Arrowroot

Arrowroot is the only binder in this encyclopaedia that was chosen for what it does not do. Egg white forms silver albumenate and changes the colour of the print; gelatin reddens a salted paper and scavenges halogen in an emulsion; citric acid makes silver citrate. Arrowroot does none of that. James Reilly’s sentence is the whole of it: starches do not react with silver salts and have no effect on the reduction of silver chloride. What arrowroot supplies is a physical layer that holds the light-sensitive material on the surface of the sheet instead of letting it sink into the fibres — and because it supplies nothing else, the formula built around it has to bring its own chemistry.

In photography

It is the binder of arrowroot paper, a salted paper with a matte surface. Reilly describes the material as a form of salted paper whose binder is a paste of boiled arrowroot starch, with surface qualities running from very matte to a dull gloss according to how much starch is laid on and how smooth the rawstock underneath is. Against a plain salted paper the gain is real and measurable: arrowroot prints are considerably more brilliant and richer-looking, with a longer density range and more delicate detail preserved. Nothing chemical happened to produce that. The starch simply kept the silver chloride in a compact layer resting on the fibres rather than dispersed among them, and Reilly sets out why that matters — light reflected from within the paper is scattered and diffused by the fibres, lowering the maximum density obtainable, whereas an image formed in a compact surface layer scatters much less and reaches a much greater maximum density.

Its inertness is what forces the citric acid. This is the most important sentence on the page for anyone reading the salting solution. Reilly divides the organic substances used in printing-out papers into two classes. The active ones — albumen, gelatin, and the organic acids citric, tartaric and oxalic — facilitate the more complete reduction of silver chloride and themselves form light-sensitive substances on contact with silver nitrate, such as silver albumenate and silver citrate. The others, of which starch is the most useful, do no such thing; they contribute by keeping the light-sensitive material on the surface and preventing a dull sunken-in appearance. So a starch-bound paper needs an active substance added on purpose, and Reilly states the consequence of leaving it out: without the citric acid the prints would be grey and flat, the hallmarks of a pure chloride image, whereas with both citric acid and starch present they take a warm purple after exposure and change to a yellowish-brown after fixing if they are not toned.

It changes how the paper is silvered. Because starch is not coagulated by silver nitrate the way albumen is, the coating stays permeable and a long float pushes silver chloride down into the fibres — undoing the one thing the binder was there for. Reilly’s floating times for arrowroot paper are accordingly short, half a minute for a light coating and at most a minute and a half for a heavy one, on a 12 per cent silver bath that itself carries 4 to 5 per cent citric acid; too long on the bath gives grey, flat prints, especially on porous papers.

And the surface it leaves decides what negative the print wants. The same quantity of reduced silver reads as a paler shadow on a matte print than on a glossy one, so a deep black on matte paper needs relatively more silver and therefore a longer exposure — and a longer exposure needs a negative with enough highlight density to keep the light tones from going too dark. That is Reilly’s reasoning for his ranking of the negative density range these papers ask for: plain salted paper the greatest, matte papers like arrowroot and matte albumen slightly less, albumen lower again. It sits beside a fact that sounds like a contradiction until the two are separated. Matte salted papers like arrowroot are the fastest of this family, ahead of plain salted paper, with albumen the slowest. Speed is how quickly the print darkens; density range is how much of the negative’s scale it can hold, and a matte surface is generous with the first and demanding of the second. The same porosity governs toning: Reilly gives 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, because a porous paper tones quickly and would rapidly overtone in a bath meant for albumen.

It sizes papers for the iron processes. Wall’s 1912 dictionary gives arrowroot three photographic uses in one entry: sizing papers, acting as the vehicle for sensitive iron salts in printing papers, and making a mounting paste. His kallitype instruction sizes the sheet with 180 grains of Bermuda arrowroot in 20 ounces of water, boiled eight to ten minutes; his platinum instruction offers arrowroot or plain starch as an alternative to gelatine and — this is the useful part — states that the choice shows in the print, a gelatine size tending to a bluish black tone and arrowroot or starch to a brownish tinge. The Getty Conservation Institute’s platinotype atlas records the same practice from the other end, noting that a paper selected for platinum printing is usually surface sized with starch or gelatin, and listing starch, agar-agar, gum arabic and gelatin among the sizing materials described in the photographic literature.

A modern chemist argues against it, for a reason that does not apply to the boiled rhizome. Mike Ware’s specification for siderotype papers excludes starch surface sizing, and his argument is precise: a paper mill’s surface sizing starch is not the familiar substance but oxidised starch, made by treating starch with hydrogen peroxide or sodium hypochlorite, and the carboxylate groups that treatment introduces bind iron(III) exactly as the carboxylates of ferrioxalate and ferricitrate do. The consequences he names are a less efficient sensitiser, a shortened exposure scale and possible sequestration of iron(III) leading eventually to staining. This is a genuine incompatibility and it belongs on this page, but it is worth being exact about what it is a claim about. Ware is describing a chemically modified industrial derivative applied at the mill; Wall was describing a native starch paste boiled and brushed on by the printer. The course reports both and does not collapse them into one.

It mounts prints, and the conservation instruction is more specific than the photographic one. Reilly’s mounting method uses a paste of boiled refined wheat starch rather than arrowroot, and the instructions are worth reading for what they say about starch pastes generally. The starch is stirred into cold distilled water, left overnight, then cooked with constant stirring — because the properties of the gel that forms, and its adhesive strength, are determined by the length and the method of cooking. No iron or steel may touch the solution at any stage. Thymol in methyl alcohol goes in as a fungicide afterwards, and the covered, refrigerated paste keeps for about a week.

And it acted photographically by accident, long before anybody put it in a salting solution. Mike Ware’s account of silver nitrate on plain paper is that the salt is not photosensitive on its own, because there is no accompanying oxidation half-reaction available to supply the electron; it becomes photosensitive in the presence of readily oxidisable substances, and on a paper carrying silver nitrate alone the oxidisable component must be the organic matter present — the cellulose of the sheet, or its starch or gelatin sizing. He makes the same point about Mungo Ponton’s dichromate prints of 1839, where the sizing agent or the paper itself, probably gelatin or starch, provided what the dichromate oxidised. On the halogen question he is careful, and the course follows him: starch sizing is among the potential halogen acceptors around a Talbot sensitiser, but Ware proposes that water and silver(I) ions together constitute the major acceptor, and concludes that the sizing agent is not essential to the photochemistry of print-out — which can be shown experimentally — while still serving to retain the sensitiser and therefore the image in the surface fibres, and possibly to protect the colloidal silver particles by surface adsorption, influencing colour and stability. That is a mechanism stated as a proposal, and it is printed here as one.

⚗What boiling actually does, and what the period manuals thought it did

A starch granule is not a lump of one substance. Malki and colleagues describe it as two polysaccharides organised together: amylose, essentially linear, built of anhydroglucose units joined by α(1,4) links, with a molecular weight of 5 × 10⁵ to 10⁶; and amylopectin, highly branched, α(1,4) along its chains with 2 to 4 per cent of α(1,6) branch points, and a molecular weight of several million. Inside the granule the short outer chains of amylopectin pair into double helices which pack into crystalline lamellae, while the amorphous regions hold the amylose and the extended amylopectin chains, and the granule is built of these two kinds of layer in alternation.

Heating that structure in water does three things in sequence. The molecules vibrate hard enough to break the intermolecular hydrogen bonds of the amorphous regions; water then hydrogen-bonds to the hydroxyl groups it has exposed on both polysaccharides, and the granule swells; and gelatinisation proper occurs as the outer chains of amylopectin lose their double-helical crystalline order. Amylose in particular leaches out of the swollen amorphous region into the surrounding water, which is where the thickening comes from. Differential scanning calorimetry on the Sri Lankan arrowroot in that study puts the onset at 75.02 °C, the peak at 77.95 °C and the conclusion at 82.43 °C — which is why the historical procedure boils rather than warms.

The nineteenth century saw the same events and explained them differently. Towler’s account of 1864 is that each granule “is regarded as a cell of concrete and insoluble material, holding within a soluble pulp”, that boiling bursts the cells so that the soluble part mixes with the water, and that the insoluble husks can afterwards be separated from the fluid portion. Reilly, writing in 1980, describes the skin on the cooled salting solution as the residue of the hulls of the burst grains and instructs that it be removed. The observation is sound in both cases and the model behind it is not: there is no soluble pulp inside a husk, only two polysaccharides differing in how readily they leach out of a structure that heat has disordered. The course states the modern account as the mechanism and the period account as what the manuals believed, because Rule 7 asks that established chemistry and historical interpretation not be printed as the same kind of thing.

Properties

Identity. The starch of the rhizome of Maranta arundinacea L., family Marantaceae — a herbaceous tuber crop whose rhizomes spread horizontally below the surface, and whose starch content is around 20 per cent of the rhizome. Towler names the same plant and the same region in 1864, and the FDA’s registry record for the plant part carries WEST INDIAN ARROWROOT ROOT and ST. VINCENT ARROWROOT ROOT among its own names, alongside the botanical synonyms Maranta indica, ramosissima and sylvatica. There is no molecular formula and no molar mass, and CAS 9005-25-8 is a family number for starch of any origin.

The granule. This is the part of the identity that does photographic work, because the size and shape of the grain decide how the cooked paste behaves. Malki and colleagues found the granules of Sri Lankan arrowroot to be oval (about 50 per cent), spherical (29 per cent) and irregular globular (21 per cent), with smooth surfaces free of fissures, averaging 44.99 ± 1.27 μm long and 31.44 ± 0.58 μm wide — large, as starch granules go, and comparable to banana and potato. Other studies of arrowroot report widths of 20 to 40 μm, and a Brazilian study an average of 56.60 μm; the authors attribute the spread to botanical origin, plant physiology and the stage of development at harvest. Towler already knew the granules were distinguishable: he describes the concentric rings visible under the microscope, and says the granule of the potato can be told from that of arrow-root.

Amylose content and crystal type. The measured amylose content was 24.95 ± 1.49 per cent, against a literature range for arrowroot of 16 to 27 per cent, which places it at the boundary of the “high amylose” band. X-ray diffraction gave an A-type pattern, from the strong double peak near 17° and 18° 2θ — although the same paper records an earlier study that found B-type, and attributes the difference to extraction process, growing conditions and genotype. A course reading period formulas should take that seriously: two jars labelled arrowroot are not guaranteed to be the same material, and the nineteenth-century preference for one starch over another was a preference between materials that had not been characterised.

Gelatinisation temperatures vary by more than the procedure allows for. The Sri Lankan sample gave an onset of 75.02 °C. The same paper collects values for arrowroot from elsewhere: 65.0 °C onset with a peak of 72.5 °C from Indonesia, 67.75 °C with a peak of 73.62 °C from Venezuela, and an onset of 62.95 °C from Brazil. That is a spread of twelve degrees in the onset alone. The historical instruction — bring the salted water to the boil and add the cream — is robust against exactly that variability, which is probably why it survived unchanged from Towler in 1864 to Reilly in 1980.

The dried layer. Reilly gives the property that makes starch usable at all as a photographic binder: when a starch paste is applied to paper and dried, the layer formed will not swell in water and will withstand the processing solutions without damage. That is the opposite of gelatin, whose reversible swelling is both its virtue in an emulsion and its weakness, and it means a starch-bound sheet needs no hardening step of any kind — no alum, no chrome alum, no aldehyde. It is also why Reilly can warn that a long float on the silver bath drives sensitiser into the fibres: a layer that does not swell is not thereby impermeable, and the starch coating stays open to water throughout.

Solubility and the paste. Insoluble in cold water and in alcohol; insoluble in hot water too, in the strict sense, since what happens on heating is swelling, bursting and leaching rather than dissolution. The least gelation concentration measured for arrowroot — the weakest suspension that set to a self-supporting gel in that study — was 8.0 per cent, and every photographic solution the course carries is well under it. Reilly’s works out at about 3.7 per cent against the 950 mL of water he states, and even that is an upper bound, because the cream carries a further quantity of cold water he does not measure. His coating advice divides the range at 2 per cent: solutions of 2 per cent or less are fluid enough to float a sheet on, and the pastes the old manuals brushed on ran 3 to 4 per cent. These are thickened liquids that dry to a film, not gels.

🔬Iodine, and a colour that did photographic work

Free iodine is the classic test for starch, and Towler gives it as such — the violet-blue colour that disappears on heating and returns on cooling. It is not only a laboratory identification. Mike Ware’s history of the calotype records that when Guillot-Saguez’s single-solution iodizing method left a coating of potassium iodide on the sheet, exposure to moist air over some days oxidised a little of it to free iodine — and on French papers, which were starch-sized, this turned the sheet blue. Guillot- Saguez turned the nuisance into an indicator: the blue was discharged when the exciting solution had completely penetrated the paper, so the printer could see when the sheet was ready.

Two conclusions follow. The first is that starch sizing was normal on continental papers and gelatin sizing on English ones — the Getty’s salt print atlas states that division directly, and Hardwich in 1864 describes the two modes as starch with saponified resin on the Continent and gelatine hardened by alum in English manufacture, the first necessarily alkaline and the second acid from the alum. The second is that a photographer of the 1840s could be relying on a binder they had never chosen and could not see.

The modern difficulty is the reverse. The Getty’s atlas notes that starch sizing is hard to detect by ATR-FTIR, because starch and the cellulose fibres of the paper are both complex carbohydrates and their spectra overlap; microanalytical tests exist but need physical sampling. A material that is difficult to prove is one whose presence in a surviving print is usually inferred.

Handling

What the course looked for and did not find is set out in the hazards note above, and the short version is that there is no GHS classification for this material to quote — PubChem holds no compound record for starch, amylose or amylopectin, and the registry records carry identity and nothing else. That is an absence of evidence and not evidence of absence, and it is why the supplier’s safety data sheet for the grade in hand is the authority rather than this page.

One control does have a number, and it is about dust. HSE’s EH40 lists Starch in Table 1 at 10 mg/m³ total inhalable and 4 mg/m³ respirable over an eight-hour reference period, with no short-term limit. That is a nuisance-dust limit, it applies to a workplace rather than to a darkroom, and the practical form it takes at a domestic scale is unglamorous: weigh the powder slowly, into a tared vessel rather than from a height, away from a running fan, and put the lid back on. The comparison the entry invites is worth resisting. Flour dust and grain dust sit at the same 10 mg/m³ in the same table but carry the Sen notation for respiratory sensitisation; the starch entry carries no notation at all, and the course states the two facts side by side without transferring the notation from one to the other.

The other two controls are physical rather than toxicological. A pan of boiling salted water into which a starch cream is being stirred becomes a thick paste that holds heat and clings, so it is treated as a scald risk in the ordinary sense: a porcelain or enamel vessel, a long glass rod or wooden implement, and no leaning over the pan. And the paste spoils, which is a reason to make it fresh rather than a hazard in itself.

Why Level A, and the one criterion it does not meet. Against the course rubric, arrowroot as a material sits at Level A: there is no classification approaching the rubric’s ceiling of irritant, harmful if swallowed or corrosive; the only exposure limit found is a nuisance-dust figure; there is no mains-voltage construction; and the waste from the starch itself is dilute rinse water. One criterion is not met, and it is better named than glossed. Level A specifies no heating of solutions above 50 °C, and the only way to use this substance is to take it well past that — a starch that has not been gelatinised is a suspension of grains, not a binder. Boiling the salting solution meets Level B’s criterion for heating or holding solutions above 50 °C.

The rubric’s own answer is the raised step: a single step may be declared at the higher level provided the page names it and states its controls where the reader meets them before the procedure, and the salting solution page does that. The hazard there is thermal rather than chemical — a litre of boiling salt solution that thickens as the cream goes in is a scald and a boil-over risk — and the controls are a vessel with more headroom than seems necessary, small additions with constant stirring, glass or porcelain rather than metal, sleeves and eye protection while it boils, and a place to cool where it cannot be knocked. Nothing in that paragraph is about the starch’s chemistry, which is the point: this material’s one exceedance of Level A is the temperature it has to reach, and not what it does when it gets there.

⚠The step after the boil, which is not a raised step but a different session

The boil is a thermal exceedance of a Level A session and is declared as one. Sensitising the coated sheet is not an exceedance at all; it is a different assessment. The dried arrowroot sheet is floated on a 12 per cent silver nitrate solution, and everything about that bath — the corrosivity, the staining, the gloves, the eye protection, the collection of every drop of rinse water for silver recovery — belongs to the silver nitrate and is written on its page and in its handling procedure. The starch changes none of it in either direction. It is not a control and it is not an aggravating factor; it is simply present.

Reading the two together is the useful part. A jar of arrowroot in a cupboard is a food starch with a nuisance-dust limit. One operation later it is a hot solution, and one operation after that it is inside a session whose hazards have nothing to do with it. The letter belongs to the operation being performed, and this substance passes through three of them in an afternoon.

Waste

Dilute starch in rinse water is oxygen-demanding organic load; a spoiled paste is fermenting carbohydrate. Neither is a reason to be careless, because the starch is rarely the only thing in the liquid. The wash water from a sensitised sheet carries silver and belongs with the silver stream. A spent fixing bath goes to silver recovery. A gold or platinum toning bath carries its metal. A paste made up with thymol or another preservative carries that substance’s classification with it. Bottle each stream separately and label it with what it contains and the date, as ILFORD instructs domestic users to do for photographic wastes. Check your local regulations; they govern.

History

A starch paper is a specific invention with a date. Reilly credits the first starch papers to DeBrébisson in 1854, made with tapioca rather than arrowroot; arrowroot displaced tapioca afterwards as the most suitable and widely used starch for the purpose. The timing was awkward. Albumen paper was new in the same years and had its own difficulties — harder to tone and fix, sometimes difficult to coat evenly — so starch papers arrived as a real advance over plain salted paper for matte work and almost completely displaced it, and the new trade in ready-salted and sized papers sold both kinds. Then albumen improved, and by the second half of the 1850s it held the paper market outright and kept it for thirty-five years.

What brought arrowroot back was taste rather than chemistry. Reilly’s account of the reversal is that glossy papers fell out of favour after about thirty years; the 1880s saw a renewed interest in matte surfaces among photographers whose interest was aesthetic rather than commercial; and by 1900 the public regarded matte papers as more artistic. Arrowroot papers revived alongside plain salted papers, helped by platinum toning, which gave them a range of browns. What ended them was economics rather than fashion: printing-out papers were expensive and slow, and the last arrowroot and plain salted papers left the market in the years after the First World War.

The sizing story runs underneath all of it and is older. Long before anybody boiled arrowroot into a salting solution, paper mills were sizing sheets with starch, and the Getty’s atlas records the division of practice — English mills preferring gelatin, French and other continental mills starch. Hardwich in 1864 describes both modes and draws a photographic conclusion from them that is worth reading as a period inference rather than as established chemistry. He says the photographic properties of a paper are much affected by the mode of sizing, because the picture is probably formed partly upon the albumen and partly in the sizing; that English gelatine-sized papers tend to red tones becoming brown or chocolate; and that continental starch-sized papers give tones that are sepia-brown after fixing and purple-black under gold. His two reasons are that starch does not, like gelatine, exert a marked action in reddening the picture, and that a starch-and-resin size is alkaline where an alum-hardened gelatine size is acid, alkalis diminishing redness and acids increasing it. The first of those is consistent with Reilly’s flat statement that starch is not an active substance; the second is an argument the course records as Hardwich’s, not as its own.

And a note on how the substance itself was understood. Hardwich’s chapter on the chemical elements and their combinations prints the formula for starch as C₂₄H₂₀O₂₀, in the equivalent notation of his day, and uses it to make a point he could state but not explain: that what he calls lignine, or cotton fibre and starch contain the same percentages of carbon, hydrogen and oxygen while differing widely in properties. He was right that the puzzle was real, and elemental analysis was the wrong instrument for it. The answer is structural rather than compositional, and this page’s own account of the granule is a description of exactly the kind of structure a percentage composition cannot see — α(1,4) chains, α(1,6) branch points, double helices packed into crystalline lamellae and alternating with amorphous ones. None of that was available to any manual quoted here. What the printer is doing when the cream goes into the boiling salt water is breaking hydrogen bonds, melting double helices and leaching amylose into the water, and every one of these authors got the procedure right without being able to say so.