Gold toned silver gelatin print

Gold does three different jobs on a silver gelatin print and Kodak’s own sheet keeps them apart. Gold Toner T-21 plates warm-tone papers to a range of browns and tones highlights and shadows at a uniform rate. Gold Protective Solution GP-1, with sodium thiocyanate, changes the image tone only slightly and is used for protection. Blue Toner T-26 pairs gold with thiourea and tartaric acid and tones the highlights first. Chloroauric acid owns the substance, and GP-1 and T-26 the two formulas the course holds.

The chemistry

Why a silver image is worth plating. Ware’s account is a lesson in particle physics as much as chemistry. A 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 gold, the silver is partially replaced by elemental gold, which coats the particles and protects them.

⚗Why every useful gold toner is a gold(I) bath

Gold in the substance photographers call “gold chloride” 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 such as acetate, borate, carbonate or chalk, in which gold(III) slowly oxidises water instead, over as much as twenty-four hours.

Note the by-product on the printing-out papers: toning leaves silver chloride in the paper, so toning is always followed by fixing.

And the ligand is also dissolving the image. This is not a defect and it matters. The formation constant for the tetrathiocyanatoargentate ion is 1.2 × 10¹⁰ — about a thousand times the diamminesilver ion’s — which is strong enough to dissolve finely divided silver. A thiocyanate gold toner both plates gold onto the image and etches the image it is plating. Kodak’s ten minutes for GP-1 is a published time rather than an arbitrary one.

Historical workflow

Fix, wash, tone, wash — and the gold bath is mixed for the job rather than kept. Kodak’s instruction to mix GP-1 right before use is the practical form of the chemistry: a reduced gold bath is a bath quietly on its way to precipitating metallic gold.

Reading a gold bath is a skill and Reilly explains it. The acid stock solution is yellow and relatively inactive; 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, not to the photographer’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. Too strong a bath deposits gold superficially, and the layer is then largely removed in the fixer.

The history runs from the daguerreotype forward. Ware records Fizeau as usually credited with discovering the gilding of daguerreotypes, announced by Arago on 23 March 1840 — while noting evidence that Himly at Göttingen had used the idea by 19 October 1839. The word toning did not enter the vocabulary until about 1855; before that, photographers spoke of colouring or gilding. The daguerreotypists’ sel d’or passed to paper printing after 1850; alkaline gold toning replaced it around 1855, and thiocyanate toners followed in 1867, achieving a more complete substitution of gold for silver and therefore a colder tone, at the price of consuming more gold.

The image

The protective bath barely changes it. Kodak’s end point for GP-1 is a very slight shift to a bluish black — ten minutes at 20 °C, or until you can see that much. That is unusually specific about how little should happen, and it is what makes gold the toner of choice where the print is not supposed to look toned.

T-21 browns a warm-tone paper, and tones highlights and shadows at a uniform rate, which distinguishes it from the blue toner.

T-26 tones the highlights first and the shadows last, and it is slow: Kodak publishes eight to forty-five minutes at 20 °C and two to fifteen at 38 to 40 °C. A five-to-one range at each temperature is not vagueness; it is what a bath that works this gradually looks like when the times are published honestly. The bath exhausts fast and unevenly across a run — the first few prints tone the most and the last few may not tone at all.

Gold in an emulsion is a different use again, and a different salt. Ilford’s 1946 patent by Waller, Collins and Dodd adds small quantities of a soluble gold salt before or during digestion, at a pH not greater than about 8, for a considerable increase in sensitivity. The quantities are so small that they make no appreciable difference to the tone of the developed image, and too much gold loses the speed gain rather than increasing it. That is Part V’s subject, not this one, and the salts the patent names are not the toning reagent.

Permanence

The claim is old, empirical and better attested than most. The Photographic Society of London’s Fading Committee of 1855 reported the causes of albumen print deterioration and recommended thorough washing and toning with gold chloride — a century before anyone could photograph a nanoparticle. Kodak’s modern general claim is that toning converts the image to an inert compound and that all its toners protect the image whether or not they produce a colour shift.

Two qualifications the course insists on.

A superficial gold layer is not protection. Reilly’s warning that too strong a bath deposits gold superficially and the layer is then largely removed in the fixer is a permanence warning as much as an economic one.

The thiocyanate is etching while the gold plates. A protective bath that also dissolves finely divided silver is a bath whose published time is part of the specification, and running it longer is not running it safer.

Hazards

Level B. Chloroauric acid — the substance sold as “gold chloride” — is deliquescent and corrosive, and its encyclopaedia entry classifies it Level B.

One arithmetic trap worth repeating. Gold(III) chloride, Au₂Cl₆, is 64.93 per cent gold; hydrogen tetrachloroaurate trihydrate is 50.01 per cent. Weigh a gram of the true chloride where the formula meant a gram of the acid and you have put in about thirty per cent more gold than intended — an expensive error in the direction that also over-tones the print. The gold(III) chloride page works it through.

Thiourea, in the blue toner, is classified at Level C in this course’s encyclopaedia, which is why T-26 is published at Level C and the protective bath at Level B. The odourless alternative to a sulfide bath is not a way around the sulfide question.

Sodium thiocyanate is a silver solvent, so the spent bath carries silver as well as gold; both are worth recovering and neither belongs in a drain.

The cost is a control in itself. Gold is expensive enough that a bath is mixed for a job and used by inspection, which is a habit worth more than most written precautions.

Where the course teaches it

Part XX, Toning Chemistry, principally in Gold, Iron-Blue and the Other Metal Toners. That lesson opens with the arithmetic this entry assumes rather than states — what it costs in silver to put one atom of gold on a print — and then makes the argument that decides which bath a printer can actually control: the complexing agent rather than the gold. It distinguishes the three published gold routes on that basis, treats gold on an already sulfide-toned print and why the order matters, and closes on where gold toning is used today and what it costs.

Assignment: The Toned Print, Level B, closes the part and is where a gold toner stops being a chapter and becomes a decision: two prints from one session, one held back untoned as the control, the exposure compensated from your own measurements, and a permanence claim you could defend to a stranger.

Two more pages of the part matter to this entry. What a Toner Actually Does to a Silver Image places gold under replacement and plating rather than conversion, which is the distinction this page turns on. And Toning, Permanence and What the Evidence Actually Supports sets Kodak’s GP-1 claim, Ware’s account of the Fading Committee and Reilly’s hedged mechanism side by side and names what kind of statement each one is — which is the honest answer to “does gold toning protect a print” and is not a number.

And the same salt, doing the same job, a century and a half earlier, is taught in Part XXII, on a printed-out image where colour and lifetime are not two subjects. That lesson distinguishes the four gold toner families chemically; this entry’s cluster and that one are the same chemistry meeting two different image structures, and reading them together is the point.

The substances and the weights are unchanged: the chloroauric acid and gold(III) chloride encyclopaedia entries, and five gold formulas — GP-1, T-26, and the acetate, borax and thiocyanate toners for printing-out papers.