Van Dyke Brown

Van Dyke Brown is the printing-out member of the iron-silver pair and the kallitype is the developed-out one. They are close enough that the names are used loosely in the field, and the difference between them is exactly one tray — which is why this pair is the clearest demonstration in the course that printing out and developing out are a choice within one chemistry rather than two different chemistries.

The chemistry

Three weighed chemicals and a starch. Photographers’ Formulary’s kit is ferric ammonium citrate (green) 9.0 g, tartaric acid 1.5 g and silver nitrate 3.8 g, dissolved as three separate solutions in 33 mL of distilled water each and only then combined, in the order A, B, C — because the separate solutions are not light-sensitive and the mixture is.

The photochemistry is the siderotype’s. Ultraviolet light acting on an iron(III) salt of an organic acid transfers an electron from the acid’s anion to the metal, giving iron(II) and carrying the acid off as carbon dioxide; the iron(II) is then a reducing agent strong enough to throw silver out of solution as the metal. The silver is not light-sensitive at all, and Ware’s redox table says why the citrate can reach it: silver’s couple is at +0.80 V and the citrate photoproduct at +0.372 V, which is enough for silver and gold and not for platinum or palladium.

⚗Why tartaric acid is there, and the honest gap beside it

Ware’s principle is that the key compound in an iron imaging system is usually an iron(III) salt of citric, tartaric or oxalic acid, and that the acid is not a spectator that happens to be acidic but half of the light-sensitive compound: two carboxylate groups and two hydroxyls on four carbons give tartrate several oxygen atoms to close around an iron(III) ion, and a chelated iron(III) is both soluble and photoactive where a bare one would hydrolyse and precipitate.

What no source read for this course states is why the Van Dyke formula uses tartaric acid rather than more citrate, when the iron is already supplied as ferric ammonium citrate. The kit gives the weight and says nothing about the reason. Practitioners’ notes circulate claiming that varying it shifts contrast and image colour, and the course found no manufacturer, standard text or conservation source that tests the claim. Treat the acid’s presence as well attested and its precise function as open — the tartaric acid page says the same.

The sensitiser is only as good as the citrate, and that is a real limitation rather than a caveat: ammonium iron(III) citrate is not a compound but a family of complexes whose iron content ranges from 14 to 28 per cent, so two printers working apart cannot be certain they hold the same substance.

Historical workflow

Coat under subdued light, dry, contact-print under ultraviolet until the image is out, wash, and fix in a very dilute thiosulfate bath. There is no developer, which is the whole of the difference from a kallitype, and it means the print can be judged by inspection.

Ware’s argyrotype of 1991 follows the same instructions and changes the silver salt, and the comparison is instructive rather than merely alternative. He replaces silver nitrate with silver sulfamate, made in situ, on the grounds that nitrate is an oxidising anion which tends to dissolve the colloidal image silver during wet processing, especially under acid conditions; the whole process then runs at pH 3.5 in a single-bottle sensitiser with a long shelf life, whose contrast is controlled by added acid. He uses 22 g of the green citrate per 100 cc. Sulfamic acid’s page carries the reasoning.

The image

Colour. Warm brown, and the mechanism is particle size rather than any added substance. Ware’s argyrotype notes state that the colour of a silver print is determined by the size of the metal particles, which is affected by the humidity of the coating during the exposure — which is why the careful modern practice controls humidity with a saturated-salt enclosure rather than leaving it to the room. The silver page carries the general argument.

Scale. A print-out scale is long and self-masking: the darkening shadows inhibit their own further darkening, so a longer density range in the negative can be accommodated by exposing longer. That is the same property the print-out platino-palladiotype trades on, and it is why print-out processes want a negative made for them rather than one made for an enlarging paper.

Surface. Silver among the fibres, no binder, matte. The paper is part of the picture.

Permanence

The iron-silver family has a reputation for impermanence, and the course’s reading — the same one it takes about the salted paper print — is that the reputation belongs mostly to short clearing and washing rather than to the chemistry.

Three mechanisms are worth separating, because the remedies are different.

Residual iron. Iron left in the paper stains it yellow and goes on doing chemistry there. This is a clearing problem, not a fixing one, and it is the family’s characteristic fault; residual iron is the diagnostic term and the troubleshooting atlas carries the confirming test.

Residual thiosulfate. The fixer is very dilute for a reason, and the salt print’s evidence applies: Ware’s account of the Scottish calotypists attributes their prints’ good survival to dilute hypo and scrupulous washing, and the fading of the Reading prints to residual thiosulfate. Over-fixing a print-out silver image also costs density, because colloidal silver is easily oxidised in the presence of thiosulfate.

The anion. Ware’s argument that nitrate attacks the image during wet processing is a design criticism of this formula, and the argyrotype is what he did about it.

Toning is the countermeasure the family shares with the salt print: gold or palladium plates the particle surface with a metal that does not tarnish. Toned kallitype covers the baths, and they are used on Van Dyke prints in the same way.

Hazards

Silver nitrate sets the level. The rubric names it explicitly under Level B; it stains skin and everything else, and the stain is the least of it.

Ammonium iron(III) citrate is Level A, an irritant, with splash goggles preferred for weighing the solid because nearly nine in ten classifying notifiers give the serious-eye-irritation statement. It is also deliquescent, and its stock solution is an excellent nutrient medium for moulds.

Tartaric acid is Level B on its own page.

The wash and the dilute fixer are silver-bearing and belong to the silver recovery stream. The sensitiser must not go onto a garden, into a soakaway or into a watercourse.

Where the course teaches it

Part XXIV, Iron-Silver Processes, in Lab: Printing a Van Dyke Brown at Level B — three solutions combined in an order that matters, which is the point this entry’s chemistry section makes and the lab makes you feel.

The lab and this entry agree on the formulation, which is worth saying because the entry was written before the part existed: ammonium iron(III) citrate (green) 9.0 g, tartaric acid 1.5 g and silver nitrate 3.8 g. The lab adds two things this entry did not hold. It names the one raised step of the session — weighing 3.8 g of solid silver nitrate — and separates it from the rest of the handling rather than averaging the hazard across the bench. And it records that the green grade of the citrate runs 14 to 18 per cent iron by weight with no supplier obliged to say which, so the supplier and lot go in the record: a sensitiser whose iron content is a range is a sensitiser whose speed is a range.

The Siderotype Principle: Iron Reduces Silver is the lesson underneath it, and it supplies the mechanism this entry assumes. The reaction is barely favourable for the free ions and comfortably favourable once the iron is wearing a ligand, so the ligand is the answer rather than the metal; print-out and develop-out sit in the same family and differ by one tray; and the absence of amplification is quantified rather than asserted.

Contrast Control in Iron-Based Printing ranks what actually moves contrast here and is candid that the traditional strongest lever is a chromium(VI) compound the course teaches in full and does not hand over. Lab: Clearing, Toning and Making an Iron-Silver Print Last is where this entry’s permanence claim is tested rather than asserted — getting the iron out and proving it has gone — and Break/Fix: The Iron-Silver Print That Would Not Clear works six failures as diagnoses, including the point at which the honest answer is to throw the bottle away.

And the argyrotype is where this process goes when its clearing has annoyed you enough. The siderotype lesson gives Ware’s attack on it in full: the fault is the nitrate, an oxidising anion present only because silver nitrate is the cheap soluble silver salt, and silver sulphamate made in the beaker from sulfamic acid and silver(I) oxide replaces it — which buys a single-bottle sensitiser that keeps, wet processing in water with a little citric acid, and contrast adjustable by acid alone. Part XXIV explains why it is not a lab: the silver oxide is not a photographic-supplier chemical, and the convenience only means something once you have felt its absence.

Two earlier parts still bear on it: the fixing chemistry of Part XI and the washing and permanence work of Part XII, both of which the very dilute thiosulfate bath here depends on being understood.