Gold-toned and palladium-toned kallitype

Toning an iron-silver print is not a colour decision that happens to help. It is a permanence decision that happens to change the colour, and the reasoning is set out in full on chloroauric acid’s page. This entry covers the two baths the alternative-process trade actually sells for a kallitype or a Van Dyke Brown, and the accessibility route the curriculum builds on the second of them.

The chemistry

Why a colloidal silver image needs help. Ware’s account is a lesson in particle physics as much as chemistry. A printed-out or lightly developed silver image is nanoparticulate, so its surface area relative to its mass is enormous, which makes it a conspicuous target for anything that attacks silver — hydrogen sulfide and sulfur dioxide from the air, and even the sulfur-bearing amino acids methionine and cysteine present in albumen and gelatin. Complete conversion of that silver to silver sulfide is what gives so many early photographs their putty-coloured, faded look. Toned with a noble metal, the silver is partially replaced at the particle surface by a metal that does not tarnish.

Gold, and the arithmetic that decides the bath. Gold in the commercial acid is gold(III). Silver oxidises only to silver(I), so each gold atom deposited costs three silver atoms — the print bleaches faster than it tones, and Reilly describes the result of a plain acid gold chloride bath as flat, lifeless and reddish. Every useful gold toner therefore works with gold(I), where the exchange is one for one and the loss is a third as great. Gold(I) is not a stable state you can buy, so making a toner is mostly the business of reducing the gold(III) you can buy. Ware distinguishes two routes: sulfur ligands — thiosulfate, thiocyanate or thiourea — which both assist the reduction and stabilise the product, and a plain mild alkaline buffer, in which gold(III) slowly oxidises water instead over as much as twenty-four hours.

Palladium, which is gentler. Where a platinum toner needs an acid bath and still attacks the silver image, a palladium toner is mild: the trade kit for printing-out paper, Van Dyke and kallitype is nothing but 1 per cent citric acid with seven to fifteen drops of palladium solution per litre, worked by inspection with continuous agitation and refreshed after ten to twenty-five prints. That is a very dilute bath by any standard in this atlas, and the difference in strength between it and a platinum toner is the reason the palladium version is the one a course can teach.

Historical workflow

Print, wash, tone, clear, fix, wash — and the order matters, because toning leaves silver chloride or another silver salt in the paper, so toning is always followed by fixing.

The gold bath tells you when it is ready. Reilly’s reading of it is the practical one: the acid stock solution is yellow and relatively inactive, and as alkali converts it to the active state it goes colourless, and that decolorisation is the best guide to the condition of the bath. A sodium acetate toner generally needs twenty-four hours to ripen. Made too alkaline it tones faster but dies faster, and can end up holding plenty of gold while toning nothing at all — with no sign of it other than the toning stopping.

Strength is matched to the paper rather than to the printer’s patience. Reilly gives 0.1 to 0.2 g of gold chloride per litre for matte salted papers, which are porous and tone quickly, and 0.4 to 0.5 g per litre for glossy albumen, with toning by inspection over three to fifteen minutes at 17 to 20 °C and constant agitation. Too strong a bath deposits gold superficially, and the layer is then largely removed in the fixer — an expensive way to achieve nothing. A modern thiocyanate kit works at a comparable strength: 50 mL each of a 2 per cent ammonium thiocyanate solution and a 0.2 per cent gold chloride solution in a litre of water, replenished at roughly 5 mL of gold stock per print, with the gold momentarily turning brown or orange on first mixing before it redissolves — which is the visible sign that the ligand has done its job.

The image

Gold shifts a warm print colder, towards purple and blue-black, and the shift is the same particle-surface effect that gives a gold-toned albumen print its rich purplish brown. Thiocyanate toners, introduced in 1867, achieve a more complete substitution of gold for silver and therefore a colder tone, at the price of consuming more gold.

Palladium gives a smoother, browner result. Ware’s general description of palladium as an image metal — lower in contrast, browner, very smooth — is about a palladium print rather than a toned one, and the course does not transfer the claim wholesale; what the toner sheets support is that the bath is gentle and worked by inspection.

Partial toning is the normal case, and it matters more than it sounds. Conversion is proportional to time and dilution, so a toned print is usually a mixture of toned and untoned silver in proportions the printer chose without necessarily knowing it. Moersch says as much of selenium on a silver gelatin print — after six minutes not all the silver has been transformed — and the same logic applies here. Protection is proportional to conversion, and conversion is partial.

Permanence

This is the process’s entire justification, and the evidence for it is old and empirical.

The Photographic Society of London’s Fading Committee, set up in 1855 after albumen prints began deteriorating within five years of the process being published, reported the causes as imperfect washing, an old or insufficiently acidic hypo bath, acidity in the mounting materials, and excessive moisture and sulfur in the atmosphere — and recommended thorough washing and toning with gold chloride. A nineteenth-century committee arrived, from failures in the field, at the two controls this course still teaches, a century before anyone could photograph a nanoparticle.

Two cautions belong beside the claim.

A superficial gold layer is not protection, because the fixer takes most of it away. Reilly’s warning about too strong a bath is a permanence warning as much as an economic one.

Toning does not fix the clearing. Residual iron in the paper is untouched by a gold or palladium bath, and it is the iron-silver family’s characteristic fault. A beautifully toned print that was cleared badly is a beautifully toned print that will stain.

Hazards

Chloroauric acid — the substance sold as “gold chloride” — is deliquescent and corrosive, and its encyclopaedia entry classifies it Level B. Note that it is not gold(III) chloride: the true chloride is 64.93 per cent gold against 50.01 per cent for the acid trihydrate, so weighing one where the formula meant the other puts in about thirty per cent more gold than intended. The gold(III) chloride page works the arithmetic.

Ammonium thiocyanate is Level B. Its datasheet says it contains no cyanide, and its older classification says an acid will make some; the entry states both.

Palladium salts are Level B and classified as skin sensitisers, despite palladium’s reputation beside platinum.

The baths are dilute and the metals are expensive, which is its own control: a process where the toner is refreshed after ten to twenty-five prints is a process nobody splashes. The spent bath carries silver and a noble metal and is collected rather than discharged.

Where the course teaches it

Part XXIV, in the Level B lab “Clearing, Toning and Making an Iron-Silver Print Last”, which closes the practical sequence of that part. Part XX owns the toning chemistry in general, with separate lessons on gold, iron-blue and the other metal toners. Neither part is yet written.

And Part XXV names it as the accessibility route. That part’s own overview states it plainly: the route through palladium alone rather than platinum, small prints, and the palladium-toned kallitype of Part XXIV as a legitimate substitute, with students taking that route able to complete every page except two labs. That makes this entry more than a variant. It is the course’s answer to the objection that noble-metal printing is only for people who can afford noble metal, and it is written into the curriculum design rather than offered as a consolation.