Arrowroot salting solution

The plain salting solution and this one differ by two ingredients and one operation, and the difference is not what it looks like. Swapping gelatin for arrowroot looks like a change of binder. It is really a change of chemistry, because gelatin takes part in the printing-out reaction and starch does not — and once the binder has stopped contributing, something else has to, which is what the three grams of citric acid are for.

Ingredient	Quantity	Form the source specifies
Arrowroot	35 g	rubbed to a cream with cold water, quantity of that water unstated
Sodium chloride	35 g	dissolved with the citric acid in the 950 mL
Citric acid	3 g	dissolved with the salt in the 950 mL
Water	950 mL, added	brought to the boil; the cream carries more, unmeasured

!Arrowroot is in the encyclopaedia because this formula needed it

The register was complete at 134 entries and held neither arrowroot nor starch, and the alias map resolved neither. Rather than point the id at some other substance, this page named arrowroot because that is what Reilly and Towler both call it, and left the reference dangling for the validator to catch — which is how the gap was found and closed. The entry now exists, beside albumen, gelatin and gum arabic, and the ingredient above is a link like any other.

What this page keeps is the reading behind it, under Function of every ingredient, from the two chapters that actually describe the material: Reilly’s on starches and Towler’s on the amylaceous substances. They are independent of each other and they agree.

Purpose

To put three things into the surface of a sheet of paper in one operation: a chloride, so that a later bath of silver nitrate can precipitate silver chloride in place; a starch binder, so that the silver chloride when it forms sits in and just under the surface instead of sinking into the body of the sheet; and an organic acid, because the binder that does the second job is chemically dead and cannot do the third.

The first purpose is the salt print’s, and it is described in full on the plain salting solution page: silver chloride is insoluble, so it cannot be dissolved and coated, and must be assembled inside the paper from two solutions applied and dried separately.

The second and third purposes are this formula’s own, and the third is the interesting one. Reilly divides the organic substances used in printing-out papers into two classes. Active substances — albumen, gelatin, and the organic acids citric, tartaric and oxalic — facilitate the more complete reduction of silver chloride and additionally form light-sensitive silver salts of their own on contact with silver nitrate, such as silver albumenate and silver citrate. Inactive substances do none of that; they contribute only by keeping the light-sensitive material at the surface and preventing a dull, sunken-in appearance. Starch is the most useful of the inactive class.

So an arrowroot paper is a paper whose binder has been chosen for a purely physical property and whose chemistry has to be supplied separately. Reilly states the consequence without hedging: the formula is essentially that of plain salted paper except that it contains citric acid, and if the citric acid were not present the prints would be grey and flat — the hallmarks, as he puts it, of a pure chloride image.

Recommended uses

Matte arrowroot prints, from a long-scale negative. The surface runs from very matte to a dull gloss depending on how much starch ends up on the sheet and how smooth the rawstock underneath is. Reilly’s judgement is that arrowroot prints can be considerably more brilliant and richer-looking than plain salted papers, with a longer density range and more delicate detail preserved.

Brushed or sponged on, not floated — and the formula decides that. Reilly gives 2 per cent arrowroot as the concentration below which a solution is fluid enough to float paper on, and this one is above it. The method he says is most recommended in the old manuals is a 3 to 4 per cent starch paste applied with a thin flat brush, and at roughly 3.7 per cent of the stated water this formula lands inside that band. Towler’s 1864 procedure, at a slightly lower starch concentration, uses a fine moist sponge. Either way the sheet is pinned flat, not laid on a tray.

On a fairly smooth rawstock, but not a plate finish. Reilly’s guidance cuts both ways here in a manner worth noticing. Arrowroot prints can be made on almost any surface and a fairly smooth paper is generally preferred — but highly calendered “plate” finishes are harder to coat evenly, and porous papers need a binder of higher starch content, because the paste must be absorbed into the fibres to a certain extent in order to adhere at all. A starch layer that sits entirely on top of the paper is a starch layer that will come off.

Where you want printing-out speed. Of the materials in Reilly’s book the matte salted papers like arrowroot are the fastest, then plain salted papers, with albumen slowest. That is a relative statement about a family of exceedingly slow contact-printing materials, but if you are printing in weak winter sun it is the one to choose.

Where you want to sensitise ahead. This is the practical headline. Arrowroot paper is sensitised on a 12 per cent silver bath that also contains 4 to 5 per cent citric acid, and that bath leaves the sheet usable for several weeks. A plain salted sheet keeps one or two days. The acid that the formula needs for its chemistry turns out, in the silver bath, to buy a working life the rest of the family does not have.

In white light, at leisure, in batches. Nothing in this solution is light-sensitive. Coat a dozen sheets on a wet afternoon and silver them when you have a negative worth printing.

When another formula is preferable

• For the simplest possible salt print, the plain salting solution. Twenty grams of salt and two of gelatin, no mortar, no boiling, no cream that separates while you work, and a three-minute float instead of a brushed coating that has to be even. It gives a flatter, greyer, more sunken print, and that is exactly the look some people want.
• For a glossy surface and a shorter 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, so the negative needs less range. Albumen is also the slowest of the three and the hardest to coat.
• For a redder, slightly more brilliant print with no boiling at all, Reilly’s citrate version of the plain salting solution — 20 g of salt, 20 g of sodium citrate and 2 g of gelatin. It reaches for the same “active organic” effect this formula gets from citric acid, in a solution you can mix in five minutes. It is set out on the plain salting solution page.
• For a paper that behaves like a modern one — a developed image, a fixed speed, an enlarger — the silver chloride contact emulsion. A far harder thing to make, answering a different question.
• If the point is a hand-coated print rather than a silver one, the classic cyanotype sensitiser is one coating, no fixer and a fraction of the cost.

Mixing

Two vessels, and they only meet at the boil. This is the shape of the formula and a flat ingredient table cannot carry it.

Rub the arrowroot to a cream, cold. Thirty-five grams of arrowroot go into a mortar with a little cold water and are rubbed with the pestle to a creamy paste, then let down with enough further cold water to make a fairly runny cream with no lumps. Reilly is specific that the mortar and pestle is the best way to do it. Towler’s wording in 1864 is the same operation: rubbed with cold water into a cream, so that all lumps have been thoroughly broken up and saturated.

The reason for the cold and the reason for the rubbing are the same, and they are in Reilly’s own chapter on starches and in Towler’s on the amylaceous substances. Starch grains are insoluble in cold water. They do not dissolve, they wet. Tip dry arrowroot into hot water and the outsides of the lumps gelatinise instantly into a skin that seals dry powder inside, and no amount of stirring afterwards gets it out. Rubbing the dry powder into a small quantity of cold water first separates the grains and wets every one of them, so that when heat arrives they all burst together.

Stir the cream to the moment of use. Reilly warns that the cream will tend to separate into its components — which is simply insoluble grains settling out of water — and that it must therefore be stirred or rubbed up right until it goes in.

Dissolve the salt and the acid in the 950 mL, and boil that. The sodium chloride and the citric acid go into 950 mL of water together and that solution is brought to the boil in a porcelain container. Towler says glass or porcelain. Neither says why, and the course’s reading is the obvious one — a boiling chloride solution in a metal pan is a corrosion problem and a contamination problem at once, and the same caution appears in Reilly’s warning that a chromated salting solution must not touch metal.

Add the cream to the boiling solution in small amounts, stirring constantly, with a glass rod or a wooden spoon. Not the other way round. Adding cold salt solution to a starch cream would drop the temperature through the gelatinisation range slowly and lumpily; feeding a thin cream into a large volume of boiling liquid bursts each portion of grains as it arrives.

Boil gently for a few minutes, then take it off the heat.

Let it cool, and lift off the skin. Reilly’s instruction is to remove the skin that forms on the surface of the cooled liquid, and he says exactly what it is: the residue of the hulls of the burst grains of starch. Towler calls the same thing the scum, and his chapter on starch explains the structure behind it — each granule is a cell of concrete, insoluble material holding a soluble pulp within, and boiling bursts the cells so that the soluble part mixes with the water while the insoluble husks stay behind. Leaving the skin in puts insoluble fragments into a coating that is supposed to be uniform.

🧮What is actually in this solution, and why every figure here is an upper bound

Reilly states a volume of water the salt and acid are dissolved in, and never states what the finished solution comes to. The cream carries more water and he does not say how much. So:

35 g ÷ 950 mL = 3.68 % w/v, and 35 ÷ 58.44 = 0.599 mol, so 0.630 mol/L

Sodium chloride, against the stated water

35 g ÷ 950 mL = 3.68 % w/v

Arrowroot, against the stated water

3 g ÷ 950 mL = 0.32 % w/v

Citric acid, against the stated water

Each is a maximum. If the cream carries 200 mL of cold water the true figures are about 3.0, 3.0 and 0.26 per cent; if it carries 400 mL, about 2.6, 2.6 and 0.22.

That uncertainty is not a footnote, because Reilly’s own rule turns on it. He states that the silver bath must be approximately four times as strong as the salting solution, and gives the worked example: a 3 per cent salting solution calls for approximately 12 per cent silver. The silver bath he prescribes for arrowroot paper is 12 per cent. Read against 950 mL this formula is 3.7 per cent chloride and wants nearer 15; read as a finished solution of about 1,150 mL it is 3.0 per cent and matches his example exactly.

The course’s reading — and it is a reading, not Reilly’s arithmetic — is that the cream is intended to bring the finished solution to somewhere around a litre, which puts the chloride at about 3 per cent and reconciles the two passages. What is not a reading is the direction: this is a substantially more chlorided paper than the plain salted one, whose 20 g per litre is 2.0 per cent and 0.342 mol/L.

0.513 mol/L × 169.87 g/mol = 87 g/L, an 8.7 % bath

Silver that would exactly consume the chloride, at 3.0 % NaCl

120 g ÷ 1000 mL = 0.706 mol/L, a 12 % bath

Silver actually supplied

So roughly a 38 per cent molar excess of silver, against about 106 per cent for the plain salted paper on the same 12 per cent bath. Both leave free silver nitrate in the sheet, which the mechanism below needs; the arrowroot paper leaves less of it, and Reilly’s instruction that starch-bound papers want strong silver baths and short floating times is the practical form of the same fact.

One caveat on all of it. These are bath concentrations, not what ends up in the paper, and a sheet brushed with a viscous paste does not take up the same volume per unit area as a sheet floated on a thin solution. Comparing this formula with the plain one as solutions is fair; comparing what the two put into a sheet is not something either source measures.

Behaviour

It is a starch paste, and it behaves like one rather than like a salt solution. It is viscous, it is turbid, it forms a skin, it separates if it stands, and it will ferment. None of that is true of the plain salting solution, and all of it changes how the tray-side of the work goes.

Its viscosity chooses the coating method, and the formula was written to a method. Below about 2 per cent starch a solution is fluid enough to float paper on, though Reilly notes that floating on starch is harder than floating on albumen or thin gelatin and puts less binder on the surface. Heavier coatings come from immersing the sheet and drawing it out between glass rods. The brushed 3 to 4 per cent paste is the one the old manuals recommend, and this formula is a 3 to 4 per cent paste. Diluting it to float it is not a convenience, it is a different paper.

It must sink in, and the amount it sinks is a property of the rawstock. Reilly is explicit that arrowroot pastes tend to be absorbed into the paper fibres and must be absorbed to a certain extent in order to adhere, so a porous paper requires a binder of higher starch content. That is the opposite of the instinct a modern coater brings to the job. Hardwich, writing in 1864 about calotype paper rather than this, records the failure mode from the other side: on a foreign starch-sized paper, unless it is re-sized with some organic substance, the solutions sink in too deeply and the picture wants clearness and definition.

Dried, the starch layer is tough. Reilly’s chapter on starches makes a point that matters for everything downstream: when starch pastes are applied to paper and dried, a layer is formed that will not swell in water and will withstand the processing solutions without damage. Unlike gelatin, which swells in every bath and must be handled wet with care, and unlike albumen, which is water-soluble until the silver bath coagulates it, dried starch simply stays put.

Which is also why it must be silvered quickly. The same insolubility means starch is not coagulated or rendered insoluble by the silver solution the way albumen is. The starch layer stays very permeable to water, so prolonged floating on the silver bath drives fine silver chloride particles down into the paper fibres and the print loses brilliance. Reilly’s numbers: half a minute may be enough for a light coating, a minute and a half at most for a heavy one, and too long gives grey, flat prints, especially on porous papers.

It spoils. Reilly publishes no keeping time and the course will not invent one; see Storage for what can be said and on whose authority.

Image characteristics

Matte to a dull gloss, and the amount of starch decides which. Reilly ties the surface to two variables: how much starch has been applied and how smooth the rawstock is underneath.

More brilliant and richer than plain salted paper, with a longer density range. This is Reilly’s own comparison and it is the reason the material displaced plain salted papers for matte work almost completely after 1854. The mechanism is the one he sets out for binders generally — the more of the image that is formed in a compact layer at the surface rather than scattered among the fibres, the less the reflected light is diffused, and the higher the maximum density that can be reached.

It still wants a long negative, but slightly less long than plain salted paper. Reilly’s ranking is unambiguous and it is a ranking of the negative density range required: plain salted greatest, matte papers like arrowroot and matte albumen slightly less, glossy albumen lower again, and every one of them far higher than a develop-out paper needs. A negative made for enlarging paper prints flat and empty on this material.

Warm purple, then yellowish-brown if it is left untoned. Reilly is precise about the conditions: in the presence of both citric acid and starch the prints take on a warm purple colour after exposure, and in the absence of toning change to a yellowish-brown after fixation. The purple is the print-out colour; the shift is what the fixing bath does to a printing-out image, and it happens on every paper in this family.

🕰Hardwich reached the same conclusion in 1864, from the paper mills

Hardwich was not writing about arrowroot salting solutions but about the sizing already in commercial rawstock, and he arrived at the same division from the opposite direction. English papers, sized with gelatine and hardened with alum, tend to red tones that become brown or chocolate in the finished print. Foreign papers sized with starch or resin produce tones that are sepia-brown after fixing and purple-black when treated with solution of gold.

He gives two reasons and they are worth separating, because one is a claim about chemistry and one is a claim about pH. First, starch and resin do not, like gelatine, exert a marked action in reddening the picture — which is Reilly’s active-and-inactive division stated 116 years earlier in different words. Second, starch sizing has an alkaline reaction while gelatine sizing is acid from the alum, and alkalies are found to diminish redness just as acids increase it.

Hardwich also ranks the three binders: the reddening action of gelatine, although greater than that of starch, is less than that produced by albumen, and the surface brilliancy is less as well. That ordering — starch, gelatine, albumen — is the ordering of this formulary’s three salting solutions, and it was published by someone who had never seen any of them.

Note what this does not settle. Hardwich’s second reason would make image colour partly a matter of pH, and this formula deliberately puts an acid into a starch layer. Whether the citric acid’s warm purple is the silver citrate Reilly describes, or the acidity Hardwich describes, or both, is not resolved by either source.

The mechanism

There are four steps and the fourth is where this formula differs from every other salting solution in the course.

Step one, before any silver: the starch gelatinises. Arrowroot is the starch of the West Indian plant Maranta arundinacea — Reilly and Towler agree on the identification — and it arrives as microscopic grains that are insoluble in cold water, in alcohol and in ether. Towler describes each granule as a cell of concrete, insoluble material holding a soluble pulp within, recognisable under the microscope by its concentric rings, and distinguishable by them from potato starch. When starch is heated in water the grains burst and a turbid paste is created; the soluble part mixes with the water to form a thick gelatinous mass, and the insoluble husks can be separated from the fluid portion. That is the whole of the boil, and it is why the skin comes off the cooled solution.

Step two: a double replacement, inside the paper. The dried sheet carries sodium chloride in and just under its surface, held there by the starch. 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

Reilly notes that the sodium nitrate has no effect on the printing process and is washed away.

Step three: light, and a reaction that wants to run backwards.

2 AgCl + light ⇌ 2 Ag + Cl₂

Photolysis, and the back-reaction that limits it

Ware’s account is that this is reversible and severely self-limiting. In a pure silver halide crystal the photolytic silver particles stop growing at around 10 nm and the total yield corresponds to an optical density of the order of 0.02 — a barely perceptible darkening. Reaching a printable density requires that photolysis continue at the crystal surface, and that requires something in the surroundings to take the chlorine away.

Step four: which of the things present takes the chlorine, and this is where starch and citric acid part company. Ware lists the candidates around a Talbot sensitiser: excess silver(I) ions, water molecules, the cellulose substrate, and probably an organic sizing agent of gelatine or starch. He proposes that the combined action of water and silver(I) ions is the major halogen acceptor and contributes the greater part of the image silver.

Cl₂ + H₂O → Cl⁻ + HOCl + H⁺

Disproportionation of the liberated chlorine in the water the paper holds

Cellulose paper carries about 8 per cent water by weight at ordinary humidity, so that acceptor is always present. The chloride it produces meets more free silver nitrate and becomes fresh silver chloride, which light breaks down again — the cycle Vogel reasoned out in the nineteenth century and Reilly quotes.

Ware also sets out a second fate for the halogen, and it is the one that matters here: it may be trapped by reaction with the substrate or some other organic component of the sensitiser, by addition to unsaturated compounds, by substitution in saturated aliphatic compounds, or by oxidation of functional groups such as alcohols. He adds an important qualification — such acceptors complete the photolysis of the crystal, but because the halide is taken out of circulation they do not promote the photolysis of the excess silver ions in the medium.

⁇Does the starch do nothing? Reilly says nothing, Ware says probably very little, and they are not saying the same thing

Reilly (1980). Starches do not react with silver salts and have no effect on the reduction of silver chloride. This is stated twice, in his chapter on binder materials and again in the arrowroot paragraph, and it is the stated reason the citric acid is in the formula.

Ware (2019). Starch appears in his list of potential halogen acceptors alongside gelatine and cellulose, but his conclusion is that the paper sizing agent is not essential to the photochemistry of print-out — which, he says, may be demonstrated experimentally — and that gelatin is known not to be an effective scavenger of halogen at print-out levels of exposure. What the sizing 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 final image.

Where they agree. Both make the binder’s principal contribution physical — holding the image at the surface — and both make it a contributor to image colour and stability. On the practical question this formula turns on, they agree: you cannot rely on starch to do the photochemistry.

Where they differ, and it is not about starch. Reilly’s active class includes gelatin and albumen, which he says facilitate more complete reduction. Ware says gelatin is not an effective halogen scavenger at these exposures. So the disagreement is over the proteins, not the starch. If Ware is right, the plain and albumen salting solutions are less chemically different from this one than Reilly’s scheme implies — and the citric acid is doing something the gelatin never did either.

What the course uses. Ware’s, for the photochemistry, on the grounds given on the plain salting solution page. Reilly’s, for the practice, because the prediction his scheme makes about this formula is testable and specific: leave the citric acid out and the prints come out grey and flat. That is an experiment, and it is under Experiments. Neither position should be quoted as established chemistry.

Where the citric acid fits. Two mechanisms are attributed to it and they are different claims. Reilly’s is that it is an active organic substance which facilitates the more complete reduction of silver chloride and additionally forms a light-sensitive silver salt of its own, silver citrate — a compound he confirms is present in the unprocessed image layer, because his chapter on fixation lists it among the light-sensitive substances the fixer has to remove. Ware’s framework offers the second: citric acid is a small molecule carrying a hydroxyl group and three carboxyls, which is exactly the class of oxidisable functional group he names as capable of trapping halogen. The two are compatible and neither source states the other’s.

Function of every ingredient

Arrowroot, 35 g — about 3.7 per cent of the stated water. The binder, and the one ingredient here with no encyclopaedia page.

What it is. The starch of Maranta arundinacea, a West Indian plant; Towler gives the same identification in 1864 and Reilly repeats it. It reaches the darkroom as a fine white powder of microscopic grains, insoluble in cold water, in alcohol and in ether. Reilly says the starches preferred for photographic purposes are those whose pastes are pure white, very viscous, odourless and of low turbidity, and that arrowroot is the most important of them, with tapioca and sago also useful. Towler groups it with starch, cellulose and gum arabic among the amylaceous, non-nitrogenous substances — which is the nineteenth-century way of saying it is a polysaccharide and contains no protein, and that is the property the whole formula is built around.

Why it is here. To keep the light-sensitive material on the surface of the sheet and prevent the dull, sunken-in appearance of a bare salted paper. That is the entire job. Reilly names starch as the most useful of the substances that do this without being chemically active.

What it does chemically. Nothing, on Reilly’s account — starches do not react with silver salts and have no effect on the reduction of silver chloride. Ware’s position is more guarded and is set out in the contested callout above; on neither account can it be relied on to do the photochemistry.

What it does physically. On boiling, the grains burst and their soluble contents form a viscous paste; on drying, that paste forms a layer which will not swell in water and withstands the processing solutions without damage. It must penetrate the fibres somewhat in order to adhere.

More or less. More starch means a heavier binder layer, more of the image held at the surface, more brilliance and a duller gloss — and a paste too stiff to coat evenly, one that sits on the sheet without adhering, and a sheet that needs a longer silvering. Less means a more fluid solution that can be floated instead of brushed, a thinner binder, and a print progressively more like a plain salted one: greyer, flatter, more sunken. Below about 2 per cent you have changed the coating method as well as the paper. Porous rawstocks need more starch than smooth ones, not less.

Substitutions the source allows. Tapioca or rice starch, used in the same manner. DeBrébisson’s first starch papers of 1854 were tapioca; arrowroot displaced it as the most suitable.

Sodium chloride, 35 g — about 3.7 per cent of the stated water. The halide, and the only ingredient that becomes part of the light-sensitive substance. Dried into the surface, it waits for the silver bath and is converted to silver chloride exactly where it lies.

Its quantity is nearly twice that of the plain salting solution, and the increase is not incidental. Reilly’s general recommendation for salted papers is to keep the chloride content at about 2 to 2.5 per cent with a 10 to 12 per cent silver bath, because lowering the chloride tends to produce prints that lack brilliance and density — and brilliance and density are precisely what an arrowroot paper is for. The maths callout above works through where this formula actually sits once the cream’s unstated water is allowed for, and how it relates to Reilly’s four-times rule.

More or less. More chloride means more silver chloride formed and a richer, denser print, but it consumes more of the silver bath and leaves less free silver nitrate to act as the chlorine acceptor — which works against print-out. That trade-off is the course’s reading of the mechanism above, not a sentence of Reilly’s. Less chloride costs brilliance and density, which he does state. Because the silver bath here is only about 38 per cent in molar excess rather than the plain paper’s 106 per cent, this formula has less room to be pushed upward than the plain one does, and raising the chloride without raising the silver is the change most likely to produce a disappointing print.

Citric acid, 3 g — about 0.3 per cent of the stated water. The “active” organic substance, and the ingredient that distinguishes this formula from the plain salting solution. It is here because the starch is inert.

Reilly’s statement of its function is unusually direct for a nineteenth-century-derived formula: if the citric acid were not present, the prints would be grey and flat, the hallmarks of a pure chloride image. With both citric acid and starch present they take a warm purple after exposure. Its two attributed mechanisms — assisting the reduction of silver chloride, and forming light-sensitive silver citrate — are set out under The mechanism, together with what Ware’s framework adds.

More or less. Reilly gives no series for the citric acid in the salting solution, and the course will not invent one. What he does quantify is citric acid in the silver bath, which is a different place in the process: 4 to 5 per cent for arrowroot paper, with the maximum preservative effect at 5 per cent and as little as 1 per cent extending usable life noticeably. He also records that on albumen paper citric acid shifts the print colour to reddish brown even under prolonged gold toning, and that the shift is stronger when the acid is in the silver bath than when it is in the binder. Whether more citric acid here would go on improving this paper is exactly the experiment below.

What it interacts with. Silver nitrate, to form silver citrate — which is a light-sensitive salt the fixer must afterwards remove, so more of it is a slightly longer fixing job. Nothing else in the formula.

Water, 950 mL added, plus an unstated quantity in the cream. Not a filler, and not one thing. The measured 950 mL is the solvent for the salt and the acid and the vehicle that is brought to the boil; its heat is what bursts the starch grains. The unmeasured cold water in the cream is a processing requirement — starch will not wet in hot water — and it is also the reason no percentage on this page can be stated exactly. Reilly requires distilled or de-ionised water for the silver bath and gives his reason, but states no such requirement here, and the reason does not apply: there is no silver in this solution for hard-water ions to precipitate.

Interactions

With the silver bath, which for this paper is not a plain silver bath. Arrowroot paper is sensitised on 12 per cent silver nitrate containing 4 to 5 per cent citric acid — so the acid appears twice in the process, once here and once there, doing related but not identical jobs. In the salting solution it supplies the activity the starch lacks. In the silver bath it additionally preserves the sensitised sheet for weeks and, on Reilly’s evidence from albumen paper, pushes the image colour redder. Anyone treating the sensitiser as interchangeable with the plain salted paper’s has removed half the citric acid from the process.

With floating time, more sharply than any other paper in the family. Because starch is not coagulated by silver nitrate the way albumen is, the layer stays permeable and a long float pushes silver chloride down into the fibres. Reilly’s half-minute to ninety-second window is a consequence of this formula’s binder, and it interacts with the coating: a heavy starch coating needs the longer end, a light one the shorter. Get it wrong and the result — grey, flat prints — is indistinguishable by eye from having left the citric acid out.

With the rawstock, in the opposite direction from the plain salting solution. There, a porous paper simply gives a flatter print and the answer is a smoother stock. Here, a porous paper needs a higher starch concentration so the paste can adhere, which means the formula itself is the variable rather than the choice of paper. The Getty Conservation Institute’s account of internal sizing is worth knowing before you start: English mills preferred gelatin, French and other continental mills starch, and the atlas notes that starch sizing is hard to detect by ATR-FTIR because both the starch and the cellulose fibres are complex carbohydrates.

With toning, which this paper reaches quickly. Reilly warns that porous surfaces such as arrowroot and plain salted paper tone much faster than glossy albumen and would rapidly become overtoned in the strong baths used for it. His figures: 0.1 to 0.2 g of gold chloride per litre for matte salted papers, against 0.4 to 0.5 g for glossy albumen, toned by inspection for 3 to 15 minutes at 17 to 20 °C with constant agitation. See the gold thiocyanate toner and the platinum toner for printing-out papers; platinum toning is what brought arrowroot papers back into favour around 1900.

With fixing, which has slightly more to remove than on a plain salted paper. Reilly’s chapter on fixation lists what is in the unprocessed image layer: the metallic silver, the binder, and the unexposed silver salts — primarily silver chloride, but depending on what was added to the salting or sensitising solutions, silver citrate and others besides. This formula adds silver citrate to that list deliberately. See the alkaline fixing bath and the sulfite washing aid.

With permanence, through particle size. Reilly stresses that the extremely small size of the image particles in these papers makes them more vulnerable to chemical attack from the residual products of fixation than a develop-out print, and that strict adherence to correct fixing and washing is the only route to optimum permanence. Nothing about the starch binder changes that.

Variants

Towler’s arrowroot paper, 1864 — the same procedure, a century earlier. The Silver Sunbeam gives the formula in apothecaries’ measure:

Ingredient	Towler, 1864	Reilly, 1980
Sodium chloride	5 drachms	35 g
Citric acid	4 grains	3 g
Arrowroot	4 drachms	35 g
Water	19 ounces, distilled	950 mL

His method is Reilly’s in every particular: dissolve and filter the salt and acid, rub the arrowroot with cold water into a cream with all lumps thoroughly broken up, boil the mixture in a glass or porcelain dish stirring all the while, remove the scum when cold, apply with a very fine soft moist sponge worked longitudinally and laterally, and take off ridges with a glass rod. The sheet is pinned to a board a trifle smaller than itself with the edges folded over the back.

🧮Converting Towler, and what the comparison shows

Taking apothecaries’ weight — 1 drachm = 60 grains = 3.888 g — and fluid ounces of 29.57 mL, so that 19 ounces is 562 mL:

300 grains = 19.4 g in 562 mL = 3.46 % w/v

Sodium chloride

240 grains = 15.6 g in 562 mL = 2.77 % w/v

Arrowroot

4 grains = 0.26 g in 562 mL = 0.046 % w/v

Citric acid

This conversion is the course’s, not Towler’s, and one assumption in it is worth flagging: if his 19 ounces are ounces by weight rather than fluid ounces the volume is about 539 mL instead and every figure rises by roughly 4 per cent, which changes nothing below.

The chloride agrees, and that is the number that matters. 3.46 per cent against Reilly’s 3.68 — within 7 per cent of each other, across 116 years, and both well above the 2 per cent of the plain salted paper. The starch is in the same band, Towler’s about three-quarters of Reilly’s, and note that at 2.77 per cent Towler’s is also above the 2 per cent float threshold, which is precisely why he reaches for a sponge. The citric acid is not in the same band at all: Reilly’s is about seven times Towler’s.

The course does not know why, and neither source explains it. Two possibilities are worth stating as possibilities. Towler’s sensitising bath is kept acid with nitric acid and is stronger than Reilly’s — he maintains it at about 70 grains to the ounce, roughly 15 per cent — so some of the work Reilly’s citric acid does may be done elsewhere in Towler’s process. Alternatively, Reilly may simply have found the higher figure better. This is a disagreement between printings, which the schema has no field for; it is open item 5 of docs/FORMULARY-SCHEMA.md.

Tapioca or rice starch, used in the same manner as arrowroot, on Reilly’s own statement. This is a substitution the source explicitly authorises, at the same quantities. DeBrébisson’s original starch papers of 1854 were tapioca.

Resin-arrowroot paper, in Reilly’s Chapter Five: an ammonia-shellac solution stirred vigorously into a 2.5 per cent arrowroot solution prepared according to the instructions of Chapter Three — that is, to this formula, diluted. It is a further step away from an active binder, and Reilly notes that citric acid has a marked effect on resin-arrowroot papers for exactly the same reason it does here.

Arrowroot-albumen paper, also Chapter Five: equal volumes of fresh albumen and arrowroot solution prepared according to Chapter Three. This one puts an active binder back in beside the inert one, and is the bridge between this page and the matte albumen print.

The plain and citrate salting solutions, on the plain salting solution page. The citrate version is the instructive comparison: it reaches the same “active organic” effect through a citrate salt in a gelatin binder, where this one uses a free acid in a starch binder.

What the modern literature describes. The Getty’s process description of salt printing gives a salt solution of around 4 per cent, often sodium citrate with ammonium chloride, and a sensitiser of around 12 per cent silver nitrate sometimes containing citric acid. Salt printing as practised today has largely moved to citrate-and-ammonium-chloride salting without a starch binder. This page publishes Reilly’s arrowroot formula, and says so.

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

Reilly gives a dichromate addition for raising contrast when a negative is too thin, and states that bichromate additions serve the same contrast-enhancing function in almost any kind of printing-out paper, in approximately the same quantity as with the plain salted paper formula — so the temptation to carry it across to this one is explicitly invited by the source.

The course does not publish its quantities. 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. Putting a Level D recipe inside a Level A page would put the two on the same footing.

What is worth knowing without the recipe: the effect falls mainly on the highlights; 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; the image colour goes browner; and Reilly’s own judgement, after years of experience by many writers, is that the best prints are made in sunlight from optimum negatives without contrast-enhancing additions. See the chromium page.

Safety

Level A. Arrowroot, table salt and citric acid, in water. All three are foods. Sodium chloride carries an aggregated classification in which the large majority of reports state it meets no GHS hazard criteria at all. Citric acid is the one substance here with hazard statements against it — serious eye irritation, and in some notifications respiratory and skin irritation — and they apply to the solid and to strong solutions, not to three grams dissolved in a litre.

The real hazard on this page is thermal, not chemical, and that is unusual for a Level A formula. A litre of boiling salt solution receiving a starch cream that thickens as it goes is a scald risk and a boil-over risk. Add the cream in small amounts, stir constantly, use a vessel with more headroom than you think you need, and keep the pan handle turned in. Glass or porcelain on a hotplate, not metal. Long sleeves and eye protection while boiling, oven gloves to move it, and let it cool where it will not be knocked.

What is not a hazard here, and why. There is no silver in this solution, so nothing stains skin and nothing has to be collected: the whole silver nitrate regime belongs to the next bath. Nothing is light-sensitive, so there is no safelight requirement and coating is done in white light. Nothing here produces a vapour or a gas — the acid is a weak solid organic acid in dilute solution, and boiling it produces steam and nothing else — so ventilation is not among the controls, beyond the ordinary comfort of not steaming up a small room. There is no oxidiser and no reducing agent. The dangerous parts of making an arrowroot print are the silver nitrate at the sensitising bench, the ultraviolet exposure, and whatever is in the toner. None of them is here.

Ordinary discipline still applies: gloves and eye protection as habit, vessels and utensils that have never held food and never will, and labelling per the labelling SOP. The fact that all three ingredients are edible is exactly why the labelling rule matters.

Storage

The made-up solution: no published keeping time, and the course will not supply one from nothing. Reilly gives none. Towler says only that all the papers prepared as directed will keep but are best when fresh, which is about the paper and not the pot.

What can be said, and attributed. A boiled starch paste is a dilute carbohydrate solution at room temperature, which is to say a growth medium; it will ferment, go ropy, or grow mould, and it will do so faster than a salt solution and about as fast as a dilute gelatin one. The precedent inside Reilly’s own book is his mounting chapter, where the starch paste is dosed with thymol as a fungicide and he notes it does not affect the paste. Applying that to this paste is an extension by the course, not an instruction from the source, and is marked as one. Note also that the solution separates on standing even when perfectly sound — that is starch settling, not spoilage, and it is cured by stirring.

The practical course: make what you will coat. The formula is cheap, the boil takes ten minutes, and a paste that has been in the fridge for a week is not worth the sheet of paper you are about to put it on. Refrigerate the remainder, use it within a few days, stir it before use, and discard anything that smells sour, has gone stringy, or has grown a haze that is not the starch skin.

The coated paper: it keeps, and it is not light-sensitive. Coat a batch, dry it hanging, press it flat, and store it between boards somewhere dry. Mark the back in pencil before coating, because after drying there is no telling which face was treated.

The sensitised paper: several weeks, which is the one place where this paper is dramatically better than a plain salted one, whose sensitised life is a day or two. The citric acid in the 12 per cent silver bath is what buys it. That clock starts at the silver bath, not here.

The dry chemicals. Arrowroot is a foodstuff and keeps as one: dry, closed, and away from damp, because starch takes up water and once damp it will support mould in the jar. Sodium chloride cakes in damp air and is otherwise indefinite. Citric acid is deliquescent, and the monohydrate effloresces in dry air, so an old jar of it has lost water and weighs light. All in labelled containers that have never held food.

Incompatibilities

Silver nitrate, absolutely and by design. The two solutions of this process must never meet except inside the paper. A splash of silver into the salting pot precipitates silver chloride and silver citrate on the spot and ruins both. Separate vessels, separate stirrers, separate benches. See incompatibilities.

Metals, while boiling. Both sources specify glass or porcelain and neither gives a reason. A boiling chloride solution is corrosive to many metals, and dissolved metal ions in a silver process are a contamination problem as well as a corrosion one. Reilly’s explicit warning that a chromated salting solution must not contact metals shows the same caution in a case where he does explain it.

Fixer, on anything that goes near the pot. A trace of thiosulfate carried back on a wet hand does nothing to the starch paste and is later a dead spot on a print. In a printing-out workflow contamination travels backwards up the process at least as often as forwards.

Heat, past the point the paste needs. Not an incompatibility in the hazard sense but in the chemical one: Reilly says gently boil for a few minutes. Prolonged hard boiling drives off water and thickens the paste past the concentration the formula specifies, and scorching a starch paste on a hotplate produces brown specks that end up in the coating.

Nothing else. There is no alkali, no oxidiser, no reducing agent and no heavy metal in this formula, and saying so plainly is more useful than a list of things that cannot happen.

Waste

This is not a silver stream, and knowing which streams are which is the point. Spent arrowroot salting solution is dilute sodium chloride and citric acid with cooked starch 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.

It does carry an organic load that the plain salting solution does not. Cooked starch is biodegradable and is a food-industry effluent rather than a photographic one, but it is a real biochemical oxygen demand, and a litre of paste poured down a domestic drain is more likely to cause a blockage than a pollution problem. Scrape the pot rather than rinsing it hot.

Where the silver actually goes is the sensitising bath, the first wash after exposure — Reilly’s clouding of that wash is excess silver nitrate meeting the ions in tap water — and the fixer. The fixing bath page and the silver-bearing waste SOP cover those. None of it applies to this pot, and blurring the distinction 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 the general chemical waste SOP.

Troubleshooting

Lumps in the finished solution. The cream was not properly rubbed up, or it was added to the boiling solution too fast. Lumps are grains that gelatinised on the outside while dry inside, and they cannot be dispersed afterwards — strain the solution through muslin if it is otherwise good, and next time use the mortar and add the cream more slowly.

A skin on the cooled solution. Expected, not a fault. Reilly identifies it as the residue of the hulls of the burst starch grains and says to remove it; Towler calls it the scum and says the same. Lift it off before coating.

The solution separates while you work. Also expected. Reilly warns that the cream separates into its components and must be stirred to the moment of use, and a cooled paste behaves the same way. Stir it; it is not spoiled.

Grey, flat prints with no real black. The classic arrowroot failure and it has three quite different causes, which is why it is worth working through them in order. One: the citric acid was left out or was stale. Reilly’s stated consequence of omitting it is precisely grey and flat prints, the hallmarks of a pure chloride image. Two: the sheet was floated too long on the silver bath. Because the starch layer is permeable and is not coagulated by silver nitrate, prolonged floating drives fine silver chloride down into the fibres and costs brilliance — Reilly says this happens especially with porous papers, and gives half a minute to ninety seconds as the window. Three: the rawstock is too porous, in which case the answer is a higher starch content rather than a smoother paper. Check them in that order, because the first is free to test.

An uneven, streaky or mottled coating. The hardest part of the process, and Reilly says so: the difficulty is keeping the coating as even as possible at every step. Brush in one direction, cross it perpendicular, let it sink in a minute or two, then work it back with a dry round brush until the surface is uniformly matte. On a textured paper, make sure the paste is brushed into the crevices. If the paste is too stiff to level, the boil went too far.

The coating lifts, flakes or comes away in the baths. The paste did not penetrate. Reilly is explicit that arrowroot pastes must be absorbed into the fibres to a certain extent in order to adhere. A highly calendered plate finish resists absorption and is the likeliest culprit; a thinner first coat that is allowed to sink well in, or a less heavily sized paper, is the answer.

Grey specks or brown flecks in the coating. Either starch hulls that were not skimmed, or a scorched paste. Skim the cooled solution; boil gently and stir.

A sour smell, stringiness, or mould on the stored solution. It has fermented. Discard it. See Storage.

The print is a good purple out of the frame and goes yellowish-brown after fixing. That is what Reilly says an untoned arrowroot print does. It is not a fault in the salting solution — it is the reason these papers are toned.

The print overtones almost instantly. The gold bath is too strong for a matte paper. Reilly’s figure for matte salted papers is 0.1 to 0.2 g of gold chloride per litre, against 0.4 to 0.5 for glossy albumen, and porous surfaces tone much faster.

Experiments

The citric acid series, which is the experiment this page exists for. Make four batches identical in every respect but the citric acid: 0 g, 1 g, 3 g and 6 g. Coat the same rawstock the same way, sensitise all four on the same 12 per cent silver bath containing 4 per cent citric acid, and print each from the same negative to the same visual endpoint. Reilly’s claim is falsifiable and specific: the 0 g sheet should be grey and flat, and the others should print warm purple. Record the exposure times, because if the acid is doing what he says it should also change the speed. This is the direct test of the active-and-inactive division on which the whole formula rests, and it is the experiment that would tell you something Reilly, Ware and Hardwich between them do not settle.

Measure the cream’s water, and settle the volume this page cannot state. Weigh 35 g of arrowroot, rub it up in a mortar to the “fairly runny cream with no lumps” Reilly describes, and measure how much cold water that took. Then make the formula and measure the finished cooled volume. You will have determined the one number the source omits, and with it the true concentration of everything in it — and whether the chloride lands on Reilly’s 3 per cent worked example or above it. Repeat it three times, because “fairly runny” is a judgement and the spread between your three attempts is the real answer.

The starch series, against the coating method. Batches at 2, 3.7 and 5 per cent arrowroot, keeping salt and acid constant. Float the 2 per cent one; brush the others. You are testing Reilly’s float threshold directly, and looking at what more binder does to surface, maximum density and the sunken look in raking light. Expect the highest-starch sheet to be the hardest to coat evenly, and note whether it also needs the longer end of the silvering window.

The silvering-time series, which this paper punishes harder than any other. One coating, sheets floated on the same 12 per cent silver bath for 30 seconds, 1 minute, 90 seconds and 4 minutes. Reilly predicts a loss of brilliance and grey, flat prints at the long end, and gives a mechanism — the starch layer’s permeability letting silver chloride migrate into the fibres. Try it on a smooth and a porous stock; he says the effect is worse on the porous one.

Arrowroot against tapioca against rice starch, which Reilly authorises as equivalents at the same quantity. Towler notes that the granules of different plants are distinguishable under the microscope by their concentric rings; if you have one, look. Compare paste viscosity, coating behaviour, surface and image colour. If they really are equivalent, this is a cheap way to find out; if they are not, you have found something the source does not record.

This formula against the plain and citrate salting solutions, from one negative. Three sheets of the same rawstock, three salting solutions, one silver bath, one exposure series, one toner. This is Hardwich’s starch-gelatine-albumen ranking made visible, and it is the single most instructive afternoon available in this part of the course.

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. Do it for this paper and for a plain salted one and check his ranking — arrowroot should want a slightly shorter negative density range than plain salted paper, and both should want far more than any develop-out paper. Put the result in a laboratory report.