Plain salted paper salting solution

Two numbers settle what a salt print will look like before any silver touches the paper, and neither of them is the exposure. One is the smoothness of the rawstock. The other is this: twenty grams of common salt and two grams of gelatin in a litre of water.

Ingredient	Quantity	Form the source specifies
Sodium chloride	20 g	dissolved in the heated portion of the water
Gelatin	2 g	swelled first in 250 mL of cold water
Water	to make 1000 mL	250 mL cold for the gelatin, the remainder heated for the salt

Purpose

To lay chloride down inside the surface fibres of a sheet of paper, in just enough binder to keep it there, so that a later bath of silver nitrate can precipitate silver chloride in place.

That indirection is not a historical quirk; it is forced by the chemistry. Silver chloride is insoluble in water and in most solvents, so it cannot be dissolved and brushed on the way a dye or a gum can. Reilly puts it plainly: it must be formed in place by first treating the paper with a soluble chloride and then treating it again with silver nitrate. Everything else about salted paper follows from the fact that the light-sensitive substance has to be assembled inside the paper out of two solutions that are applied separately and dried separately.

The second purpose is less obvious and is the one that repays study. This solution does not put in as much chloride as it can. It puts in deliberately too little — far less than the silver bath that follows can convert — and the reason is Talbot’s 1834 discovery that a paper in which chloride and silver are present in equal amounts is barely sensitive at all. The salting solution is where that deficiency is set.

Recommended uses

Plain salted paper prints, on a smooth rawstock, from a negative of long density range. Reilly offers the formula as a paper which closely approximates the printing materials of the 1840s, and it is the simplest silver process in the whole formulary: two solutions, no developer, no coating rod.

Floating, for three minutes, on the smoother side of the sheet. Reilly prefers floating to immersion for a specific reason rather than a traditional one: immersion salts both sides, and during the silver bath some of the sensitiser is then drawn through to the back, locally sensitising it and producing patches of uneven density on the face of the print. Some rawstocks take immersion without trouble and only experience tells you which.

In white light, at the sink, at leisure. Nothing in this bath is light-sensitive. The whole salting operation may be conducted in white light, and the dried sheets keep indefinitely if they are stored cool and dry. That makes salting a batch operation: salt twenty sheets on a wet afternoon and sensitise two of them a month later.

As the first half of a pair. Paper salted to this formula is sensitised by floating for three minutes on a 12 per cent silver nitrate solution, and after that it keeps for only one or two days. The salting solution has no deadline; the sensitised sheet has nothing else.

As the teaching case for the whole printing-out family. Everything the albumen and arrowroot papers do, this does in the fewest possible moving parts, and every one of their refinements is visible as a change to this one litre.

When another formula is preferable

• For a longer scale, more brilliance and finer detail on a matte surface, an arrowroot salting solution. Reilly’s is 35 g of arrowroot rubbed to a cream, with 35 g of sodium chloride and 3 g of citric acid dissolved in 950 mL of water and boiled together. Note what the higher chloride buys and what the citric acid is for: starch is not chemically active in the way gelatin and the organic acids are, so without the acid, Reilly writes, the prints would be grey and flat.
• For a glossy surface and a shorter density range in the negative, an albumenised salting solution and albumen paper. A transparent binder minimises the scattering of light by the paper fibres, which makes whites whiter and shadows denser, and a paper that does that needs less contrast from the negative.
• For a redder, slightly more brilliant print with no change of method, Reilly’s own citrate version of this same formula. It is set out under Variants.
• For a paper that behaves like a modern one — a developed image, a fixed speed, an enlarger — the silver chloride contact emulsion. It is a far harder thing to make and it answers a different question.
• If what you want is a hand-coated print and not specifically a silver one, the classic cyanotype sensitiser is one coating rather than two, needs no fixer, and costs a fraction as much.

Mixing

Two vessels, and they stay separate until the end. This is the one procedural point that a flat ingredient table cannot carry.

Swell the gelatin cold. Two grams of gelatin go into 250 mL of cold water and are left to take up water. Reilly’s own chapter on binder materials explains why the step exists: in cold water dried gelatin does not dissolve, it swells to a viscous mass, and that mass melts at temperatures above 32 °C. Tipping dry gelatin into hot water instead gives lumps with dry centres that never fully hydrate. Ware’s independent procedure for gelatin sizing is the same operation with numbers attached — swell for about thirty minutes at room temperature, then warm — and is worth reading beside Reilly’s sentence.

Dissolve the salt hot, in the rest of the water. The remaining 750 mL is heated and the 20 g of sodium chloride dissolved in it. Reilly states no temperature for this portion.

Combine, then cool to about 80 °F. The hot salt solution and the swollen gelatin are mixed, and the solution is ready when it has cooled to approximately 80 °F, which is 26.7 °C.

✕How hot is 'heat the rest of the water'? Reilly does not say, and it matters

Reilly gives no figure for the heated portion, and the course will not invent one. What can be said is what two sources state independently. Reilly’s own Chapter Two gives 32 °C as the temperature above which swollen gelatin melts. Ware’s gelatin sizing procedure warms to 40–45 °C and warns explicitly not to exceed it, otherwise the gelatin will be denatured.

The course’s reading — and it is a reading, not Reilly’s instruction — is that “heated” here means hot enough to dissolve salt readily and to melt the swollen gelatin on contact, and that boiling water is neither necessary nor a good idea. Water at 45 to 60 °C in the salt portion brings a mixed litre out somewhere in the thirties, which then has to cool to 26.7 °C before use anyway.

Watch the temperature you float at, for a reason that is mechanical rather than chemical. Reilly warns that if the salting solution is warmer than the room — which is also the temperature of the paper — the sheets will curl ferociously when they are laid on the surface. His 80 °F is above a typical room temperature, so his own two instructions pull slightly against each other. Take the 80 °F as an upper bound rather than a target, get the paper and the solution to the same temperature, and read the curling remedies under Troubleshooting.

Then float, dry and press. Mark the back of each sheet in pencil before it goes anywhere near the tray, because once a sheet is salted and dried there is no way to tell which side was treated. Lower the sheet from one corner so that bubbles are rolled out to the edges, start timing only when the sheet lies evenly and no bubbles remain, and lift it slowly from one corner after three minutes. Hang it by the two corners of the long edge, so the run-off has the shortest distance to travel; that is what limits the heavier coating that always forms towards the bottom. When dry, press the sheets flat under weights, which makes them supple enough to handle in the silver bath.

🧮Twenty grams, two grams, and the twelve per cent that comes next

20 g ÷ 58.44 g/mol = 0.342 mol/L, a 2.0 % w/v bath

Chloride, as made

120 g ÷ 169.87 g/mol = 0.706 mol/L, a 12 % w/v bath

Silver, in the sensitiser Reilly specifies for it

0.342 mol/L × 169.87 g/mol = 58 g/L, a 5.8 % bath

Silver that would exactly consume that chloride

So Reilly’s silver bath carries about twice the silver needed to convert the chloride, and a little over half of it survives the reaction as free silver nitrate in the paper. That surplus is not waste. It is the second reagent of the printing-out process, and the section on the mechanism is about what it does.

Two checks on the arithmetic. Reilly states the rule twice himself, in two forms: that the sensitising solution must be approximately four times as strong as the salting solution, and that Talbot found about six times more nitrate was necessary, with a salting solution of 2 to 4 per cent against a silver bath of 12. This formula is the six-times case exactly. And the course’s own step from bath concentrations to what is in the sheet assumes that the paper carries away comparable volumes of the two baths, which is an assumption, not a measurement; what is not an assumption is that the silver is in large excess, because both authors say so directly.

The gelatin, for scale: 2 g/L is 0.2 per cent, one fifteenth of the 3 per cent solution Ware uses to size a sheet. This is not a coating. It is a trace of binder carried in with the salt.

Behaviour

It is a solution of table salt with a trace of protein in it, and it behaves like one. There is nothing to oxidise, nothing to precipitate, nothing that goes off in an hour. The interesting behaviour is all in what it does to the paper.

It sinks in, and how far it sinks is the whole game. Reilly is explicit that with a formula carrying this little binder the choice of rawstock is extremely important and has a great influence on the results: porous rawstocks such as watercolour paper will yield very flat prints indeed, and early photographers generally chose the smoothest stocks available. The two grams of gelatin slightly inhibit the sinking of the image into the paper fibres. They do not prevent it.

Sheets curl on it, and the curl is a moisture gradient rather than a fault in the formula. Curling is caused by the swelling of the paper fibres on the underside of the sheet, so it is cured by wetting the top: a fine mist from an atomiser, or the folded-edge “boat” that resists bending, or simply holding the edges down until the sheet relaxes. Thin papers are easier.

The tray is used for sheet after sheet, and Reilly publishes no capacity for it. He describes successive floats from the same tray and tells you to check the surface for dirt and bubbles before each one, so re-use is his practice; but he gives no number of sheets per litre and no exhaustion test, and the course has none to offer. What actually depletes the bath is straightforward — every sheet carries off both salt and water — and what would settle it is a measurement nobody has published: the chloride left in the tray after a stated number of sheets of a stated stock.

No published keeping time for the made-up solution either. See Storage, where what is known about dilute gelatin solutions is set out and attributed.

It is the last operation in the process that is easy. After this comes silver nitrate, a sensitised sheet with a two-day life, an exposure judged by eye, and a fixing bath thrown away after ten prints. Salting is where a beginner should spend an afternoon getting the coating even, because it is the only step that forgives being repeated.

Image characteristics

Matte, low in maximum density, and long in scale. These three go together and they all come from the same fact: with only 0.2 per cent gelatin the image silver is formed partly within the paper fibres rather than in a compact layer on top of them. Light reflected from the print is scattered and diffused by those fibres, so the darkest areas look paler than the same quantity of silver would look in a transparent binder, and the achievable maximum density falls.

Which is why it wants a long negative. Reilly’s rule runs the other way from the intuitive one: to get a dense black on a matte paper you need more reduced silver, which means a longer exposure, which means the negative must have enough density in the highlights to stop the light tones going too dark while you make it. Plain salted papers therefore require the greatest density range of any of the papers in his book; matte arrowroot and matte albumen slightly less; glossy albumen less again; and all of them far more than a develop-out paper. A negative made for enlarging paper will print flat and empty on this material, and that is a property of the paper, not a fault in the salting.

Long scale, and unusually graceful about it. Reilly reports Hübl’s measurements: the scale length of glossy albumen paper equalled that of platinum paper, and the scale length of salted papers exceeded platinum by a considerable margin. The reason is self-masking — as silver builds in the shadows of a printing-out print, that silver acts as if it were extra density in the negative, delaying the arrival of maximum density and letting the highlight detail print in. Printing-out papers have far less tendency to the “soot and chalk” look than develop-out papers.

Reddish, and the gelatin is why. Reilly’s chapter on binders states it directly: gelatin lends a characteristic reddish colour to salted paper prints, and Talbot’s own prints showed that colour not because he put gelatin in his salting solution but because the English papers he used were already gelatin-sized by the mill. Two grams per litre added on purpose is a small, deliberate version of the accident that gave the first salt prints their colour.

And it will change twice more before you are done. A printing-out image loses density and turns yellower in the fixing bath, reddens in the first wash and firms up again on drying. That sequence belongs to the fixing bath and is described there; it is mentioned here only so that nobody judges their salting solution by a wet print.

The mechanism

Step one: a double replacement, inside the paper. The salted sheet carries sodium chloride in and just under its surface. Silver nitrate arrives, and because silver chloride is almost insoluble it precipitates immediately where the two meet.

NaCl + AgNO₃ → AgCl + NaNO₃

Silver chloride formed in situ, which is the only way it can be formed

The sodium nitrate takes no part in the image and washes out. The silver chloride cannot go anywhere: it has been made as a fine precipitate inside the fibres and inside the trace of gelatin, and where it lands is decided by how deep the salting solution soaked.

Step two: light, and a reaction that wants to run backwards. Ultraviolet and blue light reduce silver(I) to metallic silver and oxidise chloride to chlorine.

2 AgCl + light ⇌ 2 Ag + Cl₂

Photolysis, and the back-reaction that stops it

Ware stresses that this is reversible. Chlorine is a powerful oxidising agent, and if nothing removes it the print-out soon comes to a standstill as the chlorine re-forms silver chloride from the silver just made. In a pure silver halide crystal that limit is severe: the photolytic silver particles stop growing at around 10 nm and the density reached is about 0.02, which is a barely perceptible darkening. That is the whole reason a salt print needs more than silver chloride in it.

Step three: the excess silver nitrate takes the chlorine away. This is the function of the surplus computed in the maths callout above.

Cl₂ + H₂O + Ag⁺ → AgCl + HOCl + H⁺

What the free silver nitrate and the paper's own water do with the liberated chlorine

Ware’s argument is that the combined action of water — cellulose paper holds around 8 per cent water at ordinary humidity — and free silver(I) ions is the major halogen acceptor, and that it contributes the greater part of the image silver. The chloride made in that reaction meets more free silver and becomes fresh silver chloride, which light then breaks down again. Vogel reasoned to the same conclusion in the nineteenth century and Reilly quotes him: the liberated chlorine unites with the silver nitrate present to form new silver chloride, light breaks that down, and the cycle repeats. More free silver nitrate therefore means more image silver and a greater maximum density, which is the practical statement of everything above.

Step four, and this is where the salting solution earns its place. Ware sets out the surface chemistry that makes the ratio matter. A precipitated silver halide is not stoichiometric at its surface: whichever ion is in excess in the surrounding solution is adsorbed onto the crystal. Where silver ions are in excess — Talbot’s case, and Reilly’s — the crystal surface carries adsorbed Ag⁺, the excess positive charge attracts photoelectrons to the surface, and silver specks can grow there free of the constraints of the lattice, with the halogen escaping into a medium that will take it. Where halide is in excess, the surface carries adsorbed chloride and behaves like a fixed, stabilised halide instead.

That is why 20 g and not 80 g. A salting solution strong enough to convert all the silver would not make a paper with more silver chloride in it; it would make a paper whose silver chloride sits in the wrong ionic environment and barely prints.

⁇What is the gelatin actually doing? Reilly and Ware do not agree

Reilly’s account (1980). Certain organic substances are “active” in printing-out papers — he names albumen, gelatin and the organic acids such as citric, tartaric and oxalic — and they facilitate the more complete reduction of silver chloride as well as forming light-sensitive substances of their own with silver nitrate, such as silver citrate. Starch and the resins are, in his terms, not active: they keep the image on the surface and do nothing chemically, which is why an arrowroot paper needs citric acid added and a gelatin one does not.

Ware’s account (2019). Gelatin may well be the important halogen acceptor in a development emulsion, where it surrounds every crystal and the amount of halogen released is minute, but gelatin is not an effective scavenger of halogen at print-out levels of exposure. On that reading the sizing agent is not essential to the photochemistry of print-out at all, and this can be demonstrated experimentally; what it does instead is retain the sensitiser, and therefore the image, in the surface fibres, and possibly protect the hydrophobic particles of colloidal silver by surface adsorption, thereby influencing the colour and stability of the print.

What the course uses. Ware’s, for the photochemistry, because it is the more recent, is argued quantitatively, and is consistent with his own account of water and silver ions as the dominant acceptor. Reilly’s, for the practice, because his division of binders into active and inactive predicts correctly what a printer observes — an arrowroot paper without citric acid is grey and flat — and because the citrate variant below really does change the colour.

The two are not as far apart as they look. Both agree that the gelatin holds the image at the surface and that this is most of its value; both agree that the organic acid case is different from the protein case. Where they differ is on whether a protein binder contributes to the photochemistry at all, and that is a live question rather than a settled one. Neither author’s position should be quoted here as established chemistry.

Function of every ingredient

Sodium chloride, 20 g — a 2 per cent bath. The halide, and the only thing in the formula that becomes part of the light-sensitive substance. Dried into the paper, it waits for the silver bath and is converted to silver chloride exactly where it is lying.

Its quantity does two jobs at once and they pull in different directions. More chloride means more silver chloride formed, which means more silver available for the image and a richer, denser print: Reilly’s sensitising chapter recommends for most salted papers a chloride content of about 2 to 2.5 per cent with a 10 to 12 per cent silver bath, because lowering the chloride content tends to produce prints that lack brilliance and density. This formula sits at the bottom of that band. But more chloride also consumes more of the silver bath, leaving less free silver nitrate to act as the chlorine acceptor, which works against print-out — that consequence is the course’s own reading of the mechanism above rather than a sentence of Reilly’s.

Two nearby figures are worth having and worth attributing carefully, because Reilly states them of albumen paper rather than of salted paper. In his albumen chapters he writes that a low chloride content of 1 to 1.5 per cent makes a paper less sensitive and slightly more contrasty from thin negatives, at the expense of a rich dense image, and that more chloride than is necessary only results in higher silver consumption without conferring any additional benefit. The direction of both is consistent with what he says about salted papers; the numbers are not stated for this material and should not be read as though they were.

The chloride ion is also, indirectly, a contrast control: Reilly states that the chloride content of the salting solution has an effect on contrast, though large increases in contrast are only available through chromates, which this page does not publish (see Variants).

Gelatin, 2 g — a 0.2 per cent trace. The binder, at a concentration far below anything that would be called a coating. Reilly’s own description of its job is precise and modest: the amount of gelatin is only enough to slightly inhibit the sinking of the image into the paper fibres. It does that by swelling into the surface of the sheet and holding the chloride — and therefore, later, the silver chloride and the image silver — nearer the surface than bare cellulose would.

More gelatin moves the paper towards a glossier, more brilliant, higher-maximum-density material with a shorter scale, which is the direction albumen paper went in and is a different process rather than a better version of this one; it also makes the sheet harder to float and, unhardened, liable to give up gelatin into the processing baths. Less gelatin, or none, gives the grey, flat, sunken print that Reilly says results when no organic binder is added and the paper’s own sizing is insufficient — the reason he says some organic material is necessary even for a simple salted paper.

Gelatin also brings the reddish image colour, and it is the reason a beginner should not substitute freely: what the sizing on the rawstock already contains changes the answer, and the English-gelatin against French-starch difference confused photographers for most of the 1840s before Davanne and Girard explained it.

Whether the gelatin does anything chemically is genuinely disputed, and the contested callout above sets out both positions rather than picking one silently.

Water, to make 1000 mL, in two portions at two temperatures. Not a filler. The cold quarter is a processing requirement — gelatin has to be swelled cold before it will dissolve cleanly — and the hot remainder is what dissolves the salt and then melts the swollen gelatin when the two are combined. The make-up volume is the formula: everything above is a concentration, and a litre made up to 900 mL is a 2.2 per cent salting solution rather than a 2 per cent one.

Reilly requires distilled or de-ionised water for the silver bath, and gives the reason — silver nitrate reacts with chloride and hard-water ions to throw a cloudy precipitate that robs the bath of strength — but states no such requirement here. The course’s reading is that the reason does not apply: there is no silver in this solution for hard water to precipitate, and the salting bath is mostly chloride already. Tap water is what Reilly’s own text implies and the course does not contradict it.

Interactions

With the silver bath, and this is the interaction that matters most. The two solutions never meet in a tray; they meet inside the paper. Their ratio is the design of the process, and Reilly states the relationship twice — the sensitiser about four times the strength of the salting solution, and Talbot’s six times more nitrate than chloride. Change one without the other and the paper changes character: a weaker salting solution with the same silver bath gives a thinner, more contrasty, less permanent print; a stronger one with the same bath eats the free silver nitrate the print-out mechanism depends on.

With the sizing already in the rawstock. This is the classic trap of the process, and it is not a modern one. The Getty Conservation Institute’s account of salt prints records the same division Reilly does: English mills preferred gelatin as an internal size, French and other continental mills used starch. Two grams per litre of added gelatin on a gelatin-sized English sheet is a small adjustment to something already present; the same two grams on a starch-sized sheet is the only protein in the system. Reilly’s advice to salt the smoother, more heavily sized side of the sheet is the practical form of the same point.

With the atmosphere, through the paper’s own water. Ware’s mechanism has water as half of the halogen acceptor, and a sheet dried to a crisp in a hot cupboard has less of it than one equilibrated in an ordinary room. This is also why Reilly notes that a little moisture in the storage environment makes sheets easier to float.

With everything downstream, once. The chloride placed here is what the fixing bath has to remove later, as unreduced silver chloride, and the amount of it is set here and nowhere else. A heavily salted, heavily silvered sheet is a longer fixing job and a longer wash; the sulfite washing aid and the thirty-minute final wash exist because of decisions made in this tray.

With the toner, indirectly. Image colour in a salt print is decided by the binder, by the chloride level, by the free silver and only then by gold or platinum. Two prints from the same negative on differently salted paper will not tone to the same colour.

Variants

Reilly’s citrate version, printed immediately beneath this one. It is the same formula with 20 g of sodium citrate added:

Ingredient	Quantity
Sodium chloride	20 g
Sodium citrate	20 g
Gelatin	2 g
Water	to make 1000 mL

Reilly’s stated effect: the addition of a neutral citrate causes the prints to be more reddish in colour and slightly more “brilliant”, and he adds at once that the porosity of the rawstock will still be the largest single factor. Both papers take the same sensitising, toning and fixing. Mechanistically the citrate belongs to Reilly’s “active” class — an organic acid salt that both assists reduction and forms a light-sensitive silver citrate of its own. It is recorded here rather than as a separate entry because the formulary register holds no id for it; that is a bookkeeping decision and not a judgement about the formula.

The arrowroot paper, in Reilly’s Chapter Three: 35 g of arrowroot, 35 g of sodium chloride and 3 g of citric acid in 950 mL, boiled. Not a variant of this formula so much as its answer to a different question — a starch binder gives more brilliance and a longer scale but is chemically inert, so an active organic acid has to be brought in separately. It is a planned entry of its own.

What the modern literature describes. The Getty’s process description of salt printing gives a salt solution of around 4 per cent that is often sodium citrate with ammonium chloride, and a sensitiser of around 12 per cent silver nitrate sometimes containing citric acid. That is a different and later recipe from Reilly’s plain gelatin one, and it is worth knowing that the process as practised today has largely moved to a citrate-and-ammonium chloride salting. This page publishes Reilly’s, and says so.

Talbot’s own, for the historical floor. Ware’s reading of Talbot’s Notebook P puts the salting solutions between 0.6 and 2.3 per cent w/v, with a customary trough of 1.1 per cent of domestic culinary salt, against silver solutions of 11 to 23 per cent. No gelatin was added — the paper’s own size supplied it. Talbot’s own summary of his 1839 Royal Society paper mentions the great variety of effects resulting from comparatively small differences in the mode of preparation of the paper, which is as good a one-sentence justification for this page as anything written since.

☠The chromate contrast addition, which this page does not publish

Reilly gives a third salting solution containing a 5 per cent potassium dichromate stock, for raising contrast when a negative is too thin. He introduces it with a caution of his own, notes that it has no place in regular practice, and records that the judgement of years of experience by many writers is that the best salted paper prints are made in sunlight with optimum negatives and without contrast-enhancing additions.

The course does not publish its quantities here. A dichromate is a chromium(VI) salt, and under the course’s own safety classification any formula containing one is a Level D historical-study entry with no working procedure — the treatment the improved classic cyanotype already receives. Putting a Level D recipe inside a Level A page would put the two on the same footing, which is exactly what the rule exists to prevent.

What is worth knowing without the recipe: the effect falls mainly on the highlights; stability and predictability decline as the dichromate rises; the papers must be salted and stored in yellow or diffused light because the dichromate is itself light-sensitive; the solution must not touch metal; and the image colour goes browner. The technique dates from Ferdinand Hrdliczka in the mid-1890s, when printing-out papers were already losing ground. See the chromium page for why the course treats chromium(VI) the way it does.

Safety

Level A. Common salt and gelatin in water, made up at the sink in white light. Both substances are food. Sodium chloride and gelatin each carry aggregated classifications with no hazard statements, as their encyclopaedia pages record, and neither has a workplace exposure limit.

The controls that apply are the ordinary ones and they apply because of what else is in the room, not because of what is in the tray: gloves and eye protection as a matter of habit, a tray that has never held food and never will, and labelling per the labelling SOP.

What is not a hazard here, and why. There is no silver in this solution, so there is nothing that stains skin or that has to be collected — the whole silver nitrate regime belongs to the next bath and not to this one. There is no acid and no alkali, so no vapour and no route to a gas. Nothing here is light-sensitive, so there is no safelight requirement and no dim-light handling; salting is a white-light operation. Heating is involved but only to the temperature of hot tap water, so this is not a hot-work hazard either. Warm gelatin solutions are a microbiological question rather than a toxicological one, which is dealt with under Storage.

That short list is the honest one. The dangerous parts of making a salt print are the silver nitrate at the sensitising bench, the ultraviolet exposure — direct sunlight is the traditional source and it carries the ordinary sunburn and eye hazards — and whatever the toner contains. None of them is here.

Storage

The made-up solution: Reilly publishes no keeping time, and the course will not supply one from nothing. What can be said, and attributed, is that this is a dilute gelatin solution and that dilute gelatin solutions spoil. Ware, writing about a gelatin sizing solution at fifteen times this concentration, states that it may be re-used but that its storage life, even refrigerated, is limited to a few days, and that it will suffer decomposition unless preservatives are added. Reilly himself uses thymol as a fungicide in the starch paste of his mounting chapter, at 0.4 mL of a saturated solution in methyl alcohol per batch, and notes that it does not affect the paste. Applying either of those to this bath is an extension by the course, not an instruction from the source, and is marked as such.

The practical course, therefore: make what you will use. A litre salts a great many sheets, and salting is cheap, quick and repeatable. Refrigerate what is left, use it within a few days, and throw away anything that smells or that has grown a haze.

The salted paper: indefinitely, if it is kept cool and dry. This is the single most useful property of the process and Reilly states it flatly. Salt a batch, press the sheets flat, keep them between boards in a dry cupboard, and take one out when the light is right. Mark the back in pencil before salting, because after drying there is no way to tell the salted side from the other one and sensitising the wrong face wastes both the silver and the sheet.

The sensitised paper: one or two days, depending on temperature and humidity, and that clock starts at the silver bath and not here.

The dry chemicals. Sodium chloride cakes in damp air and is otherwise indefinite. Dry gelatin takes up water from the atmosphere, which is why it needs a closed container; it does not spoil dry. Both in labelled containers that have never held food.

Incompatibilities

Silver nitrate, absolutely and by design. The two solutions of this process must never meet anywhere except inside the paper. A splash of silver into the salting tray precipitates silver chloride on the spot and wrecks both — the tray becomes a milky suspension and the silver is gone. Separate trays, separate measuring vessels, separate tongs, and the salting tray kept on the wet bench well away from the sensitising bench. See incompatibilities.

Fixer, on anything that touches the salting tray. A trace of thiosulfate carried back into a salting tray on a wet hand does nothing to the salt solution but is later a dead spot on a print. In a printing-out workflow, contamination runs backwards up the process at least as often as forwards.

Metals, if a chromate is ever introduced. Reilly warns that a bichromated salting solution must not contact metals, which react unfavourably with bichromates. It is one more reason the course keeps that variant out of this page.

Heat, past the point gelatin tolerates it. Not an incompatibility in the hazard sense but in the chemical one: gelatin is denatured above about 45 °C on Ware’s account, and a solution boiled to save time is a different solution afterwards.

Nothing else. There is no acid, no alkali, no oxidiser and no reducing agent in the formula, and saying so is more useful than a long list of things that cannot happen.

Waste

This is the one solution in the salted-paper workflow that is not a silver stream, and knowing which streams are which is the point. Spent salting solution is dilute sodium chloride with a trace of protein and some paper fibre in it. It carries no silver, no heavy metal, no thiosulfate and no chromium — unless a chromate was added, in which case it is chromium(VI) waste and an entirely different problem, which is a further reason this page does not publish that variant.

Where the silver actually goes is the sensitising bath, the first wash after exposure, and the fixer. Reilly’s own description of the initial wash — the clouding of the water is the excess silver nitrate reacting with the ions in tap water — is a description of a silver-bearing stream. The fixing bath page carries the course’s finding on that and the silver-bearing waste SOP carries the container. None of it applies to this tray, and pretending otherwise would blunt the rule where it matters.

Local regulation decides what may go to drain, here as everywhere, and this course cannot tell you what it says where you are. See disposal and, for the general case, the general chemical waste SOP.

Troubleshooting

Flat, grey, sunken prints, with no real black. The commonest outcome, and usually the rawstock rather than the formula. Reilly: porous stocks such as watercolour paper yield very flat prints indeed. Try the smoothest paper you can find before you change a quantity. If the stock is right, the next suspects in order are a negative of insufficient density range, too little chloride, and a silver bath that has fallen below strength.

Blotchy patches of uneven density on the face of the print. Both sides of the sheet were salted — almost always because it was immersed rather than floated — and during sensitising some silver was drawn through to the back, sensitising it locally. Float instead, or find a stock that tolerates immersion.

Paper-white round spots with a light brown stain around them. Air bubbles were trapped under the sheet while it floated on the salting solution, so that area never received chloride; the silver nitrate that arrived later had nothing to react with and stained the paper on its own. Pure paper-white spots with hard edges and no stain are the same fault one step later, in the silver bath. Lift the sheet from a corner, burst any bubble with a toothpick, and start timing only once the sheet lies flat.

The sheet curls ferociously the moment it touches the surface. The solution is warmer than the paper. Let it cool towards room temperature, mist the back of the sheet lightly with an atomiser, fold a half-inch flap on all four edges to make a shallow boat, or hold the edges down until the sheet relaxes. Do not let droplets collect on the back.

A visible band of heavier coating along one edge after drying. The run-off ran to the bottom while it dried. Hang by the two corners of the long edge so the run has the shortest distance to travel, and weight the bottom edge with clothes pegs if it curls.

You cannot tell which side was salted. You cannot; nobody can. This is why the back is marked in pencil before the sheet goes in the tray. A sheet whose treated side is unknown can be salted again on both sides and used as a test strip, but not trusted for a print.

Blotches or light spots in the dense areas — the old manuals called it the “measles”. Not this solution’s fault. Reilly attributes it to insufficient sensitisation: a silver bath that is too weak, or too little sensitiser left in the paper. Re-float an unexposed sheet on a stronger silver solution.

A haze, a smell or a skin on the stored salting solution. The gelatin has spoiled. Discard it and make fresh; see Storage.

Experiments

The chloride series, which is the experiment this page exists for. Salt four sheets of the same stock at 1, 2, 3 and 4 per cent sodium chloride, keeping the gelatin at 2 g/L and everything downstream identical, and sensitise all four on the same 12 per cent silver bath. Print each from the same negative to the same visual endpoint. You are testing Reilly’s recommendation of 2 to 2.5 per cent for salted papers, and asking whether the trade-off he describes for albumen paper — slightly more contrast and less density below 1.5 per cent, nothing but higher silver consumption above the optimum — carries across to a plain gelatin-salted sheet. He does not say that it does. Record the exposures, because they will differ.

The gelatin series. Same salt, gelatin at 0, 1, 2 and 4 g/L. The 0 g/L sheet is the control that tests whether your rawstock’s own sizing is sufficient — Reilly says it usually is not. Look specifically at maximum density and at how sunken the image looks in raking light, and at the colour, which is where the reddish contribution should show.

The rawstock series, which will probably teach you more than either. One salting solution, five papers: a smooth hot-pressed sheet, a laid writing paper, a cold-pressed watercolour paper, a gelatin-sized sheet and a starch-sized one if you can identify them. Reilly’s claim is that porosity is the largest single factor in the result; this measures how much larger.

Measure the scale length properly, with a 21-step tablet. Reilly’s method: sensitise a test sheet, print it under the step tablet long enough that the step-1 patch matches the density of the margin outside the tablet, process normally, and count the steps you can distinguish. That count is the paper’s scale length, and the density range of the negative you should be making for it is approximately the density difference between the highest and lowest steps you can still see. It is the only way to stop guessing at negatives for this process, and it should go in a laboratory report.

Test the silver-to-chloride ratio directly. Salt one batch at 2 per cent and sensitise sheets from it on 6, 9, 12 and 15 per cent silver baths. The 6 per cent bath is close to the calculated point at which the silver only just consumes the chloride and leaves almost nothing free; if the mechanism section is right, that sheet should print out visibly more slowly and to a lower maximum density than the others, and the difference between 12 and 15 should be small. This is the cheapest available test of a hundred-and-ninety-year-old piece of chemistry.

Time the float. One sheet at 30 seconds, one at 3 minutes, one at 10. Reilly specifies three minutes and does not say what the number is protecting. Find out on your own stock, and note whether a longer float simply puts more salt in or starts putting it deeper.