Chloroauric acid

Almost nothing sold to photographers as “gold chloride” is gold chloride. Reilly puts it plainly: the gold chloride bought from a photographic or chemical supplier is always an acidic substance, technically chlorauric acid, made by dissolving gold metal in a mixture of nitric and hydrochloric acids, while true gold chloride is an unstable substance made by passing chlorine over gold leaf at high temperature and is not usually commercially available. Ware’s own list of alternative names for the trihydrate ends with the resigned note that the earlier literature called it, ambiguously and inaccurately, “gold chloride”. This page is about the substance the bottle actually contains.

In photography

As a toner, it plates the image silver with a metal that does not tarnish. Ware’s account of why is a lesson in particle physics as much as chemistry. A printed-out 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, protects them and shifts the colour, usually to a rich purplish-brown on an albumen print. The Photographic Society’s Fading Committee of 1850 reached that recommendation empirically, a century before anyone could photograph a nanoparticle.

⚗The chemistry

Gold in this 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.

[AuCl₄]⁻ + 3 e⁻ → Au + 4 Cl⁻

Gold(III) takes three electrons to reach the metal

3 Ag + [AuCl₄]⁻ → Au + 3 AgCl + Cl⁻

Toning in an acid chloride bath: three silvers spent for one gold

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 in the second equation: toning leaves silver chloride in the paper, so toning is always followed by fixing.

The bath tells you when it is ready, and Reilly says how to read 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, 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. Bostick & Sullivan’s modern 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 — and replenishes at roughly 5 ml of gold stock per print.

On silver gelatin the same salt does three different jobs, and Kodak’s G-23 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.

And it prints in its own right. In Ware’s New Chrysotype the gold salt is not a retrospective treatment of a silver image at all but the image substance: a sensitiser of three stock solutions — a ligand, 3,3′-thiodipropanoic acid, which binds the gold and moderates its reactivity; the gold itself, as either the sodium salt or this acid; and ammonium iron(III) oxalate as the light-sensitive component. Ware’s Platinomicon gives the gold couple as E(AuCl₄⁻/Au, 4Cl⁻) = +1.00 V, the most positive in his table of noble metals, which is why photochemically generated iron(II) reduces it so readily and why the untamed reaction is too vigorous to control without the ligand.

Properties

Deliquescence is the property that governs handling. Haz-Map’s compiled description calls the hydrate highly hygroscopic and deliquescent; Kodak repeats in two separate footnotes that gold chloride will liquefy rapidly in a normal room atmosphere; Ware warns that a stock bottle gains weight from the air and will ultimately dissolve entirely. That is why the reagent is sold pre-weighed in sealed vials, and why Ware devotes a numbered procedure to opening one — score the glass, wrap it in card, snap it, and rinse both halves into the measuring cylinder until the wash water is no longer yellow.

PubChem’s record carries no solubility figure for this compound, and the course does not supply one from memory. What the sources do establish is that the reagent dissolves readily enough in distilled water for a 1 per cent stock solution to be routine, which is the concentration Kodak, Reilly and Wall all assume.

Purity is a trap in the other direction. Ware warns against ordering gold salts of 99.99 per cent purity or higher on grounds of cost alone, notes that CAS numbers differ with purity grade, and points out that the stated purity refers only to the metal content. Because the water of crystallisation varies between batches, the better manufacturers give an assay of the actual gold content, and where that differs from the theoretical figure by more than one or two per cent the water used to make up the solution should be corrected rather than the weight.

Handling

The aggregated ECHA notifications classify this substance Danger, with the corrosive, irritant, health-hazard and environmental pictograms. Every one of the 200 reports gives severe skin burns and eye damage; three-quarters give serious eye damage and an allergic skin reaction; a fifth give organ damage on prolonged or repeated exposure. Ware’s own summary, abstracted from the safety data sheets, reads: corrosive, causes burns, extremely destructive of mucous membranes, may be harmful by ingestion or inhalation, may cause an allergic skin reaction.

Why Level B. The course rubric caps Level A at substances classified at most as irritant, harmful if swallowed, or corrosive at the concentrations actually handled. A corrosive solid that three-quarters of its notifiers also call a skin sensitiser goes past that, and Level B’s first criterion names exactly this case — concentrated corrosive reagents and sensitisers — while its fourth names procedures whose failure mode is a splash or a burn rather than a spoiled print. It does not reach Level C: that criterion is qualified by the words “where a fume cupboard or specialist disposal is the recognised control”, and for a non-volatile solid weighed in single grams, or a 1 per cent solution used in a tray, it is not. The classification carries no acute toxicity category above “harmful”, no carcinogenicity and no respiratory sensitisation — which is the difference between this page and the platinum salt two pages over.

⚠Safety

Gloves and splash goggles for the solid and the stock solution; gloves for the tray. Because roughly three in four notifiers classify it as a skin sensitiser, gloves are changed when contaminated rather than rinsed and reused: HSE’s HSG262 makes the point that once a person is sensitised, very small amounts of the allergen provoke a reaction and the only remedy is to prevent further exposure. Use glass and plastic only — Ware records that these compounds attack most metals and organic matter and will stain skin a spectacular purple. Keep the bottle stoppered, keep the fixer away from it, and treat a 1 per cent solution as a corrosive liquid rather than as a photographic convenience.

Waste

Three things leave the darkroom in a spent gold bath: gold that was not used, the metal it took out of the print, and the ligand that carried the gold. A toner that has stopped working still holds gold — Reilly notes that an over-alkaline bath can contain a great deal of it and tone nothing — so an exhausted bath is not an empty one. Collect it in a labelled bottle, keep it away from thiosulfate waste, and treat it as a metal-bearing stream. Twenty per cent of the ECHA notifiers classify the substance as toxic to aquatic life with long lasting effects; that alone settles the question of the drain. Kodak’s environmental guidance for amateur photographers sets out the principle that metal-bearing solutions are collected and treated, and ILFORD’s guidance to domestic users is to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard, which in the United Kingdom is the route GOV.UK’s hazardous household waste page directs you to. Check your local regulations; they govern, and they differ.

History

Gold reached photography before paper printing did. Ware records that Armand Hippolyte Louis Fizeau is usually credited with discovering the gilding of daguerreotypes with gold salts, announced to the Académie des Sciences by Arago on 23 March 1840 and published by Fizeau that August — while noting evidence that August Friedrich Karl Himly, professor of chemistry at Göttingen, had already 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, gold chloride mixed into an excess of sodium thiosulfate, passed to paper printing after 1850, and Reilly records that Le Gray’s advocacy is what made it popular. It was a poor bath: acidified by the gold chloride going into it, it sulfided the image as often as it gilded it. Alkaline gold toning, proposed by James Waterhouse around 1855, replaced it with baths of gold chloride and one or two mild alkalis; 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. Those three families — thio-ligand, alkaline buffer, and combined toning-and-fixing baths of the kind Wall reprints from Valenta — are still the three families in use.

The modern turn is Ware’s. Herschel’s chrysotype of 1842 printed in gold directly and was abandoned as uncontrollable; Ware spent six years and, by his own account, many thousands of experiments applying twentieth-century coordination chemistry to taming the gold salts, and published a working process in 1987. He is emphatic that this is not gold toning, and the distinction matters: a toner modifies an image that already exists, while a chrysotype sensitiser makes the image out of gold in the first place. The colours that come with it — pink, magenta, purple, violet, blue, green — are the same particle-size effect that gives a gold-toned albumen print its purplish-brown, read through Mie’s 1908 theory of scattering by small particles: the smallest particles absorb blue and look red, the larger ones absorb red and look blue.