Kodak GP-1

Two ingredients, a litre of water, ten minutes at room temperature. GP-1 is the simplest gold formula in the published literature and the least like a toner: its purpose is to make a print last, and the colour change is so slight that Kodak has to tell you what to look for.

It is also the one formula in this batch with a hole in it, and the hole is the maker’s.

Ingredient	Quantity	Form the source specifies
Chloroauric acid	10 mL of a 1% solution	printed as "gold chloride, 1% stock solution"; Kodak's own footnote makes the stock by dissolving 1 gram in 100 mL of water, or buying it ready made
Sodium thiocyanate	15.2 mL	printed as "sodium thiocyanate (liquid)"; Kodak states no strength for the liquid anywhere in the sheet, so this volume cannot be converted to a weight
Water	to make 1000 mL	at 20 °C; 750 mL receives the gold stock; 125 mL dissolves the thiocyanate separately; the rest makes the litre. Kodak prints one table with one make-up volume and describes the split in the paragraph beneath it.

!What Kodak did not publish, and what follows

The sodium thiocyanate line reads “Sodium thiocyanate (liquid) 15.2 mL”. Kodak states the strength of that liquid nowhere in the sheet. CAMEO carries a datasheet for a commercial sodium thiocyanate solution at 56 per cent or less, and that is the article Kodak’s formula is written around — but “56 per cent or less” is not a concentration, and the sodium thiocyanate page states the course’s position in terms: the course cannot convert that 15.2 mL to a weight, and says so rather than guessing.

So the formula is printed here exactly as Kodak printed it, and this is what that means in practice:

• If you have Kodak’s liquid, or a solution whose strength you know and can match to it, the formula is complete and usable.
• If you have the solid salt, this formula as published does not tell you how much to weigh, and no figure is supplied here. What is available instead is Reilly’s thiocyanate toner, which is published in weights and is set out under Variants.
• Do not read 15.2 mL as 15.2 g. They are not the same quantity and the substitution is not a rounding.

Purpose

To protect a finished print, and to change its colour as little as possible while doing so. Kodak’s own description is one sentence: GP-1 provides print protection while changing the image tone only slightly.

The chloroauric acid page carries the argument behind that, from Ware. A silver image is finely divided, 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 the sulfur-bearing amino acids in the gelatin itself. 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. The Photographic Society’s Fading Committee reached that recommendation empirically in 1850, a century before anyone could photograph a nanoparticle.

Recommended uses

A print that is to be kept, on any paper. Unlike T-21 and the selenium toners, GP-1 is not restricted to warm-tone papers, because it is not being asked to produce a visible colour.

A print whose colour must not change. This is the only formula in the toner library whose published end point is “a very slight change”.

After any other archival step. Kodak’s instruction is a thoroughly washed print, and the ten-minute wash afterwards is the shortest on the whole sheet.

As the gold toner to reach for when a persulfate is not acceptable. The potassium persulfate page names GP-1 by name for exactly this: where a gold toner is wanted, GP-1 made with sodium thiocyanate does the job without a persulfate in it.

When another formula is preferable

• Where a brown tone is actually wanted, T-21, which is built to change the colour of a warm-tone paper and does it evenly across the scale.
• Where a blue tone is wanted, T-26, which is the same gold with thiourea.
• Where the print is a salted paper, an albumen, a printing-out paper, a Van Dyke or a kallitype, the gold-thiocyanate toner, which is the same chemistry sold as a kit for those processes with both stock strengths published, or the gold-borax toner for the alkaline route.
• Where only the solid salt is available and the strength of Kodak’s liquid cannot be established, Reilly’s thiocyanate toner under Variants, which is published in weights.

Mixing

One litre, mixed cold, in an order that is not optional.

1. Make or buy the gold stock. Kodak’s footnote: dissolve 1 gram of gold chloride in 100 mL of water for a 1 per cent solution, or obtain a 1 per cent solution from a photographic dealer.
2. Add the 10 mL of gold stock to 750 mL of water at 20 °C.
3. Dissolve the sodium thiocyanate separately in 125 mL of water.
4. Add the thiocyanate solution slowly to the gold solution, stirring rapidly.
5. Water to make one litre.

Mix it immediately before use. That is Kodak’s own instruction and the only keeping statement it makes about this formula.

🧮What a litre of GP-1 carries, and how that compares

10 mL × 1% = 0.1 g per litre

Gold chloride

A tenth of a gram of gold salt in a litre, which is the lowest gold concentration in the formulary and which places GP-1 exactly at the bottom of Reilly’s published range for a gold toning bath — 0.1 to 0.2 g of gold chloride per litre for matte salted papers, 0.4 to 0.5 for glossy albumen. Kodak’s own T-21 works out at 0.119 g/L and its T-26 at 0.4 g/L, so the three Kodak gold formulas span the same range the conservation literature gives, with the protective one at the bottom of it.

That is not a coincidence and it is the design of the formula: the less gold in the bath, the less the colour moves, and protection is what is wanted rather than tone.

The thiocyanate cannot be put on this scale. If Kodak’s liquid were the 56 per cent solution CAMEO describes, 15.2 mL would carry something on the order of 8 to 9 g of the salt, which would put GP-1 in the same region as Reilly’s 15 to 20 g — but “56 per cent or less” is a datasheet’s upper bound and not Kodak’s figure, and the course does not build an arithmetic on it. The number above is written out only to show what kind of quantity is missing.

Behaviour

Ten minutes, at 20 °C, or until you can see a very slight change to a bluish black. That is the whole published procedure, and the end point is unusually specific about how little should happen.

The wash afterwards is ten minutes, against an hour for T-1a and T-21. Nothing in this bath needs long to leave the paper.

It is mixed for the job and not kept. Kodak’s instruction to mix right before use is the practical form of the chemistry: a reduced gold bath is a bath that is quietly on its way to precipitating metallic gold.

No capacity is published, and no exhaustion test. Kodak treats it as a one-shot bath by publishing a mixing instruction rather than a replenishment rate — which is the opposite of what it does for T-21.

The thiocyanate is also dissolving the image while the gold plates it. This is not a defect and it is important. 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. Ten minutes is a published time, not an arbitrary one.

Image characteristics

Very little change, by design. Kodak’s end point is a very slight shift to a bluish black.

Bluish rather than warm, which is the direction gold moves a neutral image.

Protection rather than appearance is the deliverable, and the chloroauric acid page carries what is known about the mechanism and what is not. Kodak’s own general claim on the same sheet is that all its toners protect the image whether or not they produce a colour shift.

The protection is partial, as every toner’s is. Gold coats the silver particles; it does not replace all of them, and the paper, the gelatin and the mount have their own failure modes.

The mechanism

The formula is a reduction, run in a beaker, in the two minutes before the print goes in.

⚗Why the thiocyanate goes into the gold and not the other way round

Gold in the salt you buy is gold(III); silver oxidises only to silver(I). A plain gold(III) bath therefore spends three silver atoms for every gold atom deposited — it bleaches the print faster than it tones it, and Reilly’s description of the result is flat, lifeless and reddish. The equations are on the chloroauric acid page and are not repeated here.

Every useful gold toner works with gold(I), where the exchange is one for one. Ware’s two routes to it are a sulfur ligand — thiosulfate, thiocyanate or thiourea — which both assists the reduction and stabilises the product, or a plain mild alkaline buffer, in which gold(III) slowly oxidises water instead over as much as twenty-four hours.

GP-1 is the thiocyanate case, and it is the fastest of the three sulfur-ligand formulas: where T-21 stands for eight hours and drops a precipitate, this one is usable at once. Kodak’s mixing order is the reason. The gold goes into three-quarters of the water first, so it is dilute when the ligand meets it; the thiocyanate is dissolved separately and added slowly to the gold, with rapid stirring, so the ligand is always the scarce reagent at the point of mixing. Reverse it — gold poured into concentrated thiocyanate — and the local excess of ligand is exactly the condition for precipitating metallic gold in the beaker.

What the bath’s colour says. Reilly’s practical test, on the chloroauric acid page, is that an acidic, inactive gold bath keeps a yellow colour and a working one is colourless, and that decolorisation is the best guide to the condition of the bath. The same page states the limit of that observation: no source the course has found assigns those colours to named complexes, so it is reported as an observation and not as a mechanism, and this page does not go further.

Why a protective solution is a weak one. Gold plated onto silver changes the colour, and the amount of colour change follows the amount of gold. GP-1 puts a tenth of a gram of gold salt in a litre because that is enough to coat the particles and not enough to recolour the print.

Function of every ingredient

Chloroauric acid, 10 mL of a 1 per cent stock, so 0.1 g per litre. The gold, and the only substance that stays behind in the print. Kodak prints it as “gold chloride”; the course’s reading of that name in the photographic literature is this substance rather than gold(III) chloride proper, and both pages carry the reasoning. The substitution is not neutral: the true chloride is 64.93 per cent gold against about 50 per cent for the acid, so a gram of the wrong solid is roughly thirty per cent more gold than the formula intended. More gold moves the print further towards a visible tone and, past a point, deposits gold superficially so that the layer is largely removed in the fixer — Reilly’s warning. Less protects less. Note the form twice over: Kodak’s quantity is a volume of a stated stock, which is what makes 10 mL a quantity at all, and the solid is deliquescent, so it is kept stoppered and dry and a jar that has been open is a jar of unknown strength.

Sodium thiocyanate, 15.2 mL of Kodak’s liquid. 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), where one silver buys one gold instead of three, and holds the gold in solution so the bath does not precipitate metal in the tray. Against silver it is a solvent — Wall’s 1912 dictionary states flatly 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 its own page shows how strong a solvent it is for finely divided silver. Those two jobs are why a thiocyanate gold toner has a published time rather than an instruction to leave it until it looks right. And this is the ingredient whose quantity Kodak did not finish stating: 15.2 mL of a liquid whose strength is not given. See the box at the top of this page. More thiocyanate, at a fixed gold, means more etching for the same plating; less risks an under-reduced bath.

Water, one litre of it, in three portions that are all part of the same litre. 750 mL to receive the gold, 125 mL to dissolve the thiocyanate separately, and the remainder to make up. The split is not a formality: it is what keeps the ligand from ever being in local excess over the gold, and it is the only mixing instruction Kodak gives for this formula.

Interactions

With fixer carried in on the print, and this is the one that spoils a gold toner. Reilly is blunt, as quoted on the chloroauric acid page: the toner solution is ruined by even a trace of fixer. Wash the print thoroughly, and never share tongs between the fixing tray and the toning tray.

With acid. Thiocyanates and acid are a combination the sodium thiocyanate page treats as an absolute rule, and the reason is on that page. No stop bath, no acid fixer, nothing acidic in the waste.

With metal, anywhere. Ware records that these gold compounds are corrosive and oxidising, attack most metals and organic matter, and will stain skin a spectacular purple, and that they will corrode a stainless steel or nickel-plated spatula. Glass and plastic throughout. Kodak’s general toning guidance forbids metal trays for every toner in any case.

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

With a previous toner. GP-1 after a sulfide or selenium toner is a different operation from GP-1 on an untoned print, and no source read for this course publishes the combination. It is not offered here.

Variants

Reilly’s thiocyanate toner, published in weights. From the conservation literature for albumen and salted paper: sodium thiocyanate 15 to 20 g, 1 per cent gold chloride 60 to 80 mL, water to 1 litre. This is the formula to use where only the solid salt is available, because every quantity in it is a weight or a volume of a stated stock.

Two things must be said about it rather than left implied. First, it is a much stronger gold bath than GP-1 — 0.6 to 0.8 g of gold chloride per litre against 0.1 — so it is a toner rather than a protective solution. Second, Reilly’s own strength rule, on the same subject, gives 0.1 to 0.2 g per litre for matte salted papers and 0.4 to 0.5 for glossy albumen, and his printed thiocyanate formula carries more gold than either. The sodium thiocyanate page notes that gap and declines to reconcile it, and so does this page: the reasonable reading is that the gold is adjusted to the paper, but Reilly does not say so and the course will not put the sentence in his mouth.

Kodak T-21, the same maker’s gold toner for warm-tone papers: a thiosulfate ligand instead of a thiocyanate one, a persulfate, a silver ripener, eight hours’ standing, four litres of bath and 43 °C. See its page; GP-1 is what its own hazard argument points at instead.

The Bostick & Sullivan kit, sold for printing-out paper and the iron-silver processes, which is the same chemistry with both stock strengths published: 50 mL of a 2 per cent ammonium thiocyanate solution and 50 mL of a 0.2 per cent gold chloride solution in a litre of water. See the gold-thiocyanate toner. It is the answer to the gap in this formula, from a different supplier.

No course variant is offered, and in particular no weight is proposed for Kodak’s liquid thiocyanate. Supplying one would be the course inventing the number its own chemical page says cannot be established.

Safety

Level B, from two Level B ingredients and no more. Chloroauric acid and sodium thiocyanate both sit there, and their pages carry the classifications, the exposure information and the first aid. Neither is restated here.

The controls that matter for this formula, taken from those two pages: nitrile gloves whenever the solid, the stock solution or the bath is handled; chemical splash goggles for the solid and the stock, because the gold’s aggregated classification gives severe skin burns and serious eye damage unanimously; plastic rather than metal for every spatula and stirrer; and hands washed before eating, because most notifiers classify both substances as harmful if swallowed.

The one absolute rule is acid. A thiocyanate and an acid do not belong in the same room as each other, and its page explains why. In a darkroom that means no stop bath in the run and nothing acidic into the waste bottle.

This bath is worked cold, which removes the whole class of hazards that T-1a and T-21 carry from being worked hot.

And the formula’s own gap is a safety matter as well as a chemical one. A reader who reads 15.2 mL as 15.2 g has not made a weaker bath or a stronger one at random; they have made a bath of unknown thiocyanate concentration. The box at the top of this page is there for that reason.

Storage

Mix it immediately before use. Kodak’s instruction, and the only one it gives.

The 1 per cent gold stock keeps and is the thing to look after. In the dark, per Reilly, because light reduces dissolved gold; away from organic matter, for the same reason; tightly stoppered and dry, because the solid is deliquescent and Kodak footnotes that it will liquefy rapidly in a normal room atmosphere.

In glass or plastic, never metal, including the closure.

Label it with the strength. A 1 per cent gold stock and a 0.2 per cent one look identical, and the difference between them is a factor of five in the most expensive ingredient in the darkroom.

Kodak publishes no shelf life for the gold stock, and none is invented here.

Incompatibilities

Acids, with the thiocyanate. Absolute.

Metals, with the gold — spatulas, stirrers, trays, tongs, cap liners.

Light and organic matter, with the gold stock, both of which reduce it.

Fixer, carried in on a print or on a pair of tongs. Reilly: even a trace ruins the toner.

Developer, in either direction, as for every bath in the darkroom.

Waste

A gold- and thiocyanate-bearing stream, in small volume. A litre of GP-1 has had 0.1 g of gold salt in it and will have picked up a little silver from the print, because the thiocyanate is a silver solvent as well as a gold ligand.

Never acidified, which is the thiocyanate’s rule and applies to the waste bottle as much as to the tray.

Not combined with a sulfide toner’s waste, and not with a ferricyanide bleach’s.

Collect it, label it, and follow the general chemical waste SOP and the disposal ruling; where the bath has toned prints it is also a silver-bearing stream. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Metallic gold precipitated in the beaker on mixing. The order was reversed, or the thiocyanate went in quickly rather than slowly, or the stirring was not rapid. Kodak’s order exists to keep the ligand from ever being in local excess over the gold.

Nothing visible happened to the print. That is close to the intended result. Kodak’s end point is a very slight change to a bluish black, and the protection is not a visible property.

The print lightened. The thiocyanate’s silver-solvent action ran ahead of the gold’s plating — a bath short of gold, or a print left much longer than ten minutes.

The bath is yellow and nothing is happening. Reilly’s test: a yellow bath is an inactive one. On this formula that most likely means the reduction did not happen, which points back at the mixing.

The toner died after one print. Expected. There is a tenth of a gram of gold in the litre and Kodak publishes no replenishment for this formula, only an instruction to mix it fresh.

Stains on the print, or a bath that stopped working immediately. Fixer, carried in on the print or the tongs. Reilly: even a trace ruins it.

Purple stains on your hands. The gold. Ware records it as a characteristic of these compounds; the gloves were the control.

Experiments

Test what “protection” means, slowly. Two identical prints, one treated in GP-1 and one not, both mounted and displayed side by side, both read for maximum density and highlight density at intervals over months. The protection claim is about what does not happen, which makes it the hardest claim in this formulary to demonstrate and the most worth demonstrating.

Find where the colour starts to move. The same paper toned for 5, 10, 20 and 40 minutes. Kodak’s ten minutes is chosen so that the tone barely changes; the point where it stops barely changing is a property of your paper.

Watch the etching. A step wedge through GP-1, read before and after. The thiocyanate dissolves finely divided silver, so the highlights — where the silver is finest — should be where the loss shows first. This is the observable consequence of a formation constant on a chemical page.

Compare the three gold ligands at matched gold. GP-1 at 0.1 g/L, T-21 at 0.119 g/L and T-26 diluted to match. Three sulfur ligands, one metal, three behaviours; nothing in this corpus isolates the ligand’s contribution.

Establish the missing number for your own bottle. If you have a sodium thiocyanate solution of known strength, GP-1’s 15.2 mL becomes a weight you can compute, and the bath becomes reproducible. Record the strength on the bottle and on the print’s back. That is the only honest way to close the gap this page reports.