Steigmann gold sensitiser stock

Two clear colourless solutions, one run into the other, and a wait until the colour that appears has gone again. That is the whole preparation, and the waiting is the part that matters: what is being made is not a mixture of gold and thiocyanate but a gold(I) complex, and the stock is not ready until the reduction has finished.

One per cent gold chloride solution — the gold

Ingredient	Quantity	Form the source specifies
Chloroauric acid	1 g	what is sold to photographers as "gold chloride" is chloroauric acid, and the two are not the same compound; see Function of every ingredient
Water	to make 100 mL	Kodak's own footnote for the same stock, on the same terms: one gram in 100 mL of water, or bought ready made from a dealer. The practitioner whose recipe this entry follows buys hers ready made and prints no preparation, so this line is Kodak's and is labelled as such.
The salt is deliquescent and will liquefy in an ordinary room atmosphere, so the weighing is the awkward part of the whole formula and the reason a ready-made stock is worth buying.

One per cent ammonium thiocyanate solution — the ligand, and the receiving solution

Ingredient	Quantity	Form the source specifies
Ammonium thiocyanate	1 g
Water	100 mL, added	at 52 °C; The source's words are "1 g in 99-100 ml distilled water at about 52 °C, cooled before use". The two readings differ by one part in a hundred and both give a one per cent solution; the upper figure is recorded here and the source's own wording is kept beside it. The warm water is for dissolving only - the solution is cooled before it is used.

Mixed in this order — the sensitiser stock, made once and kept in a dark dropper bottle

1. Start with 50 mL of One per cent ammonium thiocyanate solution — the receiving solution
2. Then add 6.0 mL of One per cent gold chloride solution

Add 6.0 ml of a 1% gold chloride solution to 50 ml of a 1% ammonium thiocyanate solution, allowing it to clear before using. Store in a dark dropper bottle.

The order is the formula. Gold run into an excess of thiocyanate meets ligand everywhere it goes; thiocyanate dripped into gold would pass through a stage in which the gold is in excess, which is the condition a precipitate comes out of. "Allowing it to clear" is the instruction that says the reduction has finished: what is being waited for is the disappearance of the transient colour, and until it has gone the stock is not the stock.

Purpose

To hold gold at a strength at which a sensitising dose is a few drops. Gold sensitisation is measured in milligrams per mole of silver, which for a domestic batch means fractions of a milligram, and no domestic balance weighs that. A stock is the only way the quantity can be handled at all.

!This course teaches gold sensitisation and does not perform it

The formula above is published, complete and traceable. What is missing is not the stock but a use the course can put its name to.

The only tested gold dose in this corpus sits inside an ammoniacal emulsion, and Part V excludes ammoniacal makes at Level B. Transplanting the gold step out of that formula and into the plain-silver make the course does run would be assembling a procedure from parts, which Rule 6 does not permit however carefully it were labelled.

So the gold is designed and not weighed. Part V’s fifth project puts it in the design half of the exercise, where Duffin’s published ranges carry it honestly as ranges, and what the student performs is a plain-silver orthochromatic make. This entry exists so that the design has a real formula to design against, and so that a reader who meets “Steigmann solution” in the literature can find out what it is.

Recommended uses

As the citable preparation behind a published dose. A recipe that says “6 to 7 drops of Steigmann standard aurous ammonium thiocyanate solution” is unreadable without this page, and unreproducible without it.

As the design object in a sensitisation specification. The exercise the course actually sets is to convert Duffin’s published range into a dose for a stated batch, choose a reading of his two figures, justify it and cost it. That requires knowing what a stock of the strength above delivers per drop, which is arithmetic this page supplies.

As the second half of a sulfur-plus-gold treatment, in the reading of the literature rather than in the tray. Duffin is explicit that gold is applied on top of a sulfur-sensitised emulsion rather than as an alternative to it, which is why the practice has the name it has.

As a worked example of cross-tier corroboration. A tested Tier 2 dose landing inside a Tier 1 published range, from sources thirty years and one continent apart, is the strongest evidential position anything in Part V reaches. It is worth studying as an example of what the course means by provenance, and the arithmetic that establishes it is under Behaviour with its assumptions on the surface.

When another formula is preferable

• When the emulsion has not been sulfur-sensitised, gold is the wrong lever. The published gain is measured over sulfur sensitisation, and a plain-silver make has no sulfur digestion to build on.
• When the object is to protect a finished print rather than to sensitise an emulsion, the gold goes in a tray and the formula is a toner: GP-1 for protection, or the gold thiocyanate toner for a printing-out paper. Those are Level B procedures the course does run.
• When speed is what is wanted and a stop will do it, buy a faster material or open the lens. Duffin’s maximum gain from gold is about a third of a log unit, and a make that has to be repeated because the digestion overshot has cost more than a stop.
• When the emulsion is for printing rather than for a camera, none of this applies. Paper emulsions in this part are deliberately slow.

Mixing

Two one per cent stocks, then one addition, then a wait.

1. Solution A, unless you buy it. One gram of gold chloride to 100 mL of water. The salt is deliquescent and liquefies in an ordinary room atmosphere, which makes this the one weighing in Part V that is genuinely difficult to do accurately; a ready-made one per cent solution from a photographic dealer is the practitioner’s own answer and is worth taking.
2. Solution B. One gram of ammonium thiocyanate in 99 to 100 mL of distilled water at about 52 °C, and cooled before use. Warm water is for dissolving; a warm solution is not what the addition is made into.
3. The addition, in the order given. 6.0 mL of A into 50 mL of B. Not the reverse.
4. Wait until it clears. The instruction is the test. Until the transient colour has gone the reduction is not complete and the stock is not the stock.
5. Into a dark dropper bottle, labelled with the substance, the strength, the date and the words gold sensitiser, per the labelling SOP.

🧮What a drop of this delivers, and every assumption underneath

The stock:

6.0 mL × 10 mg/mL ÷ 56 mL = 1.07 mg per mL

Gold chloride in the finished stock

Six or seven drops, at the conventional twenty drops to the millilitre:

6–7 drops ÷ 20 = 0.30–0.35 mL

Volume

0.30–0.35 mL × 1.07 mg/mL = 0.32–0.37 mg

Gold chloride

Into a make carrying 10.8 g of silver nitrate, which is 0.0636 mol:

0.32–0.37 mg ÷ 0.0636 mol = 5.0–5.9 mg per mole of silver

As the salt

2.5–2.9 mg per mole of silver

As gold, taking the trihydrate at 393.8 with gold at 197.0, so half by mass

Duffin’s published range is 1 to 30 mg per gram mole of silver, and the dose lands inside it on either reading. That is the corroboration, and here is what it rests on. Assumption one: twenty drops to the millilitre. A convention, not a measurement, and a dropper that gives sixteen or twenty-five moves every figure above by a quarter. Assumption two: the hydrate. Chloroauric acid is sold as the trihydrate and is 50 per cent gold by mass on that basis; a different hydrate, or the anhydrous acid, changes the gold figure but not the salt figure. Assumption three: that Duffin’s “mg of gold” means the metal. His own two statements do not obviously agree — his optimum near 3 × 10⁻⁶ mol is 0.59 mg if it is gold metal and 1.18 mg if it is the salt, and only the second reading puts the optimum inside his own stated range. The course does not settle it; a specification that uses these numbers has to say which reading it took and why, and that is the point of the exercise.

Behaviour

It clears, and the clearing is the reaction. Gold(III) run into thiocyanate does not stay gold(III). The ligand reduces it and then holds the product, and the transient colour that appears and goes is the visible part of that. A stock that has not cleared has not finished; a stock that has gone cloudy or thrown a deposit has gone somewhere else.

It keeps in the dark and not in the light. The instruction is a dark dropper bottle, and the reason is the same one that governs every gold stock in this formulary: light and organic matter both reduce dissolved gold, and a reduced gold stock is metal on the glass.

No keeping figure is published, for this stock or for either of the two solutions that make it. None is invented here. What can be said is that a stock is made in the quantity a season needs rather than in bulk, that it is dated, and that a bottle with a metallic film or a purple tinge has already answered the question.

It is expensive per gram and cheap per make. Fifty-six millilitres of stock carries 60 mg of gold chloride, and a make takes a third of a milligram of it. The economics of gold sensitisation are the economics of the first bottle, not of the dose.

What it does to an emulsion, from the published record rather than from this bench: gold applied on top of sulfur sensitisation raises speed by up to about a third of a log unit, with an optimum beyond which the curve turns down, and larger quantities are needed if the gold goes in at the completion of digestion rather than during it. Fog rises throughout a digestion while speed rises to a maximum and falls, so the endpoint that matters is the last moment before fog begins to move rather than the speed peak.

Image characteristics

Speed, and nothing else that is documented. The published gain is about one stop over sulfur sensitisation alone, and every other property in Duffin’s account of gold is a cost rather than a benefit: fog if the digestion runs on, and a curve whose maximum is easy to overshoot.

No effect on image colour is claimed by any source read here. A gold-sensitised emulsion is not a gold-toned print, and the two operations put gold in the material for entirely different reasons — one before exposure, in the crystal’s sensitivity centres, and one after development, on the silver of the image.

No effect on grain or contrast is documented at these doses in anything this course has read. That is an absence of evidence rather than evidence of absence, and the honest form of it is that the corpus records speed and fog and stops.

And there is no image characteristic to report for the stock itself, which is a colourless solution in a brown bottle.

The mechanism

Two steps, and the first one happens in the bottle rather than in the emulsion.

⚗Why the stock is aurous, and why it must clear before use

Gold arrives as gold(III): chloroauric acid is an acid of gold in its higher oxidation state, and gold(III) is the wrong species for almost every photographic purpose because each gold atom it deposits costs three electrons.

Thiocyanate does two jobs at once, and the course’s own gold chemistry has already established both: a sulfur ligand — thiosulfate, thiocyanate or thiourea — assists the reduction of gold(III) to gold(I) and stabilises the gold(I) that results, holding it in solution as a complex rather than letting it come down as metal. The alternative route, a mild alkaline buffer, reaches gold(I) without a sulfur ligand and is not what this formula does.

“Aurous” in the name of the solution is therefore a statement about what has happened, not a label. Steigmann’s solution is a gold(I) thiocyanate complex, and the instruction to allow it to clear before using is the instruction to wait until that is what is in the bottle. This is the same chemistry, and the same waiting, as the thiocyanate toners the course does run — see the gold thiocyanate toner — one operation earlier in the life of the silver.

The second step, in the emulsion, is where this page stops. What gold does at a sensitivity centre — whether it joins the silver sulfide speck, replaces part of it, or lowers the number of atoms a stable latent-image centre needs — is the subject of Duffin’s chapter on chemical sensitisation, and the course sends the reader there rather than compressing a chapter into a claim. What is safe to say, because Duffin says it, is that gold needs the sulfur: the gain is measured over a sulfur-sensitised emulsion, and the practice is called sulfur-plus-gold for that reason.

Function of every ingredient

Chloroauric acid, 1 g to 100 mL, of which 6.0 mL goes into the stock. The gold, and the whole cost of the formula. The first thing to get right is the identity: what is sold to photographers as gold chloride is chloroauric acid, HAuCl₄, and not the true chloride gold(III) chloride, AuCl₃ — the encyclopaedia gives the two separate pages for exactly this reason, and the difference is a third again as much gold per gram. Every formula in this corpus that says “gold chloride” means the acid. In this formula, at this quantity, it is supplying gold(III) to a large excess of ligand, which is the condition under which the reduction goes cleanly and nothing precipitates. More gold at a fixed thiocyanate risks an under-reduced stock and, in the emulsion, a dose past the optimum where speed falls and fog rises. Less makes the drop deliver less than the published dose without telling you. It is deliquescent, which is a practical fact with a photographic consequence: an open jar gains water, so a weighing made from a jar that has stood open is a weighing of gold chloride and an unknown quantity of water, and this is the strongest argument in the formulary for buying a solution rather than a solid.

Ammonium thiocyanate, 1 g to 100 mL, of which 50 mL receives the gold. The ligand, the reducing agent and the receiving solution, and it is present in a large molar excess over the gold on purpose. It takes gold(III) to gold(I), so that the gold is in the oxidation state every useful gold chemistry works with, and then holds the gold(I) in solution as a thiocyanate complex instead of letting it plate out. More ligand at fixed gold makes a more stable stock and dilutes the gold; less risks an incomplete reduction and a stock that throws a deposit, which is the visible form of the same failure. Note the salt: this is the ammonium thiocyanate, which is what the source specifies, and not the sodium or potassium salt that other gold formulas in this formulary use — the cation is not the active part, but a formula reproduced with a different salt is a formula the course has altered. Its own encyclopaedia entry carries the point that matters when it is stored: a datasheet will say it contains no cyanide, and its older classification says an acid will make some.

Water, to 100 mL in Solution A and 100 mL added in Solution B. Distilled in Solution B by the source’s own instruction, and warm — about 52 °C — for dissolving only, then cooled before the gold goes anywhere near it. The temperature is a solubility measure and not a reaction condition, and the cooling is part of the formula rather than housekeeping.

Interactions

With light, which reduces dissolved gold. The dark bottle is not optional and the reason is the same across every gold stock in this formulary.

With organic matter, for the same reason. A cork, a dusty funnel, a rinse that left detergent behind: all of them are reducing agents in the sense that matters to a gold(I) solution.

With acid. Ammonium thiocyanate’s own entry records the point plainly enough — the safety sheet says the salt contains no cyanide, and the older classification records that an acid will liberate some. There is no reason for acid to be anywhere near this bottle, and that is the reason.

With sulfur sensitisation, which is a dependency rather than an interaction. Gold is applied on top of a sulfur-digested emulsion. On a plain-silver emulsion with only the gelatin’s own active sulfur — one to two parts per million in an inert gelatin — there is much less for the gold to build on, and no source this course has read reports what happens if it is tried.

With the ammonia of an ammoniacal make, which is the interaction the course sidesteps by not running one. The only published dose is inside such a make, and what the ammonia is doing to the gold, the thiocyanate or the timing is not separated out anywhere in the corpus.

With the emulsion’s own halide and thiosulfate, both of which are competing ligands for silver and for gold in a bath that is already crowded. This is the reason gold added at the end of a digestion needs more than gold added during it, on Duffin’s own account.

Variants

No course variant is offered. There is nothing here to make safer by changing: the formula is two dilute solutions and the hazard is the gold salt’s own, which a bought stock removes almost entirely.

Duffin names the alternative gold salts and does not give a formula for any of them. His range is stated for “chloroauric acid or a more complex gold salt”, and his own figure 5.9 is drawn for a sodium aurothiocyanate emulsion — which is the same complex this stock makes, arrived at as a compound rather than in the bottle. The course has no preparation for it, so none is printed.

The thiocyanate toners are the nearest relatives that are not variants. The gold thiocyanate toner is 50 mL each of a 2 per cent ammonium thiocyanate and a 0.2 per cent gold solution — the same two substances, at about twice the thiocyanate and a fifth of the gold, for a bath rather than a stock. Setting the two side by side is the quickest way to see that a sensitiser and a toner are different formulas rather than different dilutions.

A course variant that removes the ammonia problem does not exist and cannot be constructed. That is the whole finding of Part V’s evidence pass on this feature, and it is recorded rather than worked around.

Safety

Level B, and it is the powder rather than the solution that sets it. Once the two stocks exist, the operation is measuring drops of a dilute salt solution into another dilute solution.

Chloroauric acid is the serious reagent on this page: signal word Danger, with the corrosive, irritant, health-hazard and environmental pictograms. Its own entry holds the classification and the handling; it is not restated here in different words. Two practical consequences follow. It is corrosive, so gloves and eye protection while the solid is handled or the stock is made are not a formality. And it is deliquescent, so the jar is opened as briefly as possible in the driest place available, and closed tightly.

Buying a ready-made one per cent solution removes the worst of it, which is why the practitioner does, and why this page says so twice.

Ammonium thiocyanate is Level B on its own page and the rule that travels with it is about acid rather than about the salt itself.

Nothing here is a gas hazard, a heat hazard or a dust hazard once the stocks are made, and the realistic exposure for anyone using a bought gold solution is a splash.

And the honest framing of the risk on this page is evidential rather than toxicological: the danger is not that the stock will hurt you but that it will be used in a procedure nobody has tested, on an emulsion it was not published for.

Storage

A dark dropper bottle, which is the source’s own instruction and the only storage direction it gives.

Label the substance, the strength, the date and what it is for. A colourless solution in a brown bottle next to another colourless solution in a brown bottle is a mistake waiting to be made, and the two on this page differ by a factor of about ten in what they cost.

Store the gold chloride solid, if you have it, tightly stoppered in a dry atmosphere. Kodak’s own footnote is that it liquefies rapidly in a normal room atmosphere, which is the same failure as an inaccurate weighing arriving a month early.

No keeping figure is published for the made-up stock and none is given here. Date it, keep it dark, and treat a metallic film, a purple tinge or a deposit as the end of that bottle rather than as sediment to be decanted from.

Make what a season needs. Fifty-six millilitres is a great many drops.

Incompatibilities

Acids, with the thiocyanate. See incompatibilities.

Light and organic matter, which reduce dissolved gold to metal.

Reducing agents of every kind — a developer, iron(II), silver metal itself. That is the mechanism the toners exploit and the last thing a stock bottle should meet.

Bare metal spatulas and stirrers, for the same reason: gold(I) in solution is looking for electrons.

Anything that has held a fixer, because thiosulfate is another sulfur ligand and a stock adulterated with it is no longer the formula.

Waste

Gold is the one reagent in this part worth recovering rather than discarding, and a spent or degraded stock carries most of what was bought.

Bottle it, label it and keep it separate from the silver stream. The general route is the general chemical waste SOP and the disposal ruling; the specific advice on a gold-bearing solution sits on the chloroauric acid page.

A thiocyanate-bearing waste never meets an acid waste, which is the one rule on this page that is absolute rather than advisory.

The quantities are small enough to be embarrassing and that is not a reason to be careless. Fifty-six millilitres of stock holds 60 mg of gold chloride, about 30 mg of gold. It is still a heavy-metal solution and still goes to a labelled container.

Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

It has not cleared. Wait longer, and check the order: gold into thiocyanate, not the reverse. If it still has not cleared, one of the two solutions is not the strength on its label — a deliquescent gold salt that gained water before it was weighed is the usual reason.

A deposit, or a metallic film on the glass. The gold has been reduced past gold(I). Light, an organic contaminant or a metal stirrer. Make it again, and this time in a dark bottle from the start.

A purple tinge to the solution. Colloidal gold. Same causes, same answer, and the colour is a better early warning than the film is.

The dose seems to do nothing. Check the arithmetic before the chemistry: at 1.07 mg of salt per millilitre, a dropper that gives twenty-five drops to the millilitre is delivering four-fifths of the dose you counted.

Fog rose and speed did not. The digestion ran past its endpoint, which is what the published curves say happens: fog climbs throughout while speed peaks and falls. This is a timing fault rather than a dose fault, and the fix is to stop earlier.

You are trying to reproduce a dose from a formula that is not the one it was published in. Stop. That is the case this whole page exists to name.

Experiments

Calibrate the dropper, which is the only measurement here that costs nothing and changes everything. Count drops of the finished stock into a small measuring cylinder to a millilitre, three times, and take the mean. Every figure on this page rests on twenty drops to the millilitre being right for your bottle.

Watch the reduction. Make a tenth of the formula in a clear tube against a white card and record what you see and how long it takes to clear, at 20 °C and again at 5 °C. The instruction says “allowing it to clear” and gives no time; you can give yourself one.

Test the order of addition. Make the formula as printed, and make a second one with the thiocyanate dripped into the gold. If the second throws a deposit and the first does not, you have demonstrated why the order is part of the formula rather than a style of writing.

Design the dose you will not perform. Take Duffin’s 1 to 30 mg per mole, choose a reading of his two figures, and compute the drops for a batch carrying 5 g of silver nitrate. Then compute what the same specification would cost in gold chloride, and what a bottle of stock would serve. A specification that cannot be costed has not been finished, and this is the exercise the fifth project actually sets.

Keep a bottle and watch it. No source gives a shelf life. Make 20 mL, store it dark, and photograph it against a white card monthly for a year. That is a small contribution to a real gap, and it is one a domestic maker can make without a single further reagent.