Lab: Salting and Sensitising a Sheet of Paper

Purpose

To make a batch of salted sheets that will keep indefinitely, a silver bath that will not, and five sensitised sheets to print with in the next session — and to close a silver mass balance that says how much silver went onto the paper, how much stayed in the tray, and where the rest of it is going to end up.

The session is split from the printing lab because there is a drying step between the two coatings and another after the second, because sensitised paper lasts a day or two while salted paper lasts for years, and because the silver nitrate handling belongs with its own hazard discussion rather than being buried in a three-hour printing session.

No quantity on this page is new. The two salting solutions and the sensitiser live in the formulary with their provenance, their variants and the function of every ingredient; the tables below are the working sub-volumes for one session and say which formula they come from.

Learning objectives

By the end of the session you will be able to:

• State what each component of a salting solution does — the chloride, the binder and the citrate — and predict what changing each one does to density, colour and surface.
• Float a sheet without wetting its back, judge the three ways it goes wrong, and say when a brush is the better instrument.
• Explain why the sensitising solution is four times the strength of the salting solution, in moles rather than in percentages.
• Prepare and label a silver nitrate solution, and say why the water has to be distilled.
• Handle silver nitrate to the controls its classification requires, and say what the black stains are and why they appear hours later.
• Recognise an exhausted silver bath from its colour and its volume, and replenish it by the published rule of thumb.
• Close a silver mass balance for one sheet, and say which of its terms you measured and which you took from a source.
• Design and run a keeping test rather than repeating somebody else’s figure for how long sensitised paper lasts.

Prerequisites

• Printing out: silver chloride made and darkened in the paper — the chemistry this session is the craft half of. Its account of the excess silver is assumed here.
• Part II’s silver nitrate lesson and the silver nitrate handling SOP, which this page cites rather than repeats, together with the spill procedure and the waste containers lesson.
• What these processes need — the paper, with one correction made below: cyanotype’s reason for demanding an unbuffered sheet does not carry over to this process, and the reason it does not is worth understanding.
• Concentration and dilution and making up to a final volume, because a per cent w/v that was made by addition rather than to a volume is not the strength on the label.
• Weighing a solid and recording a batch. Two batch records come out of this session and they are what makes next month’s comparison mean anything.

Safety classification

Level B, and the letter is carried by one substance in one operation.

The classification rubric assigns a level to the procedure as written, and here the assessment is short. Silver nitrate carries H272 (may intensify fire; oxidiser) and H314 (causes severe skin burns and eye damage) with the aquatic statements, under the signal word Danger, in the classification PubChem aggregates from the harmonised European entry. A 12 per cent solution handled wet, over trays, at face height, with sheets lifted dripping out of it, meets the Level B criteria for concentrated reagents and for eye hazard.

The salting step by itself is Level A. Sodium chloride, gelatin and sodium citrate in water, handled in white light, with nothing volatile and nothing corrosive; Reilly notes that all salting operations may be done in white light because the paper is not light sensitive until it has been on the silver. The page is classified for the operation that governs, and the two halves are kept visibly separate in the procedure so a reader can see which controls belong to which.

⚠What is not a hazard here, and why saying so is not a licence

The dry crystal is not the same object as the coated sheet, and neither is a photochemical hazard in the bottle. Ware makes the chemical point: pure silver nitrate in isolation is not photosensitive, because the reduction of the silver ion has no available oxidation half-reaction to supply the electron — nitrate is fully oxidised and water needs a larger potential than the silver couple provides. It becomes photosensitive only in the presence of a readily oxidisable substance, which on your bench means the paper. Bostick & Sullivan state the practical consequence for their own kit: the silver nitrate is not light sensitive as long as it is kept separate, and can be handled under most room lighting. The Photographers’ Formulary sheet adds the qualification that matters for storage rather than for handling — in strong light it does decompose, forming a black precipitate, so the bottle is brown and lives in a cupboard.

Ventilation is not among the controls for the silver, because nothing here evolves a vapour. Two of the three solutions are made cold; the gelatin one is warmed to about body heat. Neither is volatile and neither gasses off. The airflow section below says what the control actually is.

There is no chromium(VI) in this session. Both bought kits ship a dichromate contrast solution and both explain how to add it to the salting solution. This course does not use it, does not print the quantity and does not have it in the room; the chromium ruling is why, and the chemistry lesson gives the four contrast levers that are used instead — which are the ones Reilly himself prefers.

And there is no free ammonia. The historical ammonio-nitrate sensitiser is discussed on the sensitiser’s formulary page and is not made here. Reilly’s reason for refusing one particular operation is worth carrying even though this page does not go near it: an ammoniacal silver bath that is boiled down can form fulminating silver, which is explosive, and he declines to recommend the procedure to anybody.

None of that softens the eye hazard. It is the strongest thing the substance carries, and the controls below are not proportionate to how much of it there is.

!A raised step this page does not take

Reilly’s method for measuring the strength of a used silver bath is a Volhard titration — sodium thiocyanate run in against a ferric ammonium sulfate indicator, with the silver solution acidified by 6 molar nitric acid — worked out for photographers by Irving Pobboraysky and in use on albumen sensitising baths as early as 1875. It is the accurate answer and it is the one a serious printer should eventually learn.

This page does not run it, because concentrated nitric acid and a thiocyanate on the same bench would put the session past Level B on the rubric, and because the classification page’s own advice is that a raised step which can be avoided should be. What the page uses instead is Reilly’s published rule of thumb, which needs no reagents at all, together with his warning about what it costs: the errors are cumulative, so it is not suitable for continuous use or for a large volume of printing.

Hazards

Hazard	Where it arises	Control
Severe eye damage (H314) from splashed silver nitrate solution	Making up the bath, floating and lifting sheets, brush coating, every transfer	Chemical splash goggles, not spectacles, from the moment the jar is opened until the bench is washed down. Reilly’s specific warning is that sheets dripping silver solution pose a special hazard to the eyes, and the eyewash SOP is read before the session rather than during it
Skin burns (H314) from the solid or a concentrated solution	Weighing; a spill left in contact	Nitrile gloves, sleeves, apron. Photographers’ Formulary’s own first-aid line: wash with cold water then soap and water, and treat any wound as a heat burn
Black staining of skin, and argyria on repeated absorption	Any contact, however small; the stain appears over hours	Gloves every time and wash immediately after contact, which is Reilly’s instruction. The stain is silver bound to skin protein and being reduced — a marker of exposure, whose importance is the systemic condition repeated absorption can cause
Oxidiser (H272) — may intensify fire	The dry solid, near paper, blotter and towel	Jar closed and away from combustible material, and never mop a spill of the solid with paper towel and leave it in a bin. The solid spill SOP governs
Permanent staining of everything else	Every surface, garment, floor and tray	Bostick & Sullivan put it in capitals: it stains all surfaces black. Blotter down, a tray under everything, and the drying line over a covered floor and not over a walkway — Reilly’s own rule
Reaction with metal	Brush ferrules, clips, tongs, trays, spoons	Nothing metal touches the silver: Reilly specifies a brush bound with thread rather than a metal ferrule, because silver nitrate reacts with the ferrule and stains the print, and advises against a stainless steel tray
Aquatic toxicity (H400 and H410 in the aggregated classification)	Every drop of every solution and rinse	Nothing to the drain. All of it to the silver-bearing waste container, labelled under the labelling SOP
Hot water scald	Dissolving the gelatin at about 38 °C	Trivial at that temperature; named because the step exists

Required PPE

Chemical splash goggles for the whole of the sensitising half. Sealed goggles, not safety spectacles. This is the one item on the list that is not negotiable and not proportionate to quantity: Reilly’s warning is that silver nitrate can cause permanent, irreversible eye damage, and the highest-risk moment is a wet sheet being peeled off a tray at chest or face height with both hands occupied.

Nitrile gloves, changed when contaminated. HSE’s guidance on managing skin exposure puts gloves last in the hierarchy rather than first, and this session takes that seriously: the primary controls are the covered bench, the tray under everything and not touching wet sheets by the face. The gloves are what stops the exposure the other controls miss. Reilly asks for tightly fitting gloves — a loose glove that lets solution run inside the cuff while a sheet is held above the tray is worse than none. A glove that has been in the silver does not go back into the salting solution.

An apron, and long sleeves. Not for skin protection so much as because the stain is permanent and appears the next day on the shirt you did not think you had splashed.

Ordinary safety spectacles for the salting half are enough, and the page says so rather than demanding goggles for everything: a control that is obviously unnecessary is a control people stop obeying.

No respiratory protection is specified, because no step here raises a dust or a vapour. If you grind or crush caked silver nitrate crystals rather than tipping them, you have invented a dust hazard the page did not plan for; do not.

Ventilation

The control here is separation and containment rather than airflow, because nothing in this session produces a vapour or a gas. Silver nitrate is not volatile, the solutions are made at or a little above room temperature, and the only warm step is dissolving gelatin at about 38 °C, which evolves water and nothing else.

What the room does need is the ordinary through-draught HSE asks for wet photographic work, on a lipped and wipeable surface — and, specifically for this process, stable air rather than moving air. Reilly’s working conditions for these papers are 18 to 20 °C at about 45 per cent relative humidity, and a draught across a floating sheet dries its upper surface unevenly while the lower one is still taking up solution, which is a coating fault rather than a hazard.

The separation that does matter is between this bench and every other process. Reilly asks for a wet fabrication area and a dry printing area, and rigorous cleanliness where they share one room. What that prevents is cross-contamination: a trace of fixer on a tray puts silver sulfide in the next print.

Materials

• Ten sheets of 100 per cent cotton rag paper, unbuffered, hot-pressed, cut to 8 × 10 in — enough to salt a batch and sensitise five. Bostick & Sullivan specify 100 per cent cotton rag, unbuffered, archival, hot pressed, at least 50 lb for larger images; the Getty Atlas describes the historical choice as good quality handmade or mould-made watercolour paper. Reilly’s requirement is all-rag, for chemical purity and for wet strength — ordinary paper falls apart under coating, sensitising and four washes.
• Distilled or de-ionised water, about 2 L. Not optional for the silver solution; see Chemicals.
• Blotting paper or clean newsprint, plastic clothes pegs (Reilly notes papers stick to wooden ones), paper tape, and a soft pencil.
• Kaolin, about 10 g, if you intend to keep the silver bath rather than working it out in one session.
• A step wedge or a small opaque object for the keeping test, so that the two test strips carry the same pattern.
• A notebook, because two batch records and a mass balance come out of this evening and none of them survives being remembered.

✕The paper rule you must not carry over from Part XXI unchanged

Part XXI insists on an unbuffered paper, and the reason it gives is specific to cyanotype: an alkaline reserve hydrolyses the iron(III) sensitiser and destroys the finished Prussian blue outright. That reason does not transfer to a silver chloride image, and this course has read no source stating that a calcium carbonate buffer destroys a salt print.

Buy unbuffered anyway, and know which of these you are relying on. The reason that is sourced is Reilly’s: the paper must be of the highest purity, all-rag, because impurities react with the coating; and it must have enough wet strength for four immersions. The reason that is inference is the course’s own: silver nitrate is applied in large excess and will react with a carbonate reserve to precipitate silver carbonate in the sheet, which is silver spent on nothing and a potential source of stain — the same reaction Reilly names as the cause of the cloudy precipitate when a silver bath is made with hard tap water. That inference is marked as one, and the practical recommendation is the same either way, so nothing turns on it.

Chemicals

Two solutions, from two formulary entries, at the sub-volumes one session needs.

Half a litre of Reilly’s plain salting solution:

Chemical	Quantity	Form
Sodium chloride	10 g	Pure, non-iodised. Reilly’s own text calls it table salt; iodised salt puts silver iodide in your paper, which is a different and far slower halide
Gelatin	1 g	Swollen in cold water first. Only enough to slightly inhibit the sinking of the image into the fibres, which is Reilly’s own description of its job
Water	to make 500 mL	Tap water is acceptable here and not in the sensitiser; see below

The optional citrate variant, from the same formulary page, adds 10 g of sodium citrate to that half litre. Make it as a second bottle if you want the comparison, which is the most instructive single variable in the session: Reilly’s stated effect is that a neutral citrate makes the prints more reddish in colour and slightly more brilliant.

The sensitiser — 100 mL of the salted paper sensitising solution:

Chemical	Quantity	Form
Silver nitrate	12 g	Colourless crystals. Reilly states that technical or ACS grade is sufficiently pure for this work and that the highly purified grades needed for emulsion making are unnecessary. Record supplier and lot
Distilled or de-ionised water	to make 100 mL	Not negotiable. A make-up volume, not an addition

Why the silver bath is four times the salting solution, and what that means in moles. Reilly’s rule is that the sensitising solution must be approximately four times as strong as the salting solution, so a 3 per cent salting solution wants about 12 per cent silver. Talbot’s own version was that about six times more nitrate was needed, which is the same statement by weight rather than by concentration ratio. Work it in moles and the number becomes intelligible.

🧮Six times by weight is about twice by moles — and that is the point

Molar masses: silver nitrate 169.87 g/mol, sodium chloride 58.44 g/mol.

120 g/L ÷ 169.87 g/mol = 0.706 mol/L

Silver nitrate in the 12 % sensitiser

20 g/L ÷ 58.44 g/mol = 0.342 mol/L

Sodium chloride in the 2 % salting solution

0.706 ÷ 0.342 = 2.06

Molar ratio, silver to chloride

So a six-fold excess by weight is close to a two-fold excess by moles. If the sheet takes up similar volumes of the two solutions, about half the silver applied reacts with chloride to make the light-sensitive substance and about half stays in the sheet as free silver nitrate. That is the excess the chemistry lesson says is the mechanism, expressed as a number.

The assumption is the weak part and it is stated deliberately. Equal volume uptake is an assumption, not a measurement: a floated sheet and a brushed sheet take up different amounts, and the salting solution is applied to a dry sheet while the silver goes onto a sized, salted one. No source read for this course measures the two uptakes separately. The mass balance below asks you to weigh the sheet, which is how you replace the assumption with a number.

Why distilled water for the silver and not for the salt. Reilly gives the reaction: tap water carries chloride and hard-water ions such as carbonates, and silver nitrate meeting them throws down a cloudy white precipitate of silver chloride, silver carbonate and the rest, which robs the bath of strength and makes sensitising difficult. The salting solution has no silver in it and nothing to precipitate, so tap water is fine there. It is the same reaction you will watch happen deliberately in the first wash of the printing lab, where the milky cloud coming off the print is the excess silver nitrate meeting exactly those ions.

One variant to know about and not make tonight. Adding 5 per cent citric acid to the silver bath gives Reilly’s maximum preservative effect, and as little as 1 per cent extends the usable life noticeably — but it changes the print as well as the keeping, pushing the colour reddish-brown even after prolonged gold toning. Make a plain bath first, so the keeping test has something to compare against.

Equipment

A glass tray for the silver, larger than the sheet. Reilly’s instruction is explicit and it is a chemical one: the tray and all containers for the silver solution should ideally be glass, a stainless steel tray is not recommended, and if a plastic tray must be used it should be new, because plastic holds on to chemicals and a tray that once held developer or fixer will stain the prints. A Pyrex baking dish is his own suggestion for a serviceable substitute.

A second tray, labelled, for the salting solution, and it never holds anything else either.

A balance reading to 0.01 g, checked by the balance and thermometer check. Reilly’s own view is that a student balance reading to 0.5 g is adequate for this work; the course weighs more precisely than that not because the formula needs it but because a recorded weight is what makes a comparison possible later.

A 100 mL graduate and a 500 mL graduate, a beaker, a glass stirring rod and a funnel, verified under the glassware SOP.

A brown glass bottle of 100 mL for the silver, and a plain bottle for the salting solution, both labelled under the labelling SOP with substance, strength, date and batch number.

A wide, flat, soft brush with no metal ferrule, or a coating rod, or both. The ferrule rule is Reilly’s and it is chemistry rather than fussiness: silver nitrate reacts with the metal band and prints the reaction as a stain. His alternative is a Blanchard’s brush — a strip of wood or acrylic about 100 mm wide with several folds of clean flannel stretched over it, with the working edge sanded round.

A timer, a line strung at 5 to 7 degrees from horizontal with plastic pegs, blotting paper for the low corner, and a light-tight box or drawer for the sensitised sheets.

Your working light. Reilly’s specification for these papers is a yellow 60 W incandescent bulb, with his own remark that there is no need for the dim light of the safelights modern papers require and that the level should be bright enough to work comfortably by. That is a nineteenth and twentieth-century practice restated by a conservator, not a measured limit, and the page treats it as such — see the keeping and light tests below.

Nothing metal touches anything. No steel tongs, no metal clips, no chipped enamel, no ferrules.

Estimated cost

Band ££, and one line dominates it. The capital is a balance, two trays, a bottle and a brush, all of which Parts II and XXI already asked for; the only new item is a glass tray reserved for silver. What this session adds to the shelf is a jar of silver nitrate, the most expensive consumable purchase in the course to date, and the planner has the dated figures. Every other consumable here is cheap: salt, gelatin, sodium citrate and citric acid together cost less than a single sheet of the paper they go on.

Estimated consumables cost

Every price with a number is the planner’s own dated UK figure and every quantity comes from the Materials and Chemicals sections above. Most of the rows have no number, and that is the finding rather than a blemish.

Consumed	This session	Sourced price	Cost this session
Silver nitrate	12 g into the bath; about 10 mL of solution, or 1.2 g, actually leaves on five sheets	£59.95 per 25 g to £112.90 per 10 g, checked 5 September 2026	£28.78 to £135.48 for the 12 g made up; £2.88 to £13.55 of it goes onto paper
Nitrile gloves	2 pairs	£6.64 to £14.99 per box of 50 to 100	£0.27 to £0.60
Photographic gelatin	1 g	£17.45 per 100 g	£0.17
Cotton rag paper, unbuffered, 8 × 10 in	10 sheets salted, 5 sensitised	Not priced. Hot-press cotton watercolour paper is one of the gaps src/data/prices.json already names	—
Sodium chloride, pure non-iodised	10 g	Not priced	—
Sodium citrate, if the variant is made	10 g	Not priced	—
Citric acid, if the preservative variant is made	5 g	£10.00 per 250 g of the monohydrate	£0.20
Distilled water	about 2 L	Not priced	—
Kaolin	about 10 g	Not priced. It is one of the gaps src/data/prices.json names, and this row is where that gap came from	—

The subtotal is a floor, not a total, because five of the nine rows have no sourced price. What the table does establish is the shape of the cost, and it is the opposite of Part XXI’s: there the paper was the expensive part and the chemistry was pennies. Here the silver is the expensive part by a factor of ten or more, which is why the mass balance below is a cost exercise as much as a chemistry one, and why every drop is collected.

Equipment is deliberately absent: a balance is not consumed and neither is a glass tray, and a page that quietly counts them has stopped measuring what the consumables calculator needs.

Waste streams

Three streams, and one rule.

• The spent silver bath, when you finally discard it, and any bath spoiled by contamination. This is the most concentrated silver solution the course ever produces.
• Rinse water from the tray, the rod, the brush and the graduates. Dilute, and still silver.
• Blotting paper, spoiled sheets and trimmings. Solid, and Reilly notes that print trimmings and spoiled prints were a recognised source of recoverable silver in the nineteenth century, burned for their ash.

The rule: nothing containing silver goes to the drain, at any dilution. The aggregated classification carries acute and chronic aquatic hazard statements, and dilution is not a treatment. Everything goes into the silver-bearing waste container, labelled and dated, and the general chemical waste SOP governs what happens to it after that. Check your local regulations; they govern.

One incompatibility to design the bench around. Silver waste and fixer waste both contain silver and are both collected, but they must not be topped into the same unlabelled container casually: silver nitrate meeting thiosulfate throws down silver sulfide as a black sludge, which is the same reaction that makes a black stain in an unwashed print. The unlabelled container SOP exists because this is exactly how it goes wrong.

Alternative route

This page declares requiresDarkroom, and the declaration deserves an honest reading: what is required is the ability to exclude white light from a room and to store sheets in the dark, not a plumbed darkroom. Reilly’s own requirement is exactly that — the capacity to eliminate white light from the work areas, with a yellow incandescent bulb to work by; Bostick & Sullivan’s domestic version is a windowless room, or shaded windows. A kitchen at night with the blind down and a yellow bulb in the fitting satisfies both, and the salting half needs nothing at all, since it is done in white light and the sheets then keep indefinitely.

No room that can be darkened at all. Sensitise inside a light-tight box, or under a cloth, and dry the sheet in a closed drawer. This is a real degradation and the page says so: you lose the ability to watch the sheet as it takes up the solution, which is one of the two things you are supposed to be learning tonight. What you can still do is the mass balance, which is a balance and a notebook.

No line to hang sheets on. Lay them face up on a clean plastic screen or on fresh blotting paper, changed between sheets. You lose Reilly’s controlled runoff — the inclined line that collects the drainage at one corner where it can be blotted.

Cannot weigh, or cannot keep silver nitrate at home. A bought kit is a legitimate route and it changes only the first part of the session. Bostick & Sullivan supply the 12 per cent silver solution ready made and a proprietary salting solution whose composition they do not publish; Photographers’ Formulary supply the weighed solids and 13 g of silver nitrate to make up. Everything from the coating onward applies unchanged, and both kits’ own instructions are cited on the sensitiser’s formulary page.

And one thing has no alternative. There is no version of this session that does not put silver nitrate on paper, and no substitute chemistry that makes a salted paper print. If handling silver nitrate is not possible for you, the honest route is to read this page and the chemistry lesson and to print a cyanotype instead — which teaches the coating, the contact frame and the print-out judgement, and none of the silver chemistry.

Preparation

A day before. Cut and mark the paper. Reilly’s instruction about sides is worth following exactly: examine the sheet before salting to see whether there is a difference between its faces — usually one is smoother and better sized — and coat the smoother one. If there is a watermark, the front is up when the watermark reads correctly. Then mark the back of every sheet in pencil, because once the sheet is salted and dry there is no way to tell which side was treated.

Two hours before. Bring the paper into the working room so that its temperature and moisture match the room’s. Reilly’s reason is mechanical: if the salting solution is warmer than the paper, the sheets curl ferociously when they are laid on it. A sheet at the room’s own temperature and humidity floats flat.

The bench. Two trays, labelled, in their own places. Blotting down. The drying line strung at 5 to 7 degrees over a covered floor, and not over a walkway. The waste container open and labelled before anything is mixed rather than after the first spill.

The reading. The safety data sheet for your own silver nitrate, recorded under the SDS recording SOP with its version and date. Read the eye statement and the first-aid section before you weigh anything, and locate the eyewash.

Procedure

Seven stages. Attended bench time is about two and a half hours, and stages 2 and 6 contain drying you cannot hurry. Stages 1 to 3 are Level A and are done in white light. Stages 4 onwards are Level B and are done under the yellow working light.

The session, and where the light changes

1. 11. Make the salting solution20 min — white light, Level A. Gelatin swollen cold, salt dissolved hot, combined and cooled to about 27 °C
2. 22. Salt ten sheets and dry them40 min plus drying — white light, Level A. Float three minutes, drain, hang, press flat. These sheets keep indefinitely
3. 33. Make and label the silver bath15 min — white light is acceptable for the solution itself, but goggles from here on
4. 44. Sensitise: two sheets by floating, two by brush or rod30 min — yellow working light, Level B. Time from when the bubbles are gone; lift slowly
5. 55. The keeping test: two more sheets, marked and set aside10 min — one to be used at once in the next session, one after a stated interval
6. 66. Dry in the dark45 min unattended — hung at an angle, low corner blotted, and not over-dried
7. 77. Clean up, close the mass balance, and put the bath away25 min — the tray, the rod, the bench, the bottle, and the arithmetic

Stage 1 — The salting solution (20 minutes, white light)

Follow Reilly’s plain salting solution at half a litre. Swell the gelatin in about 125 mL of cold water and leave it; heat the remaining water and dissolve the salt in it; combine, stir until the gelatin has gone into solution, and let it cool.

Reilly’s working temperature for the bath is about 27 °C. What matters more than the number is that the solution and the paper should be at the same temperature: a bath warmer than the room curls the sheets on contact.

If you are making the citrate comparison, make it as a separate half litre with the sodium citrate added, and label both bottles. Do not make one and adjust it.

Stage 2 — Salting, by floating (40 minutes plus drying, white light)

Floating a sheet, and the three ways it goes wrong

1. 1Bow the sheet and roll it down, corner to corner — the old manuals say middle-first, which traps a diagonal line of bubbles
2. 2Peel it off slowly from one corner — lifted properly, hardly a drop leaves the sheet while it drains
3. 3Curling: bath and paper at different temperatures — solution reaches the back, and the back sensitises
4. 4Bubbles: time only from when they are all broken — pierce with a toothpick, or lift the corner and replace the sheet

Drawn from Reilly's chapter three. The timing rule is his and it is the one most often missed: the float is timed from the moment all the bubbles have been broken and the sheet lies evenly on the surface, not from when it touched down.

1. Fold a quarter of an inch over on two opposite corners as handles. Reilly’s own refinement, and it gives control the old bowing-by-the-diagonal method does not.
2. Bow the sheet, set one corner down and lower the other slowly so the contact rolls across. The old manuals say to lower the middle first; Reilly explains why that is worse — it traps a diagonal line of bubbles running to the two corners you were not holding.
3. Break every bubble. Look through the sheet if the stock is thin enough; otherwise lift a corner and look. Pierce a stubborn one with a toothpick and replace the sheet.
4. Start the timer now, not before. Reilly: time the float from the point at which all bubbles have been broken and the sheet rests evenly on the surface. Three minutes for these formulae.
5. Lift one corner and peel the sheet off slowly. A sheet lifted properly loses hardly a drop while it hangs over the tray to drain, and drips fewer bubbles back into the bath for the next sheet.
6. Hang by the two corners of the long edge, so the runoff has the shortest distance to travel. That is the single largest cause of uneven coating: the solution runs downhill and leaves the lower half thicker.
7. When dry, press the sheets flat under weights. They become more supple and much easier to handle at the sensitising stage.

When to use a brush instead. Reilly’s arrowroot instructions are the model: a starch salting solution thick enough to matter is too viscous to float well, so it is brushed on with a wide flat brush, first in one direction and then in perpendicular cross-strokes, allowed to sink in for a minute or two, and then evened out with a dry round brush until a uniform matte surface is obtained. Photographers’ Formulary offer a third method for their gelatin-citrate-chloride solution and recommend it for beginners: immerse the sheet for about 30 seconds, removing air bubbles, drain and dry. Immersion salts both sides, which costs nothing at this stage — the sheet is not yet light sensitive and there is no silver in the solution — and is exactly what you must not do at the next one.

=Why floating rather than immersion, when it comes to the silver

Reilly’s reason is not about the salting step at all — it is about what the sensitising step does to a sheet that has salt on both faces. If both sides are salted, some of the silver solution is drawn round to the back during sensitisation, which locally sensitises areas of the back and produces patches of uneven density that show as a blotchy appearance on the face of the print. He notes that some rawstocks tolerate immersion salting perfectly well and that only experience tells you which, which is a good reason to keep one immersed sheet in the batch and find out.

Salted sheets keep indefinitely in a cool, dry place. That is the reason this is a batch operation: salt ten and sensitise five.

Stage 3 — The silver bath (15 minutes)

Goggles on before the jar is opened, and they stay on until the bench is washed down.

Weigh 12 g of silver nitrate under the weighing SOP, tipping rather than scraping. Dissolve in about 70 mL of distilled water and make up to 100 mL — a final volume, not an addition; the difference is Part II’s and it changes the strength on your label.

Into a brown bottle. Photographers’ Formulary give the reason for the brown glass and it is not the one people assume: silver nitrate is not itself light sensitive, but in strong light it decomposes and forms a black precipitate, so the bottle lives in a dark place. Label it with substance, per cent w/v, date, batch number and the lot number of the solid, under the labelling SOP and the batch record SOP.

Stage 4 — Sensitising (30 minutes, yellow working light, Level B)

Two sheets by floating, two by brush or rod, so that you have the comparison.

By floating. Pour enough silver solution into the glass tray to float on — this is where the silver goes, and where the mass balance gets interesting. Float exactly as in stage 2, with three differences.

• The sheet must be dry but not bone dry. Reilly is specific that albumen and salted papers should not be excessively dry at the time of sensitising: a conditioned sheet takes up the solution evenly and is less stiff to handle. His conditioning method is to leave the paper overnight in a damp place, or for a few hours in a closed box with a dish of water in it.
• The float is timed and short. Reilly’s average for these papers is two and a half to three minutes, and a three-minute egg timer is his own suggested accessory. For a starch-bound paper it is far shorter — half a minute for a light arrowroot coating and at most a minute and a half for a heavy one — because too long a float on a permeable starch layer lets fine silver chloride sink into the fibres and gives grey, flat prints.
• Nothing must reach the back. A drop of silver on the back sensitises it, and the patch prints through as a blotch.

By brush or rod. Bostick & Sullivan’s drop table is the practical guide: 40 drops of each solution for an 8 × 10 in image, 24 for 6 × 9, 18 for 5 × 7 and 12 for 4 × 5, coated horizontally then vertically. Their two calibration rules are the useful part — if you are still spreading solution after 30 seconds, reduce the volume by a quarter, and with a rod, more than six passes means the same. Reilly adds that brush coating uses slightly stronger solutions than floating, that the brush must be washed in distilled water and reserved for this alone, and that it will discolour without that mattering.

Watch the sheet while it takes up the solution. The dry salted sheet is white or faintly cream; as the silver soaks in, the surface takes a faint grey or straw cast where silver chloride is forming. That is the reaction, not fogging. A distinct brown or purple at this stage is not normal: that is spontaneous reduction, and it means the working light is too strong or the paper was already old.

Lift slowly, drain, hang, blot the low corner. Reilly inclines the line 5 to 7 degrees so the runoff collects at one corner, and blots the drops that form there, because blotting is essential to speedy drying and even sensitisation.

Stage 5 — The keeping test (10 minutes)

Do not take a keeping figure on trust, including Reilly’s. Sensitise two extra sheets, mark them K1 and K2 with the date and the hour, put a small opaque object or a step wedge on each so that both carry the same pattern, and store both in the same closed box in the dark.

• K1 is printed in the next session, at the shortest practical interval.
• K2 is printed after a stated interval you choose — 48 hours, or a week — under conditions you record, and printed in the same session as another fresh sheet so that the comparison is against something rather than against a memory.

Record the room’s temperature and relative humidity, because both change the answer. What you are testing against is Reilly’s published figure: albumen and salted papers remain usable only for one or two days after sensitisation, and in extremely humid, warm conditions they may yellow in 8 to 12 hours or even less. He also gives the failure sequence, which is what to look for: the yellow is spontaneously reduced metallic silver in a very finely divided state; as the reduction continues the paper turns deep reddish brown; ultimately, over several months in dark storage, it turns black with a shiny greenish bronzed surface. And he offers the practical redemption — slightly yellowed paper may still be usable, because the fixer removes some of the finely divided silver from the highlights.

Stage 6 — Drying in the dark (45 minutes, unattended)

Hang in the dark, or in the yellow light and then into a closed box. Do not over-dry. Reilly is explicit that most salted papers will not produce satisfactory prints if they are too dry, and that some moisture is needed for good sensitivity and contrast at the printing stage — a point the printing lab returns to, because a bone-dry sheet in a printing frame is also a slow one.

Store the dry sheets flat, interleaved with soft tissue as Photographers’ Formulary recommend for salted paper, in a closed box, in the dark, and use them the next day. Reilly’s own conclusion is that for best results it is necessary to sensitise, print and process all on the same day.

Stage 7 — Clean-up and the bath (25 minutes)

Deal with the bath first, while you still have gloves on.

Judge it by colour and by volume. Reilly’s account of what happens to a silver bath in use is exact. The strength falls because silver nitrate reacts with the chloride in the paper. The volume falls because each sheet mechanically carries solution away. And the colour goes from water-clear to brown and eventually to almost black, for a reason that has nothing to do with either: organic matter from the paper’s sizing or binder always partly dissolves into the bath, reacts with some of the silver, and eventually reduces it spontaneously to metallic silver, which is what colours the solution. His rule is that the bath may be used slightly discoloured but not when it is nearly black.

To decolorise it, shake it with about 15 g of kaolin per litre, which adsorbs the organic matter and settles overnight; Reilly leaves the kaolin permanently in the storage bottle. Filtering does the same job faster and is necessary rather than optional once a surface scum has appeared, because scum prints as a metallic marbled sheen.

To replenish it, use the rule of thumb rather than a titration: replace the lost volume with a silver solution twice as strong as the original. Reilly’s worked example is a litre of 12 per cent fallen to 850 mL, replenished with 150 mL of 24 per cent; citric acid, if present, is replenished on volume lost alone. His estimate of how long a 12 per cent bath lasts is two or three batches — say thirty 11 × 14 prints — before a determination of strength is really needed, and he prefers starting at 12 rather than 10 per cent so the bath spends longer in its comfortable range.

📓The mass balance, and how to close it

This is the point of the session and it takes five minutes if you weighed things as you went.

What to weigh. The bottle of silver solution before and after; the tray before and after; each sheet dry before sensitising and dry after. That last pair is the one people skip and it is the only direct measurement of what actually went onto the paper.

The four terms.

m(applied) = mass gained by the dry sheet, in grams of solution × 0.12

Applied to the sheet

m(reacted) ≈ half of the applied silver, if the two solutions were taken up in similar volume — see the molar calculation above, and mark it as an assumption

Reacted with chloride

m(bath) = the mass the bottle and tray still hold, at whatever strength the bath is now

Left in the bath

m(recoverable) = m(applied) − m(image)

Destined for the wash and the fixer

And the term you cannot measure tonight. The image silver. Two published figures bracket it and they do not agree: Reilly reports that only about 6 to 8 per cent of the silver initially present in a sensitised sheet is used to form the image, and Ware, from chemical analyses of salt prints made in 1855, puts the retained fraction at about 2 per cent, with a coating weight of about 0.1 g/m² — which for an 8 × 10 in sheet works out at about 5 mg of image silver.

They are not measuring the same thing. Reilly is describing a working practice with albumen and salted papers at full strength; Ware is reporting an analysis of finished nineteenth-century prints, which had been fixed, washed and in most cases toned. The course reports both and averages neither. Write both into your balance as bounds, and note which of your own terms are measurements and which are assumptions. A balance with one honest gap in it is a result; a balance closed with a guess is not.

Then the bench. Everything that touched silver is rinsed into the waste container, not into the sink: tray, rod, brush, graduate, funnel, gloves-off last. Wipe the bench while the residue is still wet — dried silver nitrate on a work surface is a stain you will find on a sheet three sessions from now. The rod and the brush are labelled and reserved for this process, and the reason is that a rod that has been in an iron sensitiser and goes into silver puts a reduction product straight into your next print; cross-contamination between alternative processes is its own diagnostic entry for a reason.

Expected observations

During salting. The solution is clear and slightly viscous. A sheet that beads rather than wets is heavily sized; a sheet that darkens and goes limp at once is porous and will give a flat print. Reilly’s diagnostic for the second case can be applied to the finished print: if a print looks better by transmitted light than by reflected light, the solutions sank too deeply into the fibres.

The dry salted sheet looks like paper. There is nothing to see, which is why it must be marked.

During sensitising, under the yellow light: a faint grey or straw cast appearing over ten or twenty seconds where the silver meets the salt. Even, with no visible edge.

The dry sensitised sheet should be pale cream to faint grey. Reilly’s failure colours, in order of increasing damage: yellow is the first spontaneously reduced silver, deep reddish brown is the same process further on, and black with a greenish bronzed sheen is a sheet finished by months in dark storage.

In the tray, the bath will be visibly less full after five sheets and may show a first faint straw colour. Neither is a fault; both are data.

What is happening chemically

Three reactions, all of them worked through properly in the chemistry lesson. What matters at the bench is which stage each belongs to.

Stage 4 is a double replacement, and it goes to completion where the two solutions meet.

NaCl + AgNO₃ → AgCl(s) + NaNO₃

Silver chloride precipitated in place; the sodium nitrate stays soluble

Half the silver, or thereabouts, is consumed by it. The rest stays in the sheet as free silver nitrate and is the reason the paper works at all.

Stage 7’s brown bath is a reduction, and the reducing agent is the paper. Reilly’s explanation is that organic matter from the sizing or the binder dissolves into the bath, reacts with some of the silver and eventually reduces it to the metallic state, and the colloidal metal is what colours the solution. Kaolin removes the organic matter rather than the silver, which is why it can be used over and over.

And the yellowing of a stored sensitised sheet is the same reaction, in the paper instead of in the bath. Reilly names it in as many words: the yellow colour is spontaneously reduced metallic silver in a very finely divided state. Ware’s plasmon account explains why yellow specifically — silver nanoparticles of about 10 to 20 nm in a low-index medium transmit yellow. The sheet is printing itself out, slowly, without light.

Data to record

Use the lab notebook and the batch record SOP.

Two batch records, one per bottle. Substance and strength as per cent w/v; what was weighed and what it was made up to; supplier and lot number of the solid; the water used; date and batch number; and observations over time.

The session log, one line per sheet.

Column	What goes in it
Sheet	Number, and which side was coated
Paper	Maker, product, surface, weight, size
Salting	Which solution, plain or citrate; method, float or brush or immersion; time; temperature
Dry mass before sensitising	Grams, from the balance
Sensitising	Method; float time or drop count; bath strength and batch number
Dry mass after sensitising	Grams, same balance, same day
Solution taken up	The difference, and the silver nitrate it represents
Room	Temperature and relative humidity, both stages
Appearance	Dry colour, evenness, any marks, and where on the sheet

The bath log. Starting volume and strength; volume remaining at the end; colour, described rather than named; kaolin added or not; replenishment made, with what and how much.

And the two keeping-test sheets, with the hour they were coated, the hour K1 was printed, the interval agreed for K2, and the storage conditions for both.

Analysis

First, decide what you may claim. One or two sheets per condition, hand coated, one paper, one evening: every honest conclusion has the form “on this paper, at this strength, with my coating”.

Second, compare the two coating methods on the sheet rather than on the print. Divide the mass taken up by the coated area and you have grams of solution per square metre, which is the only figure that compares a float with a rod fairly. Expect the float to draw more from the tray than ends up in the sheet — that difference is inventory rather than waste, and it is why the balance has a bath term.

Third, look for the two coating faults Reilly names by their appearance, because their causes are different and the sheet tells you which is which.

• Round, paper-white spots with hard, definite edges: air bubbles trapped under the sheet during sensitising. Those areas were never sensitised at all.
• Round spots with a light brown stain: air bubbles under the salting solution. The area was never salted, so the silver nitrate that arrived there had nothing to react with and has simply stained the paper.
• Blotches and mottling over larger areas, not round: solution reached the back of the sheet during floating.
• The “measles” — light or blotchy patches in what should be the densest areas of a finished print: insufficient sensitisation, from a too-weak silver solution or too little residual sensitiser in the paper. Reilly’s remedy is to re-float or re-brush the unexposed sheets with a stronger solution; nothing can be done for a sheet already exposed.

Fourth, do the cost arithmetic, because it is a real result and it will change how you work.

🧮Sheets per 100 mL, and silver per print, from your own coverage

From the supplier’s published figure first, as a benchmark: Bostick & Sullivan state that 100 mL of their 12 per cent silver nitrate makes approximately fifty 8 × 10 inch prints, which is

100 mL ÷ 50 prints = 2 mL per 8 × 10 in sheet

Their implied coverage

2 mL × 0.120 g/mL = 0.24 g of AgNO₃

Silver nitrate on one sheet, at 120 g/L

0.24 g × (107.87 ÷ 169.87) = 0.152 g of silver

As silver metal (Ag 107.87 of AgNO₃ 169.87)

At the planner’s dated UK figures — £59.95 for 25 g and £112.90 for 10 g, both 5 September 2026 — that is £0.58 to £2.71 of silver nitrate per 8 × 10 sheet by drop coating.

Now put your own measured uptake in place of the 2 mL. If your sheets took 3 mL, your cost is half as much again and your yield drops from fifty sheets per 100 mL to thirty-three. If you floated rather than brushed, the sheet figure may be similar while the session figure is far worse, because the tray holds solution you did not use.

And set it against the image. Of the 0.152 g of silver you applied, Reilly’s 6 to 8 per cent puts 9 to 12 mg in the picture and Ware’s 2 per cent puts about 3 mg there, with his coating weight of 0.1 g/m² giving about 5 mg for a sheet this size. Somewhere between 2 and 8 per cent of the most expensive substance in the course is doing the job; the rest is in the tray, the wash and the fixer, and the only question is whether you collected it.

Fifth, read the keeping test as a test rather than a confirmation. If K2 prints indistinguishably from a fresh sheet, you have learned something about your storage and not that Reilly was wrong; if it prints yellow and flat after twelve hours, check the temperature and humidity you recorded before you blame the paper.

Troubleshooting

What you see	Most likely cause	What to change, and the test
Sheet curls violently on the salting bath	Bath warmer than the paper	Match the temperatures; or mist the back very lightly; or fold half an inch up on all four sides to make a shallow “boat”. Reilly gives all three, and warns against large droplets on the back
Mottled, blotchy pattern over broad areas after drying	Solution reached the back during floating; or the sheet was salted by immersion on a stock that does not tolerate it	Two changes, tested separately: float one sheet more carefully with the corner handles, and float a second that was salted by floating rather than immersion
Round paper-white spots with hard edges	Air bubbles under the sheet during sensitising	Break every bubble before starting the timer; lift a corner and look, or blow gently across the surface before floating
Round spots with a light brown stain	Air bubbles under the salting solution	Same technique, one stage earlier. The stain is silver nitrate that met no chloride
Streaks in one direction	Brush coating applied unevenly	Cross-stroke: horizontal, vertical, then a dry brush. Or change to a rod
Sensitised sheet dries yellow	Spontaneous reduction: the sheet is old, was stored warm and damp, or saw too much light	Check the age, then the store’s temperature and humidity, then the working light. A slightly yellowed sheet is still usable — the fixer takes some fine silver out of the highlights
Sensitised sheet dries brown or purple	The same, much further along, or white light reached it	Cover the drying box. A sheet that browns within the hour under your working light says that light is too strong, and the light test below settles it
Silver bath goes brown within one session	Organic matter from the sizing or binder, reduced to colloidal silver	Normal. Kaolin at about 15 g/L overnight, or filter. Only a nearly black bath is unusable
A metallic marbled sheen on an exposed print	Surface scum on the silver bath	Filter. Check the surface for scum before floating any sheet
Weak, flat print with poor maximum density from a well-exposed sheet	Insufficient sensitisation — the “measles” — from a weak bath or too little residual sensitiser	Re-float or re-brush unexposed sheets with a stronger solution, then replenish by the rule of thumb
Faint brown spots on the back of a finished print	Silver reached the back and printed out	Floating technique, and the drying line: a sheet dripping onto the one below does exactly this

Clean-up

In this order, and gloves come off last.

1. Return the salting solution to its labelled, dated bottle. Photographers’ Formulary say the excess may be kept for future use; it grows mould eventually, which is what the date is for.
2. Return the silver bath to its brown bottle with the kaolin, and shake.
3. Rinse the tray, the rod, the brush and the glassware into the silver waste container, not into the sink. A first rinse into waste and a second into the sink is defensible; the first rinse into the sink is not.
4. Wash the brush in distilled water, hang it bristles down, labelled. It will discolour, which Reilly says does not affect its performance if it is kept clean.
5. Wipe the bench, the balance and the floor under the drying line while the residue is wet.
6. Wash your hands immediately — Reilly’s instruction, and the control that addresses the systemic risk rather than the immediate one.

Storage

Salted paper: indefinitely. Cool, dry, flat, interleaved, in the dark or not — it is not light sensitive. Reilly’s own word is “indefinitely”, and it is the reason to salt in batches.

Sensitised paper: one to two days, and 8 to 12 hours in warm humid conditions. A tightly closed container in a cool, dry, dark place slows it, because air and moisture both accelerate the decomposition. That is Reilly’s figure; your keeping test is the point of measuring it.

The salting solution: a labelled, dated bottle, cool. It contains gelatin and will grow mould, exactly as Part XXI’s ammonium iron(III) citrate solution does.

The silver bath: brown glass, dark cupboard, kaolin in the bottom, dated. Bostick & Sullivan give their unopened solutions at least five years; a bath that has been used is a different object, and its life is set by its silver content and its colour rather than by a date.

Never in a food refrigerator, which both supplier sheets say and which is worth repeating because cool storage genuinely does help sensitised paper.

Disposal considerations

The chemistry first. The waste from this session is silver nitrate in water, at strengths from 12 per cent down to a rinse, plus dissolved sodium and ammonium nitrate, dissolved organic matter from the paper, and a little colloidal silver. The aggregated European classification for silver nitrate carries the aquatic hazard statements, and the reason is silver’s toxicity to aquatic organisms rather than the nitrate’s.

The general practice is recovery rather than disposal, and Reilly’s account of how the trade did it is the clearest short description available. Most of the recoverable silver is in the first and second changes of wash water, which were saved; the silver was precipitated out of them by adding either sodium chloride or sodium carbonate, giving insoluble silver chloride or silver carbonate that settles so the water can be decanted off. Fixing solutions were evaporated to a sludge, and print trimmings and spoiled prints were burned and the silver — and gold — recovered from the ash. A typical gallery recovered about 60 per cent of the silver it consumed.

And the honest qualification, which is Reilly’s own. Smelting and re-nitrating silver are not operations for a home laboratory, and for a printer using a small amount of silver recovery is often not economically feasible, because refiners have minimum charges and the shipping is the customer’s. That does not make the drain an option; it makes accumulation and labelled storage the realistic route, as Part XII’s silver recovery lesson sets out.

How this differs from fixer recovery. Part XII recovers from a thiosulfate solution, where the silver is locked in a soluble complex and has to be brought out electrolytically or by metallic replacement. Here it is a free ion in a nitrate solution and a chloride will precipitate it. Different chemistry, and the two streams are labelled separately even though both are silver-bearing.

The caveat that is not a formality. The course publishes no jurisdiction-specific disposal instruction. Local regulation governs, it differs between jurisdictions, and it is the only document that can answer the question for your drain. The disposal ruling says why.

Questions

1. Why must the salted sheet be dry before it is sensitised? Give the mechanical reason, and say what happens to the bath if it is not.
2. Your salting solution is 2 per cent sodium chloride and your silver bath 12 per cent. What is the molar ratio, what fraction of the applied silver can react, and what assumption does your answer depend on?
3. Why distilled water for the silver and not for the salt? Name the products you are avoiding, and say where you will let that reaction happen deliberately.
4. One sheet shows round paper-white spots with hard edges; another shows round spots with a light brown stain. What was different, and at which stage did each occur?
5. Your silver bath has fallen from 1000 mL to 880 mL and gone pale brown. What two separate things have happened, and what do you add to fix each?
6. How many 8 × 10 sheets from 100 mL of 12 per cent silver at your own measured coverage, and what is the silver cost per print? Then say why floating is worse than brushing even when the coverage per sheet is the same.
7. Reilly’s working light is a yellow 60 W bulb and he says there is no need for a modern safelight’s dimness. Is that a measurement, a convention, or a practitioner’s recommendation? Design the test that would turn it into a measurement on your bench.

Further experiments

Citrate against no citrate, on one exposure. Two sheets from the same batch, one salted with the plain solution and one with the citrate variant, sensitised together, printed together in the next session and measured together. Reilly’s stated effect is more reddish and slightly more brilliant; this turns his sentence into your own densities and your own description of the colour.

Float against brush against immersion, weighed. Three sheets, three methods, the uptake measured on the balance for each, and the coated area divided out. You will end up with grams per square metre for your own hand, which is the number this whole session exists to give you.

A deliberate reversal, once. Sensitise a spare strip with silver nitrate first, dry it, then brush salt solution over half of it and dry again. Expose both halves together in the next session. The chemistry lesson predicts that the salted half will print out feebly and then stop, because chloride in excess adsorbs on the crystal surface and repels the photoelectrons. It is the cheapest demonstration in the part of why the order of the two coatings is chemistry rather than convention.

The working-light test. Coat one sheet, cut it into four strips, and leave them under your yellow working light at a measured distance for 5, 15, 45 and 120 minutes with an opaque card over half of each. Develop nothing; just look, then fix them in plain hypo and look again. The interval at which a visible edge appears is your working-light limit, and it is a measurement rather than a convention.

And two lots of paper, side by side. The single largest variable Reilly names is the porosity of the rawstock. One hot-pressed and one cold-pressed sheet, coated and printed together, turns that sentence into something you have seen.

Recap

Salting is a batch operation and sensitising is not. Salted paper keeps indefinitely; sensitised paper keeps a day or two, and less when it is warm and damp. That asymmetry is why this part is two sessions.

Six times by weight is twice by moles, so about half the silver you apply reacts and about half stays free — the excess the process depends on, and the reason the sheet is a corrosive object as well as a light-sensitive one.

Float for evenness, brush or rod for economy. Time from the last broken bubble, lift slowly, hang by the long edge, blot the low corner. Distilled water for the silver, and no metal near it.

A silver bath fails in three ways at once: strength to the chloride, volume to the paper, colour to organic matter reduced to metal. Kaolin answers the third and the double-strength rule of thumb the first two.

Between 2 and 8 per cent of the silver ends up in the picture, on two published figures that disagree. Everything else is in the tray, the wash and the fixer — which is why it is all collected, and why the mass balance is the point of the evening.

Check your understanding

Question 1. Your salting solution is 2 per cent sodium chloride and your sensitiser is 12 per cent silver nitrate. Roughly what fraction of the applied silver can react with chloride?

• About one sixth, because the silver is a six-fold excess by weight
• About one half, because a six-fold excess by weight is close to a two-fold excess by moles
• Nearly all of it, because the reaction goes to completion
• It cannot be estimated without knowing the paper

Show the answer and why

Answer: About one half, because a six-fold excess by weight is close to a two-fold excess by moles

Molar masses do the work: 120 g/L of silver nitrate is 0.706 mol/L and 20 g/L of sodium chloride is 0.342 mol/L, a ratio of about 2.06. So a six-fold excess by weight is only a two-fold excess in the units that matter for a reaction, and about half the applied silver stays in the sheet unreacted. That answer assumes the sheet takes up similar volumes of the two solutions, which is an assumption rather than a measurement - weighing the sheet before and after each coating is how you replace it with a number.

Question 2. Why must the sensitising solution be made with distilled or de-ionised water?

• Because tap water is alkaline and hydrolyses the silver nitrate
• Because chloride and hard-water ions in tap water precipitate silver chloride and silver carbonate in the bath, robbing it of strength
• Because dissolved oxygen in tap water oxidises the silver
• Because the chlorine used to treat tap water fogs the paper

Show the answer and why

Answer: Because chloride and hard-water ions in tap water precipitate silver chloride and silver carbonate in the bath, robbing it of strength

Reilly gives the reaction and then closes a satisfying loop with it. Tap water carries chloride and carbonates, so silver nitrate made up in it throws down a cloudy white precipitate that takes silver out of the bath and makes sensitising difficult. The same reaction is what you deliberately watch in the first wash of a print, where the milky cloud coming off the sheet is the excess silver nitrate in the paper meeting exactly those ions in the wash water.

Question 3. A floated sheet shows a mottled, blotchy pattern across large areas after drying. Give the two most likely causes and the change that tests each.

• A weak silver bath, or too short a float; test by lengthening the float and by replenishing
• Solution reaching the back of the sheet during floating, or a stock that does not tolerate immersion salting; test by floating one sheet with corner handles and by floating rather than immersing the salting step on another
• Air bubbles, or dust on the surface of the bath; test by filtering the bath
• Over-drying, or too high a room humidity; test by conditioning the paper

Show the answer and why

Answer: Solution reaching the back of the sheet during floating, or a stock that does not tolerate immersion salting; test by floating one sheet with corner handles and by floating rather than immersing the salting step on another

Both causes produce the same visible fault by the same route, which is why they have to be separated by changing one thing at a time. Reilly explains that when a sheet is salted on both sides - or when silver runs round the edge during floating - some of the silver solution is drawn to the back during sensitisation, which locally sensitises areas of the back and shows through as blotchy patches on the face. Weak sensitisation gives a different appearance, the "measles": light or blotchy patches specifically in the areas that should be densest.

Question 4. A silver bath has fallen from 1000 mL to 850 mL and is pale brown. What has happened, and what do you do?

• It has lost volume to the paper and lost strength to the chloride; replenish with 150 mL of a solution twice the original strength
• Organic matter dissolved from the paper has reduced some silver to the colloidal metal, which is what colours it; shake with about 15 g of kaolin per litre and let it settle overnight, or filter
• The silver nitrate has decomposed in the light and must be discarded
• The bath is exhausted and unusable at this colour

Show the answer and why

Answer: It has lost volume to the paper and lost strength to the chloride; replenish with 150 mL of a solution twice the original strength, Organic matter dissolved from the paper has reduced some silver to the colloidal metal, which is what colours it; shake with about 15 g of kaolin per litre and let it settle overnight, or filter

Three things happen to a working silver bath and they are independent. Strength falls as the silver reacts with chloride in the paper; volume falls because each sheet carries solution away; and colour appears because organic matter from the sizing or binder dissolves in, reacts with silver and eventually reduces it to the metal. Reilly's rule of thumb - replenish the lost volume with a solution twice as strong - deals with the first two together, and kaolin deals with the third. A slightly discoloured bath is usable; only a nearly black one is not.

Question 5. Which of these is a measurement, and which are conventions or recommendations?

• The one-to-two-day keeping time of sensitised paper is a measurement; the yellow 60 W working light is a measurement
• Neither is a measurement of a limit: Reilly gives the keeping time as accumulated practice and the yellow bulb as a working practice, and this course asks you to establish both on your own bench
• Both are measurements published with their conditions
• The working light is a measured limit and the keeping time is a convention

Show the answer and why

Answer: Neither is a measurement of a limit: Reilly gives the keeping time as accumulated practice and the yellow bulb as a working practice, and this course asks you to establish both on your own bench

This distinction is the reason the session includes two tests. Reilly states that albumen and salted papers remain usable only one or two days after sensitising, and 8 to 12 hours in extremely humid warm conditions, but he gives no measured light level and no controlled ageing data; and his working light is a yellow 60 W incandescent bulb chosen because it is bright enough to work by, not because a limit was established. Both are the recommendations of an experienced practitioner writing a conservation handbook, which is a good reason to start there and no reason to stop there.

Question 6. From the supplier's own figure, 100 mL of 12 per cent silver nitrate makes about fifty 8 by 10 inch prints. How much silver metal is that per sheet, and roughly how much of it ends up in the image?

• About 0.15 g of silver per sheet, of which the two published figures put between 2 and 8 per cent in the image
• About 0.15 g of silver per sheet, nearly all of which forms the image
• About 2 g of silver per sheet, of which about half forms the image
• About 0.02 g of silver per sheet, all of which forms the image

Show the answer and why

Answer: About 0.15 g of silver per sheet, of which the two published figures put between 2 and 8 per cent in the image

Two millilitres per sheet at 120 g/L is 0.24 g of silver nitrate, and multiplying by the mass fraction of silver in silver nitrate, 107.87 over 169.87, gives about 0.152 g of silver. Reilly reports that only about 6 to 8 per cent of the silver initially present in a sensitised sheet forms the image; Ware, from analyses of nineteenth-century salt prints, puts the retained fraction at about 2 per cent with a coating weight of about 0.1 g per square metre, which is about 5 mg for a sheet this size. The two are not measuring the same thing and this course reports both rather than averaging them.

Question 7. Why does this page specify a brush with no metal ferrule and a glass rather than a stainless steel tray?

• Because metal scratches the paper surface
• Because silver nitrate reacts with the metal, which both weakens the bath and prints as a stain in the finished picture
• Because glass is easier to clean
• Because a metal ferrule holds the bristles too tightly for even coating

Show the answer and why

Answer: Because silver nitrate reacts with the metal, which both weakens the bath and prints as a stain in the finished picture

It is chemistry rather than fussiness. Reilly specifies a wide flat Japanese brush whose bristles are bound with thread rather than held in a metal ferrule, and gives the reason directly: the silver nitrate will react with the metal of the ferrule and cause stains in the print. He extends the same reasoning to trays - glass ideally, a stainless steel tray not recommended, and a plastic tray only if it is new, because plastic holds on to whatever it held before and a tray contaminated with developer or fixer will stain the prints.

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