Gold-thiocyanate toner

The gold toner the printing-out processes actually use, published by a supplier who states both stock strengths and the working dilution — which makes it the only complete thiocyanate gold formula in this formulary. Kodak’s GP-1 is the same chemistry with one of its two quantities unfinished; this one can be computed end to end.

Solution A — the thiocyanate stock

Ingredient	Quantity	Form the source specifies
Ammonium thiocyanate	2 %
Water	to make 500 mL	The kit supplies this ready made as 500 mL of a 2 per cent solution. The supplier gives the strength and the volume and not a weight, and that pair is the quantity.

Solution B — the gold stock, and the replenisher

Ingredient	Quantity	Form the source specifies
Chloroauric acid	0.2 %	printed as "Gold Chloride solution 0.2%"; the course's reagent for that name in the photographic literature is chloroauric acid
Water	to make 500 mL	Supplied ready made as 500 mL of a 0.2 per cent solution. Only 50 mL goes into the first litre of bath; the rest is the replenisher, at about 5 mL per print.

Mixed in this order — the working toner

1. Start with 1000 mL of water — Clean tap water or distilled water, in the tray.
2. Then add 50 mL of Solution A
3. Then add 50 mL of Solution B — The gold often turns a momentary brown or orange as it goes in and should redissolve quickly.

In one tray, measure out 1000 mL of clean tap water or distilled water and add 50 mL of solution A and 50 mL of solution B. This is your toning bath.

Eleven hundred millilitres in all, carrying 0.1 g of gold chloride and 1.0 g of ammonium thiocyanate - so 0.091 g/L of gold and 0.91 g/L of thiocyanate. The supplier gives volumes rather than parts, and they are recorded as volumes.

Purpose

To plate a printed-out silver image with gold, before it is fixed, so that the fixer does not take the image away with the unexposed silver. The supplier’s own list of what the kit does is worth taking in order, because the first three items are not about colour at all: to enhance permanency, to preserve the full tonal range, to prevent loss of values through the fixing stage, and to change the tonal colour to a cooler blue with some maroon or purple shades possible.

That third item is the one that separates an alt-process gold toner from a silver-gelatin one. A printed-out image is finely divided silver with no gelatin binder to speak of, and a plain hypo fixer will thin it noticeably. Gold plated onto it first is what survives the fixer.

Recommended uses

Printing-out paper, which is what the kit is sold for, toned before fixing.

Van Dyke, kallitype, albumen and salted paper, which the same sheet covers explicitly. Follow the standard rinsing and clearing instructions for the process, but do not fix before toning — with one exception the supplier names: some sources say Van Dykes are better toned after fixing and rinsing, and if staining is a problem that is the thing to try.

Where the tone is to be chosen by eye rather than by a clock. There is no published time; the colour path runs from the untoned red-brown through warmer shades to a cool blue, and the print is pulled where you want it.

Where a bath is to serve a run of prints. It is replenished rather than discarded, and the supplier publishes two replenishment schemes for two intentions.

When another formula is preferable

• On a silver gelatin print, for protection rather than colour, Kodak’s GP-1, which is the same ligand chemistry at 0.1 g/L of gold and a published ten-minute treatment.
• Where the alkaline route is wanted, the gold-borax toner, which reaches gold(I) without any sulfur ligand at all and gives a different colour path.
• Where a saturated blue is wanted on a silver gelatin print, Kodak’s T-26 — at the cost of a Level C reagent, and with the toning running from the highlights down rather than the shadows up.
• Where gold is simply too expensive for the job, a sulfide or selenium route; but note that on a printing-out paper the gold is doing something those cannot do, which is holding the image through the fixer.

Mixing

Two supplied stocks and a tray of water, mixed cold, in this order.

1. 1000 mL of clean tap water or distilled water, in the tray.
2. 50 mL of Solution A, the 2 per cent ammonium thiocyanate.
3. 50 mL of Solution B, the 0.2 per cent gold chloride.

The supplier notes what to expect and what not to worry about: the gold often turns a momentary brown or orange colour when it first goes in, and should quickly redissolve into solution.

A second tray is made up at the same time: 100 g of sodium thiosulfate in 1000 mL of water, as the fixer. It is not part of this formula and has its own entry in the plan, but the two trays are set out together because the print goes straight from one to the other.

🧮What a litre of this bath carries, and why the number is familiar

50 mL × 0.2% = 0.1 g, in 1100 mL = 0.091 g per litre

Gold chloride

50 mL × 2% = 1.0 g, in 1100 mL = 0.91 g per litre

Ammonium thiocyanate

About a tenth of a gram of gold salt per litre, which is where Reilly’s rule puts a bath for matte salted paper — 0.1 to 0.2 g per litre — and within a tenth of Kodak’s GP-1 at 0.10 g/L. Three independent sources, a manufacturer’s sheet, a conservation text and a supplier’s kit, all land in the same place for a gold toning bath.

The ligand-to-gold molar ratio is the number that says how a gold bath is built. Ammonium thiocyanate at 76.12 g/mol gives 12.0 mmol/L; the gold, as the acid at about 339.8 g/mol, gives 0.27 mmol/L. The ligand is in about forty-four-fold molar excess — far more than Kodak’s T-26 at elevenfold — which is what allows this bath to hold its gold through a whole run of prints and to be topped up rather than remade.

The 5 mL per print the supplier assumes is worth converting too: 5 mL of a 0.2 per cent stock is 10 mg of gold chloride per print, which is a tenth of what the bath started with. That is the real cost of gold toning, stated as a number.

Behaviour

No time is published and that is deliberate. The supplier’s instruction is to watch, and the colour path is the control: for cooler, bluer tones let the print tone until no further change is observed; for warmer red and brown tones pull it earlier.

The toner moves through the print from the shadows. The supplier says so directly — the toner moves through the print, usually in the shadows first, subtly changing the colour of those areas, and when more dramatic changes start to appear the toning is nearly complete. Set that against Kodak’s T-26, which tones the highlights first: two gold baths, two ligands, two directions through the tonal scale.

You cannot see the result until the print is dry. The supplier is explicit: the true final colour and density are not visible until after fixing, rinsing and washing, and the print will get noticeably darker as it dries. Learning this toner is learning to read a wet print against a dry one.

The bath slows before it stops. The supplier’s second replenishment scheme is built on that: watch the prints and add 25 mL of gold stock when the toning slows to an impractical speed.

The intermediate wash before toning is part of the process. Two to three minutes in gently running water, to remove excess silver nitrate and prepare the surface to absorb the toner evenly. The supplier describes what you see — the print lightens and shifts towards red, and the excess silver reacts with the chlorine in tap water to form a purple mist that floats off the paper.

Image characteristics

Cool blue at the end of the path, red and maroon and purple along it. The supplier’s summary is a shift towards a cooler blue with some maroon or purple shades possible.

Shadows first, which means a partially toned print is toned where the silver is densest.

The full tonal range is preserved through the fixer, which is the claim the whole formula is built on.

Faint until dry, on some processes. The supplier notes that on some types of print the toning will be faint until the print is fully dried.

The mechanism

The same reduction as every useful gold toner, by the ligand route.

⚗Three silvers or one, and why a thiocyanate

Gold in the salt you buy is gold(III) and silver oxidises only to silver(I), so a plain gold(III) bath spends three silver atoms for every gold atom deposited — it bleaches the print faster than it tones it. The equations are on the chloroauric acid page and are not repeated here.

A useful gold toner works with gold(I), one silver for one gold. Ware’s two routes are a sulfur ligand — thiosulfate, thiocyanate or thiourea — which both assists the reduction and stabilises the product, or a mild alkaline buffer, which is the gold-borax route.

This is the thiocyanate case, and the forty-four-fold molar excess of ligand computed above is what makes it a keeping bath rather than a mix-and-use one: there is enough thiocyanate to hold gold(I) in solution through a run of prints and through repeated replenishment.

Why the ligand is also dissolving the image. The sodium thiocyanate page gives the formation constant for the tetrathiocyanatoargentate ion as 1.2 × 10¹⁰ — about a thousand times the diamminesilver ion’s — and concludes that this is strong enough to dissolve finely divided silver, so a thiocyanate gold toner both plates gold onto the image and etches the image it is plating. On a printed-out paper, whose silver is about as finely divided as photographic silver gets, that is not a small effect, and it is the reason the supplier’s instructions are about watching rather than timing.

Why toning comes before fixing here and after it on a silver gelatin print. A printed-out image is not protected by a developed grain structure; the hypo will thin it. Gold plated onto the image first is what survives the bath. On a developed silver gelatin print the image is robust and the toner is a finishing step.

Why the gold flashes brown and then redissolves. The supplier reports the observation and gives no mechanism, and none is offered here: it is what a concentrated gold solution meeting a dilute one does before it is properly mixed, and the sheet’s point is that it is expected and transient.

One thing the ammonium salt does not change. Kodak’s GP-1 uses the sodium salt, this kit the ammonium salt, and Reilly’s formula the sodium salt. The active species is the thiocyanate ion in every case; the counter-ion differs. No source read for this course claims a photographic difference between them, and none is claimed here.

Function of every ingredient

Ammonium thiocyanate, 2 per cent in Solution A, 0.91 g per litre of working bath. The sulfur ligand, and it does two opposed jobs at once. Against gold it is a reducer and a stabiliser: it takes gold(III) to gold(I), so that one silver buys one gold instead of three, and holds the gold in solution as a thiocyanate complex so the bath does not precipitate metal. Against silver it is a solvent — Wall’s 1912 dictionary states that the alkali thiocyanates are used as solvents of the silver haloid salts as well as in compounding toning solutions, and the formation constant on the sodium thiocyanate page shows how strong a solvent it is for finely divided silver. More ligand at a fixed gold means more etching for the same plating; less risks an under-reduced bath, which bleaches. It is present here at about forty-four times the gold by moles, which is what makes the bath keep. The supplier ships it as a made-up solution, and the strength is the quantity: 2 per cent, not a weight.

Chloroauric acid, 0.2 per cent in Solution B, 0.091 g per litre of working bath. The gold, the only substance that ends up in the print, and the only ingredient a print consumes. The supplier prints it as “gold chloride”, and the course’s reading of that name in the photographic literature is this substance rather than gold(III) chloride proper; the two are about 50 and 64.93 per cent gold, so they are not interchangeable by weight. More gold tones faster and further, and past a point deposits gold superficially so that the layer is largely removed in the fixer, which is Reilly’s warning and exactly the failure this formula exists to avoid. Less is what happens as the bath is used, and it is why the supplier publishes two replenishment schemes. Note that only a tenth of the supplied bottle goes into the first litre of bath: the rest of Solution B is the replenisher, and that is a design feature of the kit rather than a leftover.

Water, a litre of it in the tray, and two hundred millilitres of it already inside the stocks. The dilution is what makes 0.1 g of gold into a bath a print can be moved around in. Clean tap water or distilled water, per the supplier — and note that the intermediate wash before toning is where tap water’s chlorine does something visible, reacting with the excess silver to throw a purple mist off the paper.

Interactions

With fixer, absolutely and in both directions. Reilly’s warning for gold toners, on the chloroauric acid page, is that the toner is ruined by even a trace of fixer. The supplier’s version is practical: the print need not be washed between the toning bath and the fixer, but be careful not to contaminate the toning bath with the fixer. Separate tongs, and the trays in the right order.

With acid. A thiocyanate and an acid do not belong in the same room, and the sodium thiocyanate page carries the reason. No stop bath, nothing acidic in the waste.

With metal. Ware records that these gold compounds are corrosive and oxidising, attack most metals and organic matter, and will stain skin a spectacular purple. Glass and plastic throughout.

With light and organic matter, both of which reduce dissolved gold; Reilly’s instruction for a gold stock is to keep it in the dark.

With the excess silver nitrate on a freshly printed sheet. The supplier’s intermediate wash exists for this: free silver in the paper would otherwise take gold out of the bath before the image gets any.

With the print’s own drying. The supplier’s warning that the toning may look faint until the print is fully dry is a property of the process rather than a defect.

Variants

Kodak GP-1, the same ligand on silver gelatin: a 1 per cent gold stock at 10 mL to the litre with a sodium thiocyanate liquid whose strength Kodak never published, treated for ten minutes at 20 °C. See its page — and note that this kit is the answer to the gap in that formula, because both of its strengths are stated.

Reilly’s thiocyanate toner, in weights. Sodium thiocyanate 15 to 20 g, 1 per cent gold chloride 60 to 80 mL, water to 1 litre — a much stronger bath at 0.6 to 0.8 g of gold per litre, which is above Reilly’s own strength rule. The sodium thiocyanate page notes that gap and does not reconcile it, and neither does this page.

The gold-borax toner, the alkaline route, which the Photographers’ Formulary kit uses for the same papers at 0.16 g/L of gold and 6 to 12 minutes at 21 °C. See its page. Two suppliers, two routes, the same processes.

No course variant is offered. The formula is a supplier’s own published dilution of its own published stocks, and there is nothing in it the course could improve on without changing what it is.

Safety

Level B. Both ingredients sit there: ammonium thiocyanate and chloroauric acid. Their pages carry the classifications, the exposure information and the first aid, and none of it is restated here.

The supplier’s own instruction is short and is worth quoting: wear gloves and minimise exposure to the toning bath.

From the chemical pages, the controls that matter for this formula: nitrile gloves whenever the stocks or the bath are handled; chemical splash goggles for the gold stock, whose classification gives severe skin burns and serious eye damage unanimously; plastic rather than metal for every spatula, stirrer and tray; and hands washed before eating.

The one absolute rule is acid, for the thiocyanate’s sake.

This bath is worked cold, which removes the class of hazards the hot toners carry.

Nothing here is weighed. Both stocks arrive as solutions, which is the single largest practical difference between this formula and every other gold toner in the formulary: there is no powder to raise.

Storage

Both stocks keep; the working bath is kept and replenished.

The gold stock in the dark, per Reilly, and away from organic matter, both of which reduce dissolved gold. In glass or plastic, never metal, including the closure.

Label both bottles with their strengths. A 2 per cent thiocyanate and a 0.2 per cent gold are a factor of ten apart and are both clear colourless liquids.

The supplier publishes no shelf life for either stock or for the bath, and none is invented here. What it does publish is a replenishment rate, which is the practical form of the same information.

Incompatibilities

Acids, with the thiocyanate. Absolute.

Fixer, in the toning bath — the supplier’s own warning.

Metals, with the gold: trays, tongs, stirrers, cap liners.

Light and organic matter, with the gold stock.

Sulfide and selenium toners, in the waste bottle.

Developer, in either direction, where one is in use at all.

Waste

A gold- and thiocyanate-bearing stream that has also picked up silver, because the ligand dissolves finely divided silver as well as plating it.

Never acidified.

Kept separate from the fixer’s waste, which is a much larger silver stream in its own right, and from any sulfide or selenium toner’s.

Collect it, label it, and follow the silver-bearing waste SOP, the general chemical waste SOP and the disposal ruling. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

The gold went brown or orange when it was added and stayed that way. The supplier expects the flash and expects it to redissolve quickly. A bath that stays coloured has not mixed, or the water was not clean.

The print thinned badly in the fixer. The toning did not go far enough, or the intermediate wash was skipped so the gold went to free silver rather than to the image. Preventing that loss is the formula’s first purpose.

The bath tones the first print and not the fifth. Exhaustion, and it is expected: about 5 mL of gold stock per print. Replenish.

The toning slowed to an impractical speed. The supplier’s second scheme: add 25 mL of Solution B.

Stains, or a bath that stopped working suddenly. Fixer contamination. Reilly: even a trace ruins a gold toner.

Van Dykes stain when toned before fixing. The supplier names this specifically and gives the remedy: try toning them after fixing and rinsing instead.

The toned print looks nothing like it did when wet. Expected. The colour and density are only readable dry, and the print gains significant density as it dries.

Uneven toning across the sheet. The intermediate wash prepares the surface to absorb the toner evenly; skipping it, or letting the print into the bath unevenly, is where mottle comes from. Cover the whole print quickly and agitate from the first second.

Experiments

Plot the colour path. Identical prints pulled at intervals through one bath, all fixed, washed and dried together, then compared. The supplier’s claim is a continuous path from red-brown to cool blue, and the useful output is your own set of reference prints for the points along it.

Measure what the gold saves. Two identical printed-out sheets, one toned and one not, both fixed for the same ten minutes and read for maximum density. The formula’s first purpose is preventing loss of values through the fixing stage; this is that claim as a pair of numbers.

Test the direction. A step wedge toned partially in this bath and another in Kodak’s T-26. One is said to tone the shadows first and the other the highlights first; reading both step by step shows whether the ligand really is what decides it.

Track the replenishment. Tone a run of prints, adding 5 mL of gold after each, and record the toning time for each print. The supplier assumes a print consumes about 5 mL; the point at which the times start to climb says whether that assumption holds for your paper and your print size.

Compare the two routes on one sheet. The same paper, half toned in this bath and half in the gold-borax toner, at their published strengths and times. Sulfur ligand against alkaline buffer, one metal, and no source in this corpus puts them side by side.