent remained in the final image, a coating weight of about 0.1 g per square metre. An 8 by 10 inch sheet is 0.052 m², so on that coating weight the image holds about 5 mg of silver.

The arithmetic above put 63 mg on that sheet. Even if a developed-out process is severalfold more efficient than a printed-out one — which no source read establishes — the great majority of the silver you coat leaves in the developer, the rinse, the clearing baths and the fixer. That is why those are silver-bearing waste rather than rinse water, and it is why an image made of a few milligrams of metal with nothing over it is as fragile as Ware says it is: “a piece of silver metal the size of a small pinhead weighs about 10 mg, so it would take three typical prints to supply even this much.”

The mechanism

The kallitype is a siderotype, so the first half of its chemistry is the same as the cyanotype’s and the platinotype’s: ultraviolet light reduces iron(III) to iron(II) at the expense of the organic ligand, which is oxidised and leaves as carbon dioxide. Ware attributes the observation to Döbereiner in 1831 and writes the solid-state reaction as it has been written ever since.

UV + Fe₂(C₂O₄)₃ → 2 FeC₂O₄ + 2 CO₂

Ware's equation for the photolysis of solid iron(III) oxalate

Two electrons cross from oxalate to iron, and the ligand is destroyed doing it. Ware gives the mechanism first suggested by Hatchard and Parker: absorption produces a radical anion by electron transfer from one oxalate ligand to the iron(III), which is thereby reduced, and the radical then goes on to reduce a second, unexcited complex.

The photoproduct is the problem, and it is the whole reason this page needs a developer. Iron(II) oxalate is barely soluble — Ware gives 0.022 g per 100 cc of water — so it stays exactly where the light made it and cannot go and find anything. Ware’s statement for the platinotype applies here without change: the photoproduct “cannot reduce platinum(II) or palladium(II) salts in aqueous solution to the metal unless it is solubilised by complexation”.

FeC₂O₄ + C₂O₄²⁻ → [Fe(C₂O₄)₂]²⁻

Ware: oxalate ions dissolve the immobile photoproduct into a mobile complex

That is what a kallitype developer is. A sodium citrate, Rochelle salt or borax bath supplies an anion that takes the iron(II) into solution; only then can it reach the silver(I) sitting beside it in the paper. Wall’s 1924 formulary opens its kallitype entry with the same statement in one sentence, decades before anyone wrote the complexes out: “the ferrous salts dissolve in the developer, and reduce silver nitrate to the metallic state at the points where the ferric salt has been reduced by light.”

[Fe(C₂O₄)₂]²⁻ + Ag⁺ → [Fe(C₂O₄)₂]⁻ + Ag

The image: one electron from iron(II) to silver(I)

Silver(I) takes one electron, so one iron(II) makes one atom of silver. Ware makes the general point about why these ratios matter — platinum(II) needs two electrons and therefore two iron(II) atoms, and “these ratios are important in formulating the sensitizer solution correctly.”

Deeper: why the same two metals print out in a Van Dyke and do not here

The Van Dyke Brown sensitiser contains iron(III), silver(I) and an organic acid, exactly as this one does, and it makes a visible picture in the printing frame with no developer at all. The difference is one ligand, and it acts in two ways at once.

Mobility. The Van Dyke’s iron is held by citrate, and Ware records that the nature of the iron(II) photoproduct in a citrate system “remains unknown” — but whatever it is, it is mobile enough in the few per cent of water that air-dried paper holds to reach the silver during the exposure. The kallitype’s iron is held by oxalate, and its photoproduct is a solid of stated, very low solubility. It is not that the kallitype is slower to reduce silver; it is that in a dry sheet it largely cannot.

Driving force. Ware tabulates the redox potentials, and they run the other way.

Couple	Potential
Iron(III)/iron(II) trisoxalato complex	+0.02 V
Iron(III)/iron(II) citrato complex	+0.372 V
Silver(I)/silver	+0.80 V
Platinum, tetrachloroplatinate(II)/platinum	+0.73 V
Palladium, tetrachloropalladate(II)/palladium	+0.62 V
Gold(III), tetrachloroaurate/gold	+1.00 V

A reduction runs when the couple being oxidised sits below the couple being reduced, and the gap is the driving force. The oxalato-iron couple is 0.78 V below silver; the citrato-iron couple is 0.43 V below it. That is the course’s subtraction from Ware’s published values, and it is the thermodynamic reason to expect the oxalate route to reduce silver harder and further — which is what King reports when he says a kallitype’s shadows are richer than a Van Dyke’s.

The same table bears on a second question, and it is worth being careful about how much it settles. Ware states that the citrato-iron(II) complex “does not reduce platinum(II) or palladium(II), although it will reduce gold(III) and silver(I), under the printing conditions”, and introduces the table as showing that “the ease of reduction is reflected in the relative values of the redox potentials”. Palladium and platinum are indeed the two hardest of the four to reduce, and against the citrato-iron couple their margins are the two smallest — 0.25 V and 0.36 V, against 0.43 V for silver and 0.63 V for gold.

But those margins are still positive, so thermodynamics alone does not forbid a citrate sensitiser from reducing palladium. Ware’s own qualification is “under the printing conditions”, and he notes separately that these reductions proceed only if the noble metal complex is kinetically labile enough, giving hexachloroplatinate(IV) as the standard case where the thermodynamics permit a reduction that “is too inert kinetically to yield a platinum image within the short time of a few minutes that is available”. The citrate’s own page reaches the same conclusion and declines to offer the potentials as a complete account of the division.

What the oxalato couple has is margin. At +0.02 V it sits at least 0.60 V below every one of the four metals, which is a far larger driving force than the citrate has against any of them. That is the chemical reason the platinotype had to wait for ferric oxalate, and it is why a kallitype and a platinum print are the same operations with a different metal in the bottle while a Van Dyke is not.

⚗Why it prints out a little, and why the little matters

A kallitype does show a faint image after exposure, and the reason is that the sensitiser is not pure neutral ferric oxalate. Photographers’ Formulary states that the ferric oxalate it supplies “contains a slight excess of oxalic acid”, and Ware’s measurement of what such an excess does is that the customary 2 g per 100 cc converts about a tenth of the sensitiser to the trisoxalatoferrate(III) anion — whose photoproduct is the soluble oxalato-iron(II) complex rather than the insoluble solid.

UV + 2 [Fe(C₂O₄)₃]³⁻ → 2 [Fe(C₂O₄)₂]²⁻ + C₂O₄²⁻ + 2 CO₂

Ware's stoichiometry for the photolysis of the trisoxalatoferrate(III) anion

So a fraction of the iron(II) is mobile from the moment it is made, reduces silver during the exposure, and gives the faint brown image on a yellow ground that Wall describes and that a printer uses to judge coverage. It is a tenth of the sensitiser doing what all of a Van Dyke’s does, which is why the image is faint, and it is the clearest available demonstration that the print-out and developed-out routes are one chemistry with a solubility problem in the middle.

This is an inference across two processes. Ware’s tenth is measured on a platinotype sensitiser, and King’s Solution B adds none — which is not the same as saying it contains none, because whatever came in with the powder came in unmeasured. What follows is not trivial: two bottles both labelled “20 % ferric oxalate” may print out to different extents and neither label says so.

Deeper: what the fixer does, and why the print gets weaker in it

The fixing bath is 5 per cent sodium thiosulfate on all three sheets — an eighth of the strength of a plain hypo fixing bath — and it is used for two to five minutes. That is not caution for its own sake. Ware’s three pitfalls of thiosulfate-fixing a plain-paper silver image are all live here.

One: it removes the silver the light did not use. Unreduced silver leaves as the soluble argentothiosulfate complex.

Ag⁺ + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻

Unused silver leaving as a soluble complex

Two: it dissolves the image if you let it. Ware: “the colloidal silver constituting the print-out image is easily oxidised by air in the presence of thiosulphate ions. Over-long immersion in the fixer bath, or the use of too concentrated a solution of thiosulphate, or the presence of oxidising impurities in the water, can cause serious loss of image density if the print has access to the air.” Bostick & Sullivan’s observation that warm water accelerates the bleaching, and the Formulary’s that acidic water does, are two instances of that one sentence. So is Ware’s separate note that silver sulfide forms from thiosulfate and nanoparticle silver “especially under acidic conditions”.

Three: the silver has to be washed out before the thiosulfate arrives. “The excess silver nitrate present must be washed out with water, in which it is highly soluble, before applying the thiosulphate, otherwise the image will be seriously stained with brown silver sulphide.” In this process the developer, the rinse and the clearing baths do that job between them, which is one reason the kallitype’s tray sequence is longer than a Van Dyke’s and not shorter.

Why toning before fixing is the real answer. King’s explanation is the clearest: “images toned before fixing with gold, platinum or palladium will not fade in the fixing bath. The major reason for fading, or image recession during fixing, is bleaching of the silver. An image toned with one of the more noble metals will not fade or recede in fixing because the silver has been replaced with metals that do not bleach.” Ware records King’s stronger claim — that toning “does not just coat or encapsulate the silver particles with the more noble metal, but totally replaces them with it” — and records it as a claim, which is how this course carries it too.

Deeper: the excess iron, and why clearing decides whether the print survives

Iron is why kallitypes have the reputation they have, and the arithmetic shows there is more of it here than the electron bookkeeping needs.

🧮The iron-to-silver ratio, and the width of the uncertainty in it

Silver is easy. 10 g of silver nitrate at 169.87 g/mol is 0.0589 mol per 100 mL of Solution A, so 50 mL of A carries 0.0294 mol of silver.

Iron is not easy, because ferric oxalate has no reliable formula weight. Ware sets four side by side: 375.76 anhydrous, 447.81 as the tetrahydrate, 465.83 as the CRC Handbook’s pentahydrate and 483.84 as the hexahydrate the large suppliers sell. Iron is 2 × 55.845 = 111.69 of each of those, so the iron content of the powder runs from 23.1 to 29.7 per cent by weight.

10 g × 0.231 to 0.297 = 2.31 to 2.97 g of iron = 0.0413 to 0.0532 mol

Iron in the 50 mL of Solution B that meets 50 mL of Solution A

The molar ratio of iron to silver in the mixed sensitiser is therefore between about 1.4 and 1.8 to one — a real excess, where the Van Dyke’s equivalent arithmetic lands between 1.0 and 1.3. And the width of that range is not a rounding: King does not say which hydrate to weigh, and no source read for this page does. Two printers following the same instruction with two suppliers’ powders may put 29 per cent more iron on the paper than each other, which is exactly the kind of uncontrolled variable that makes a process acquire a reputation for being temperamental.

Not all of that iron is reduced by the exposure, and none of what is not reduced belongs in the finished print. Ware’s clearing chapter says what happens to it if it stays: some iron(III) “may bind chemically to the hydroxylic functions of the cellulose”; above pH 4 it hydrolyses to a polymeric colloidal hydroxide that lodges in the fibres; and if it is not removed before the print dries it transforms irreversibly into iron(III) oxyhydroxide, the mineral goethite, “which is quite insoluble in dilute acids”. Calcium makes it worse in both of the places calcium is found — hardness in the water and a chalk buffer in the paper — because calcium oxalate is insoluble and its precipitation drives the iron complex apart.

Ca²⁺ + [Fe(C₂O₄)₃]³⁻ + 2 H₂O → CaC₂O₄ + [Fe(C₂O₄)₂(H₂O)₂]⁻

Ware: calcium strips oxalate from the iron complex and starts the hydrolysis

King’s diagnosis of the same problem is a practitioner’s rather than a chemist’s and reaches the same place: “it is impossible to remove all residual ferrous iron from the paper, and if any at all remains it will eventually cause the silver to oxidize, ultimately leading to fading. This may take several decades but is, I believe, almost certain to happen.” That is the argument for toning, and it is the argument for the EDTA and sulfite clearing sequence that Ware’s own chapter sets out and that the course publishes as its own entry.

Function of every ingredient

Ferric oxalate, 20 g in Solution B, giving a nominal 10 per cent w/v in the mixed sensitiser. What it is: an ill-defined polymeric iron(III) oxalate, sold to photographers far more often as a solution than as a powder, whose water content varies with the supplier and whose formula weight therefore varies with it. Photographers’ Formulary is blunt that “the photographic term ‘ferric oxalate’ is a misnomer, which has given rise to a considerable amount of confusion in the photographic literature”: two commercial forms exist, which it calls tripotassium and tri-hydrogen ferric oxalate, both are photosensitive, but “only the acidic form is sufficiently photosensitive to be useful in photography”, and it does not recommend the green tripotassium solid because its photo-activity is low. Why it is here: it is the only light-sensitive substance in the formula and the only reason the paper responds to light at all. What it does: absorbs near ultraviolet — the Formulary puts the sensitivity around 460 nm — and its iron(III) is reduced to iron(II) while an oxalate ligand is oxidised and lost as carbon dioxide. Photographic consequence: it sets the speed, and it sets the ceiling on how much silver can ever be reduced, because every atom of image silver needed an iron(II) to make it. More of it: faster printing, a higher possible maximum density, and more iron to clear out afterwards — which is the fault that ends kallitypes. Less of it: slower, cleaner highlights, a lower ceiling. What it interacts with: the silver nitrate, with which it can form a precipitate of silver oxalate; alkali, which hydrolyses the iron(III) to the hydroxide; calcium, which strips its ligand; heat, which the supplier limits to 50 °C; and light, which is the point of it and also the reason the bottle is brown. Two warnings from the sheets themselves: the solution “slowly degrades, with a resulting increase in print fog”, so no more should be mixed than two or three months will use; and the Formulary publishes a test any printer can run for exactly that fault, dissolving two crystals of potassium ferricyanide in about 2 mL of water and adding one drop of the ferric oxalate under safelight — good material darkens only slightly and looks yellow-brown to orange in room light, while a blue cast of Prussian blue means iron(II) is already present, and the deeper the blue the worse the material.

Silver nitrate, 10 g in Solution A, giving a nominal 5 per cent w/v in the mixed sensitiser. What it is: the standard soluble salt of silver, molar mass 169.87, colourless crystals that blacken in light and on contact with organic matter, an oxidiser and a caustic. Why it is here: it is the image, and it is in this form for the least interesting of reasons — Ware’s judgement is that “without exception all the iron-silver processes to date have used the most commonly available soluble salt of the metal”. What it does: nothing until the developer arrives. It is not light-sensitive in this formula; it waits for a mobile iron(II) and is reduced to metallic silver, one silver atom per iron(II). Photographic consequence: it sets the maximum density, and the size the particles grow to sets the colour. More of it: the Formulary’s sheet is the only source that comments, and it is careful — “Most research shows that the best print density will result from our recommendation of equal proportions. Some studies show that slightly better densities can be achieved with slightly more silver nitrate solution”, with no proportion named, so none is given here. Less of it: a weaker print and a lower ceiling. What it interacts with — and this is the criticism the argyrotype was built on: the nitrate anion is an oxidiser, and Ware’s objection is that it “tends to dissolve the colloidal image silver during wet processing, especially under acidic conditions”. The clearing bath is 3 per cent citric acid. It also reacts with oxalate to give silver oxalate (see Safety), with thiosulfate to give brown silver sulfide if the silver is not washed out first, with skin protein to give a stain that is metallic silver and cannot be washed off, and with metals, so nothing metallic goes near it.

Water, twice, and it is a make-up volume both times. Why it matters: King’s instruction for each stock is to dissolve the solid in about three-quarters of the final volume and then make up to 100 mL, and that final step is what turns a weight into a stated concentration. All three sources specify distilled water in capitals. What it does here: it is the solvent, it sets both strengths, and it is the reason the ratio is meaningful. What happens with more or less: the proportions are the formula, and a kallitype has no development stage in which a mis-made stock can be compensated — the developer is dissolving iron, not building density. What it interacts with: every later bath, because the pH and the hardness of the water used for the first rinse and the clearing baths decide how much iron leaves the paper, and that decides whether the print is still there in fifty years.

What the formula does not contain, and what each omission costs.

• No wetting agent in either bottle. Bostick & Sullivan add one at the bench: “If you have Tween 20, 1 drop of 10% Tween 20 per emulsion for an 8x10 print may be used as a spreading agent”, and their coating note says the paper should be “liberally coated but not running wet”, then left one to two minutes because “the Tween 20 will help even the coating out and pull it into the paper”. Without it the sensitiser sits on the surface of a hard-sized paper rather than in it.
• King’s Solution B adds no oxalic acid. The Formulary’s supplied solution has some, Ware’s standard preparation has 2 g per 100 cc, and Wall’s 1912 one-bottle sensitiser lists oxalic acid as an ingredient in its own right. What the acid does is established: it makes the polymeric solid dissolve, and Ware measured its second effect, converting about a tenth of the sensitiser to the anion whose photoproduct is soluble. What is not established anywhere the course has read is how much is in a given bottle of commercial “20 % ferric oxalate”, or what King’s carries. Treat it as an uncontrolled variable and record your supplier.
• No contrast control. Every published one is a chromium(VI) salt; see Variants.
• No preservative or humectant of any kind, and no source read for this page suggests one.

Interactions

Silver(I) with oxalate, in the shot glass. The two solutions are kept apart until the moment of coating and this is why. Photographers’ Formulary’s older one-bottle kallitype sheet, which mixes them permanently, says what happens: “It is very common for a precipitate of silver oxalate to form. This precipitate does no harm. If your sensitizer does form a precipitate, just be sure you do not transfer it to the paper you will be coating.”

2 Ag⁺ + C₂O₄²⁻ → Ag₂C₂O₄

Silver oxalate, the precipitate the one-bottle version throws

Every milligram of silver that goes down as the oxalate is a milligram not available to be reduced, and you no longer know the strength of what is left. The two-bottle arrangement is the answer to it, and the reason there is no keeping time on this page for the mixed sensitiser is that nobody keeps it.

Light with the iron(III), which is the exposure, and nothing else in the mixture is doing it. Both stock solutions are light-sensitive on their own — the Formulary says so of both, and stores both in dark brown bottles — but only the iron’s photochemistry produces anything a photographer wants.

Iron(II) with silver(I), which is the picture, and it needs the iron(II) to be in solution. In a dry coated sheet it largely is not, which is what the developer is for.

Atmospheric oxygen with iron(II), which is the competing reaction. Ware’s general statement for the iron processes is that photoreduction alone leaves nothing permanent “because the oxygen of the air will re-oxidise the iron(II) back to iron(III)”. In a kallitype the iron(II) sits in a dry sheet until it is developed, which makes the delay between exposure and development a variable, and no source read for this page quantifies it.

The developer’s anion with the photoproduct, which is development. Citrate, tartrate or borate takes the iron(II) into solution; the choice of anion changes the rate; the rate changes the particle size; the particle size is the colour. That chain is the course’s reading of an observation every source states and none explains, and it is flagged as such on the kallitype process page too.

Alkali with the excess iron(III), which is the design fault Ware names. This is the passage that explains most of the process’s reputation, and it is worth quoting whole:

“But nitrate is an oxidising anion, and tends to dissolve the colloidal image silver during wet processing, especially under acidic conditions. To minimise this loss of image the Kallitype process employs alkaline-buffered developers of high pH, e.g. Borax. Alas, these create a new problem, because they cause hydrolysis of the excess iron(III) in the sensitizer and the deposition of insoluble ferric hydroxide in the image, which ultimately causes it to fade.”

The kallitype is caught between the two. An acid bath protects the iron and attacks the silver; an alkaline bath protects the silver and ruins the iron. King’s method is one way through — a near-neutral sodium citrate developer followed at once by an acid clearing bath, and toning before the fixer so the silver is no longer silver by the time the thiosulfate arrives. Ware’s way through was to design the problem out of the sensitiser altogether, which is the argyrotype.

Alkaline water with the first rinse. King: “If the first rinse is alkaline, ferrous hydroxide compounds may be formed in the paper, making complete clearing difficult or impossible.” The Formulary says the same of the water used before the clearing baths: “Water with a base ph will make the print difficult to clear.”

Iron(III) with cellulose, which is the permanence problem, and calcium with all of it. Set out under The mechanism.

Thiosulfate with the image silver, which finishes it and then destroys it. A little sulfiding deepens the colour; complete conversion drops the density of a nanoparticle silver image by a factor of about thirty on Ware’s measurement.

Gold(III) with free oxalate, if you add a gold salt to the sensitiser. King publishes a gold additive for the coating solution. Ware, writing about a platinum sensitiser built on the same iron chemistry, records that “gold(III) will quite rapidly oxidise any free oxalate ions arising from the partial dissociation of trisoxalatoferrate(III)”, and gives the reaction as two tetrachloroaurate ions oxidising three oxalates to six carbon dioxides and depositing two atoms of gold. He found that in a platinum sensitiser this happens fast enough that the gold “will be decomposed before it can even be coated or exposed, and will simply impart a fog of nanoparticle gold to the paper”. That is Ware’s finding about a different sensitiser, and the course reports it here as a reason to expect trouble rather than as a finding about the kallitype; King’s own warning about metal additives — that “the image is more likely to stain and the print will be much more difficult to clear” — is consistent with it.

Noble metal toners with the image, before the fixer. Both suppliers’ toner kits name kallitype prints on their title pages, King publishes five toner formulas of his own, and every source agrees on the order: develop, clear, tone, then fix.

Variants

⁇Who invented it, when, and what survived

The invention is not in dispute; the process’s reputation is. Ware’s account is that the kallitype was “invented in 1889 by Dr. William Walker James Nicol, an academic chemist at Mason College, Birmingham, and it was named by him as kallitype (Greek: καλλι = beautiful)”. King says the same year and calls Nicol “widely considered to be its inventor”. Wall’s 1912 dictionary credits “Mr. W. W. Nicol” without a date. The course states 1889.

The dispute is over whether the prints lasted. Ware reports that Dick Stevens, surveying many major American collections, “found in total only one historic kallitype”, and that a wide enquiry of Ware’s own turned up only a handful positively identified in collections. He quotes Paul Anderson’s 1913 verdict — “the [kallitype] image is so unstable that the process should be used for only the most ephemeral work” — and Child Bayley’s, that a batch turned out of a drawer “bore no sign to distinguish the front of the paper from the back”. Against that, Ware records Stevens’s objection that Child Bayley’s practice may have been uncontrolled and that Anderson “reported no laboratory tests on the permanence of kallitype, so his condemnatory conclusion does require further substantiation”. A third position, that many surviving kallitypes are misidentified as platinotypes, Ware calls a canard originating in complaints about matte silver bromide paper, with “no substantive evidence”.

What the course says. The chemistry of the failure is well established and is set out above: residual iron(III) oxidises colloidal silver. The historical question of how many nineteenth-century kallitypes survive is contested, and this page does not settle it. What both sides agree on is the remedy that modern practice adopted — thorough clearing and noble-metal toning — and Ware’s own note that “current practitioners of kallitype wisely hedge their bets by toning their prints with platinum, palladium, or gold.”

Nicol’s kallitype, the original arrangement. King’s account is that “in Nicol’s original patent, the print was developed in a silver nitrate bath. He patented several revisions in the early 1890s and in one of the formulas recommends making silver nitrate part of the sensitizing bath rather than adding it to the developer. This last revision is the method used by most contemporary kallitype printers.” So the formula on this page is Nicol’s own second thought rather than a modern departure — on King’s authority, which is the only source read that says it. Where the silver is put is the whole structural difference, and it is the same difference that separates Herschel’s argentotype from the Van Dyke.

The one-bottle Photographers’ Formulary sensitiser, catalogue 07-0070. The older of the supplier’s two kits mixes the whole thing permanently: 2 g of solid silver nitrate stirred into 30 mL of the 20 per cent ferric oxalate, with the instruction that “the sensitizer must ripen for a few (2-3) days before use” and that it should be kept in the darkroom and stirred occasionally, “especially important if a precipitate formed during mixing”. It is a materially different sensitiser and not a repackaging of the one above.

	Two-solution, equal parts	One-bottle 07-0070
Ferric oxalate	10 % w/v	20 % w/v
Silver nitrate	5 % w/v	about 6.7 % w/v
Molar iron to silver	about 1.4 to 1.8 : 1	about 2.1 to 2.7 : 1
Made	in the coating vessel, per sheet	once, and ripened 2 to 3 days
Silver oxalate	little time to form	“very common”, and disregarded
Coating rate	about 2 mL per 8 by 10	about 4 mL per 8 by 10

The percentages and ratios in the right-hand column are the course’s arithmetic against the sheet’s own 30 mL, which is the volume the solid silver goes into and not a make-up volume the sheet states. Two things follow that are worth noticing. The one-bottle version is twice as concentrated in iron and puts about twice the volume on the sheet, so it delivers roughly four times as much iron per print — which, given that residual iron is this family’s characteristic failure, is a real difference and not a formatting one. And the ripening period is the supplier telling you that something happens in the bottle over two or three days; no source read says what.

Metal additives to the sensitiser. King publishes four, at about one part of a working solution to nine parts of sensitiser: gold(III) chloride for “a warm brown-olive tone”, potassium chloroplatinite for a neutral black, sodium chloropalladite for a warm black, and a fourth this course will not describe as a procedure (below). He is careful about the cost: “the employment of metal has one important negative effect. The image is more likely to stain and the print will be much more difficult to clear.” Ware’s finding about gold(III) and free oxalate, under Interactions, is a reason to expect the gold additive in particular to behave badly.

☠The mercury additive, which this course does not give

King’s fourth additive is mercury(II) chloride, for “a warm olive tone”, and he adds that “results can be somewhat unpredictable” and that the solution must be handled “with maximum care because mercuric chloride is a hazardous substance.”

Mercury is one of the substances the course’s Level D ruling covers: it is taught as chemistry and history and never given as a home procedure. No quantity, no working solution and no proportion for it appears on this page, and the course does not recommend the additive at any strength. Mercury(II) chloride is acutely toxic by every route, cumulative, and turns every bath and every rinse downstream of it into mercury waste requiring a licensed disposal route.

If what you want is the olive tone, King’s own gold additive is the nearest published alternative and carries no mercury; if what you want is a warm black, palladium toning after development does it and is the route every modern source recommends anyway.

☠The dichromate contrast control, which this course also does not give

Every source read for this page controls contrast with a chromium(VI) salt, and each puts it in a different place. Bostick & Sullivan supply 25 mL of a 5 per cent ammonium dichromate “contrast booster” to be added to the sensitiser. Photographers’ Formulary’s traditional kit supplies 5 g of potassium dichromate for a 10 per cent solution added to the developer, and its sheet is emphatic in bold type that it goes there and not into the sensitiser. King adds 1 to 16 mL of a 5 per cent potassium dichromate solution per litre of developer, and says that this is what widens the usable negative range from about 1.2 to about 2.2, with too much costing printing time and giving the image “a granular look”. The Getty atlas records the same practice for the platinotype, with potassium chlorate as the other historical oxidant.

The course’s chromium ruling is that chromium(VI) is not used, at any level, anywhere. Potassium dichromate’s notified classification carries twelve hazard statements, among them H340 for genetic defects, H350 for cancer, H360 for reproductive toxicity, H330 for fatal by inhalation and H410 for long-lasting aquatic toxicity, and its own page classifies it safety level D. Photographers’ Formulary’s own sheet acknowledges that “all chromium compounds are potential carcinogens” and that a spillage on skin “will cause a chemical burn, which will appear as ulceration” — and then directs the excess down a drain with copious water, which this course does not follow.

So no dichromate figure is offered as a procedure here, and the honest consequence is that this page publishes no contrast control at all. What is left is the negative: King’s own statement is that the best negative for kallitype has a density range of about log 1.8, reachable in-camera by developing about 50 per cent longer than normal, or set directly when the negative is made digitally. The course does not invent a chromium-free substitute, because no source read establishes one, and Rule 6 forbids guessing at it. Potassium chlorate is not offered either: it is a strong oxidiser with its own fire hazard, the Getty atlas names it for the platinotype rather than the kallitype, and no source read gives a quantity for it in either.

No course variant of these proportions is offered. There is nothing here to make safer by changing the weights — the hazards are the silver nitrate and the oxalate, and reducing either simply gives a weaker print — and the two better-documented alternatives, the argyrotype and a true palladiotype, already exist as separate entries with their own evidence.

Safety

Level B, and two ingredients put it there. The course’s rubric names silver nitrate explicitly under Level B, and ferric oxalate is classified Level B on its own page: the notified GHS classification is thin — one notifier, H302 and H312 — so the course classifies it upward rather than downward, and applies to it the precautions established for oxalic acid and the oxalate salts.

• Silver nitrate is both an oxidiser and a caustic, as both kit sheets put it. Solid on the skin can give a chemical burn, washed with cold water then soap and water and treated as a heat burn. Dilute solution gives a brown to brown-black stain which is “silver metal bound to the protein of the skin and cannot be washed off”; let it wear off rather than attacking it chemically. Never put solid silver nitrate in a wastepaper basket — it is an oxidiser in contact with paper. The handling SOP and the spill SOP carry the procedure.
• Oxalates are systemic poisons and they are the half of this formula people underrate. The Formulary’s own sheet says of the potassium oxalate in its clearing bath that it “is an anticoagulant (prevents blood clotting) and a poison”, and instructs tongs or gloves. Chemical Safety Card 0529 records for oxalic acid that exposure “may affect the calcium balance”, that repeated exposure can produce kidney stones and slow-healing ulcers, and that the substance is corrosive to eyes, skin and respiratory tract. Weigh the ferric oxalate without raising dust, wear nitrile gloves and splash goggles, keep an eyewash within reach, and eat nothing at the bench.
• Gloves at every stage, including coating. The Formulary’s New Kallitype sheet opens its closing notes with “Wear gloves and eye protection when mixing chemical solutions or coating paper”, and adds “Do Not use containers or utensils for food or beverages if they have been used to mix photo chemicals.” The glove guidance covers the choice.
• This is an ultraviolet process and the light is a hazard people forget. Ten to twenty minutes under a 275 or 300 watt sunlamp at 12 to 18 inches is a skin and eye exposure, and a lamp that does not feel hot is worse than sunshine because nothing warns you. Enclose the unit, use a timer, and follow the UV unit SOP. The Formulary adds a second reason to keep the distance: “The lamp generates considerable heat so use care not to place the lamp too close.”
• The dichromate that comes in the traditional Formulary kit and in the Bostick & Sullivan kit is not used at all under the chromium ruling, and the mercury additive is not used at all under the Level D ruling. Both are covered under Variants. If your kit contains a dichromate it still has to be labelled, stored away from anything reducing, and disposed of through a licensed route rather than down a drain.

⚠Silver and oxalate in the same vessel: what the hazard actually is

This formula does something the chemical encyclopaedia otherwise forbids. Every oxalate page in this course carries the same incompatibility line — the NIOSH pocket guide lists silver compounds among the incompatibilities of oxalic acid, and Chemical Safety Card 0529 names explosive silver oxalate as the reason — and here a silver salt and an oxalate are deliberately mixed.

The aggregation is not reassuring on the substance itself. PubChem’s GHS record for silver oxalate carries the signal word Danger with the explosive pictogram, and 104 of 109 company reports give H201, mass explosion hazard, alongside H302, H312 and H410; five of the 109 say it meets no criterion. The precautionary codes attached are the ones written for explosives.

What this course concludes, and what it does not. The incompatibility rule that governs the rest of the encyclopaedia is a storage and waste rule: oxalate bottles and silver bottles live apart, and their wastes never share a container. What this formula does is different in three respects that matter — the two are combined in dilute aqueous solution, in millilitres, and used within minutes. The Formulary’s judgement on the wet precipitate that forms in its one-bottle version is that it “does no harm”, which is a manufacturer’s statement about a manufacturer’s product and is evidence.

No source read for this page evaluates the risk of a dried residue at photographic scale, and the course does not invent one. What it does instead is name the precaution that follows from the uncertainty: do not let this mixture dry to a solid anywhere except in paper. Rinse the shot glass, the pipettes and the brush immediately after coating; wipe and rinse spills at once rather than letting them evaporate; never let a residue accumulate in a bottle neck or a tray corner; and do not scale the mixing up from millilitres. A coated 8 by 10 sheet carries about 100 mg of silver nitrate and 200 mg of ferric oxalate dispersed through cellulose, on the arithmetic under Mixing, and is not the same object as an evaporated puddle in a beaker.

⚠What is not a hazard here, and why

Nothing in this formula is heated and nothing is volatile. Both stocks are made at room temperature — and the Formulary forbids warming the ferric oxalate above 50 °C for reasons of photo-activity rather than safety. No step evolves a gas you could smell. Carbon dioxide is the only gas the chemistry produces, it is liberated inside the paper over a quarter of an hour, and its total amount is bounded by the iron on the sheet — at most about a millimole for an 8 by 10, on the arithmetic under The mechanism, and that only if every iron atom on the sheet were reduced, which only a fraction ever are. Ventilation is therefore not the governing control here, and the reason to have an openable window or ordinary extraction is dust while weighing rather than vapour while working.

There is no cyanide chemistry, which distinguishes this bench from a cyanotype’s, and the ferricyanide that appears on this page appears only in a two-crystal quality test in a shot glass.

There is no strong acid or alkali in the sensitiser as mixed. That changes downstream: the Formulary’s traditional kit uses 28 per cent ammonia in its fixing bath and says to mix it in a well-ventilated area and never to sniff the bottle, which is the one genuinely respiratory hazard in the whole process — and it belongs to the fixer, not to this page.

Storage

Two brown bottles, dark and cool, each labelled with its strength, its supplier and the date it was made. The Formulary’s instruction is that both solutions are light-sensitive, that both are supplied in dark brown bottles “to prevent the solutions from losing their photosensitive properties”, that neither should stand with the cap off, and that if they are not to be used in one session they go in a cool dark place.

The ferric oxalate solution is the one with a life. King: the powder “lasts indefinitely, but once mixed with water will slowly degrade, with a resulting increase in print fog. To avoid this fogging, mix no more solution than you expect to use in two to three months.” That is the only keeping figure any source read gives for either bottle, and the fog it names is the reason to run the ferricyanide test under Function of every ingredient before mixing a batch of sensitiser rather than after printing with it.

Buy the powder rather than the solution if you can weigh it, on that reasoning — and accept in exchange that you will not know which hydrate you have.

Do not store the mixed sensitiser at all. No source publishes a keeping time because no source keeps it, and the reason is under Interactions.

Coated paper: an hour or two, and no more. The Formulary states it directly and the practical consequence is a workflow one — coat what the session will print and no more. Neither the light nor the air is the whole reason; a sheet carrying a silver salt and an iron salt in intimate contact is a sheet in which slow reactions are happening in the dark, and no source read for this page says which of them governs the two-hour limit.

Keep the silver nitrate away from everything, in dark glass, tightly closed, away from organic material and away from the oxalate. The storage rotation SOP covers the general practice and labelling the rest.

Incompatibilities

Alkali, at every stage after the exposure. Above pH 4 iron(III) hydrolyses to a colloidal hydroxide that lodges in the fibres and will not come out once the print has dried. King’s warning about the first rinse and the Formulary’s about the water before the clearing baths are the same chemistry arriving twice.

Buffered paper. An alkaline reserve of calcium carbonate in the sheet is that hydrolysis waiting for you before you have coated, and the calcium brings its own problem: calcium oxalate is essentially insoluble, so a chalk-buffered paper and an oxalate sensitiser are chemically at odds. Ware decalcifies such papers in dilute acid and washes them before coating.

Hard water, for the same reason, and because it often carries dissolved iron of its own.

Ordinary photographic fixer. The Formulary’s traditional sheet states it plainly: “Do not use a standard photographic fixing bath; the finely divided, unprotected silver metal will be etched from the print.” The New Kallitype sheet is more permissive and says so carefully — “the available literature does not recommend it but we suggest that a weak alkali… or ph neutral… fix solution diluted from 1:9 (10%) to 1:19 (5%) is suitable if a sodium thiosulfate fix is unavailable” — which is a supplier hedging rather than a contradiction. A rapid fixer at working strength is several times too strong and an acid one adds Ware’s second problem on top of the first.

Metal, in trays, tongs, clips and bottle caps. H290, may be corrosive to metals, appears in silver nitrate’s aggregated classification, on a minority of notifications rather than as a consensus, which is why its own page records the percentage; and a silver solution in contact with a base metal will simply plate out on it. The Formulary warns separately that a dichromate solution corrodes a metal cap.

Every other alternative process on the same brush. A brush that has coated a cyanotype carries iron and ferricyanide; a brush that has coated a Van Dyke carries citrate. Cross-contamination between alt processes is a real and diagnosable fault, and the cheap answer is one brush per process, labelled.

Potassium ferricyanide and any silver bleach, which will attack the image on contact. The two crystals used for the ferric oxalate test go in a shot glass that is then rinsed away from everything else.

Oxalate waste and silver waste, which are the same waste here and are dealt with next.

Waste

Almost every bath carries both metals, which makes the segregation rule awkward and worth stating carefully.

The developer carries iron dissolved out of the paper and silver that was reduced in solution rather than in the sheet — King’s own reason for replenishing is the iron accumulating in it, and the Formulary describes the black deposit that settles out of it. It is reused across a session and discarded at the end, so the volumes are small and concentrated.

The first rinse and the clearing baths carry the bulk of the iron, as ferrous and ferric complexes of citrate or citric acid, along with unreduced silver on its way out of the paper.

The fixer carries the rest of the silver as the argentothiosulfate complex, in a bath discarded after each session.

The compatibility rule the encyclopaedia states everywhere — that oxalate waste and silver-bearing waste never share a container — cannot be followed literally here, because this process produces waste that is both. What the course does instead is take the more restrictive route for the mixture: collect it as silver-bearing waste, keep it away from strong oxidisers, keep the container closed so that it cannot evaporate towards dryness, and do not decant it into a general oxalate bottle that also receives spent platinum developer. The reasoning is under Safety and the general practice is in the segregation SOP and the general chemical waste SOP.

Silver is worth recovering rather than discarding; silver recovery is the reason, and the arithmetic under Image characteristics is the scale.

Nothing goes on a garden, into a soakaway or into a watercourse. Silver nitrate’s aggregated classification carries H410, very toxic to aquatic life with long-lasting effects, on 99.8 per cent of notifications and H400 on 99.6 per cent; silver oxalate’s carries both as well. Silver salts are among the most aquatically toxic things a darkroom produces.

Jurisdiction governs. The course publishes no jurisdiction-specific disposal instruction anywhere and makes no exception here; the kit sheets’ own advice to wash solids down a drain with copious water is not followed. Label what is in the container, follow the disposal ruling, and check your local regulations.

Troubleshooting

The highlights are yellow or grey and will not clear. The family’s signature fault, and there are two different residues in it. Unreduced silver comes out in the developer, the rinse and the fixer; iron chemisorbed to the cellulose does not, and needs an acid or chelating bath while the print is still wet. Check first that the water is neutral or slightly acidic and not hard. The full entry carries the EDTA and sulfite sequence. The Formulary adds a paper diagnosis: “If the print is taking longer than 10 minutes to clear your paper is probably too absorbent and it will be necessary to size it before applying the sensitizer”, and King’s limit is stricter still at four to five minutes.

Clearing got harder as the session went on. The developer, not the paper. King: “if the developer is not replenished, the accumulation of ferrous iron will make it increasingly difficult to clear the print during processing.”

The print bleached in the fixer. Expected, to a degree, and the two published answers are to tone before fixing or to overprint by a stop or two. If it has gone further than that, suspect a fixer over 5 per cent, a bath warmer than about 20 °C, acidic water, or too long an immersion. The dedicated entry covers the chemistry.

Brown staining that appeared the moment the print entered the fixer. Silver that had not been washed out meeting thiosulfate, which gives silver sulfide directly. Lengthen the rinse before the fixer rather than shortening the fixer.

Streaks and watermarks in the direction the developer ran. The developer did not arrive all at once. Blotchy kallitype development is the entry, and the Formulary’s tilted-tray technique under Behaviour is the fix.

Black specks in the highlights. The deposit the Formulary describes forming next to the dark tones and settling where it is not wanted. Agitate; filter the developer; renew it sooner.

The print is grainy and the exposure has got much longer. If a dichromate is in your developer, that is King’s own description of too much of it — and it is a reason to stop using it quite apart from the course’s chromium ruling.

Blotchy or streaked coating. Paper and technique rather than chemistry. The entry covers the mechanics; the sources’ own advice is a light touch, a good hake or artists’ brush, and a surfactant on a hard-sized sheet.

The whole print is weak however long it is exposed. Check, in this order: the age of the ferric oxalate solution, using the ferricyanide test; whether the coated sheet stood for more than an hour or two before printing; whether it was force-dried with heat; and whether the negative is anywhere near the density range the process needs.

Overall fog, with the masked areas grey rather than white. Two candidates and they are distinguishable. Iron(II) already present in the ferric oxalate solution before you coated will show in the ferricyanide test as a blue cast. Heat during drying is the other, and King names it: “DO NOT force dry with heat, which may cause fogging.”

The sensitiser has gone cloudy in the glass before you could coat. Sensitiser gone cloudy is the entry; on this formula the first candidate is silver oxalate, and the answer is to mix smaller amounts and coat faster rather than to filter and hope.

Two batches of ferric oxalate behave differently. They may genuinely be different substances: the iron content of the powder runs from about 23 to 30 per cent depending on the hydrate, and no supplier is obliged to tell you which you have. Record supplier and batch beside your exposure times and re-test whenever either changes.

The print faded in a drawer. Residual iron oxidising the image silver is the leading cause and it is a clearing failure rather than a fixing one. Residual thiosulfate is the second. An untoned kallitype is, on King’s view, eventually going to do this whatever you do.

Experiments

Weigh the same nominal solution two ways. Make Solution B from a supplier’s powder, and a second one from a different supplier’s, at the same 20 g per 100 mL. Coat one sheet with each, expose them together under one step tablet, develop and clear together, and read the results. If the hydrates differ the iron loadings differ by up to 29 per cent, and this is the cheapest way to find out whether that is visible. It is also the experiment that tells you whether your exposure times will survive changing supplier.

Add oxalic acid to Solution B and see what it buys. Three bottles from one powder: King’s as published, one with 1 g of oxalic acid per 100 mL, one with 2 g, which is Ware’s own standard for a siderotype solution. Compare how long each takes to dissolve, how much print-out image appears in the frame, and where the maximum density lands. Ware’s prediction, from the platinotype, is that the acid raises the print-out fraction; whether it also changes the developed result on a kallitype is the gap this page has had to leave open.

Test the developer time honestly. One exposure, five identical sheets, developed in the same fresh sodium citrate for 30 seconds, 1, 2, 5 and 10 minutes, then cleared, fixed and washed identically. Read the maximum density and, separately, how long each takes to clear. If King’s argument is right the density will plateau early and the clearing time will go on falling; if the Formulary’s is right the density will still be climbing at two minutes. This is a disagreement between two sources that one afternoon can settle for your own materials.

Tone before and after the fixer. Two prints from one exposure, one toned before fixing and one after, in the same fresh bath for the same time. Read both wet and dry. Every source instructs the first order; this is how you see what the second costs.

Time the fixer against density. Four prints from one exposure, fixed for one, two, five and ten minutes in the same fresh 5 per cent bath. You should see the density fall monotonically, unlike a print-out process where it improves first. Finding your own point of acceptable loss is worth more than any published number.

Water against water, and then wait. Three prints from one exposure, cleared in citric acid but rinsed first in tap water, in distilled water, and in tap water with enough citric acid to bring it to about pH 4. Judge the highlights at once, and again after a month on a windowsill. This is the permanence experiment of the whole iron-silver family and it costs an afternoon plus a month of patience.

Compare it against its cousins. One negative, three prints: this sensitiser, a Van Dyke Brown and a classic cyanotype. Measure the exposure scale and the maximum density of each and record the silver each one cost. King’s claim that the kallitype has the deeper shadows is testable in an afternoon, and the arithmetic under Mixing says it should also be the cheaper of the two silver prints.