Sodium chloride

Common salt does not make an image and is not sensitive to light. What it does is put a chloride ion into a sheet of paper where a silver ion is about to arrive — and photography on paper began there.

In photography

Salting paper. Reilly’s account of the salted-paper process starts from a difficulty: silver chloride is insoluble in water and in most solvents, so it cannot simply be brushed on. It has to be formed in place — treat the paper with a soluble chloride, then treat it again with silver nitrate.

NaCl + AgNO₃ → AgCl + NaNO₃

Reilly's double replacement: the sensitive salt made in the paper's own fibres

The silver chloride is thrown down as a cloudy white solid where the two solutions meet, and the sodium nitrate stays dissolved and washes away. Reilly’s plain salted paper is 20 g of sodium chloride and 2 g of gelatine in a litre of water, floated for three minutes; a variant adding 20 g of sodium citrate gives prints more reddish and slightly more brilliant. The Getty atlas describes the same step in its own words: the paper is soaked in a salt solution, which “can be sodium chloride but is often sodium citrate and ammonium chloride, usually around 4% by weight”, then sensitised with silver nitrate at around 12 per cent.

⚗The chemistry

Why so much more silver than salt? Reilly reports that Talbot found papers with chloride and silver nitrate in equal amounts hardly light-sensitive at all, and that about six times more nitrate was needed; he settled the salting solution at 2 to 4 per cent and the silver at 12 per cent, roughly the four-to-one ratio Reilly gives as the working rule. The nineteenth-century explanation Reilly repeats is Hermann Vogel’s: as light dissociates each unit of silver chloride, the liberated chlorine unites with the free silver nitrate to form new silver chloride, which light breaks down in turn, and the cycle repeats. Without the excess silver there is nothing to accept the chlorine.

How much chloride changes the print. Reilly gives the numbers for albumen, where the chloride is beaten into the egg white rather than floated on the paper. A low chloride content, 1 to 1.5 per cent, makes a less sensitive paper that tends to give slightly more contrasty prints from thin negatives — but the gain costs the rich, dense image, so 1.5 to 2.5 per cent is normal. Above what is needed, more chloride only raises silver consumption without conferring any additional benefit. The chloride content of a salting solution also affects contrast, though Reilly is clear that large increases need chromates, a technique he says has no place in regular practice.

Elsewhere. Kodak’s 1928 primer has sodium chloride as one of the two salts an emulsion maker reaches for — “the necessary bromide or chloride, e.g., potassium bromide or sodium chloride, is then added to the solution” — chloride emulsions being the slower contact-printing papers. Its account of Solio is the printing-out exception: a chloride emulsion made with an excess of silver nitrate, which is why it darkens rapidly in light and is toned and fixed instead of developed. Wall’s 1912 dictionary lists salt for precipitating silver from print washings and for salting positive papers; his 1924 formulary uses 6.6 g of it against 35 g of ferricyanide as the chloride bleach for indirect sulfide toning.

Properties

Colourless transparent crystals or a white crystalline powder, relative molecular mass 58.44. A gram of it therefore carries far more halide ions than a gram of potassium bromide (119) or potassium iodide (166) carries of theirs — worth remembering when a historical formula is written in one halide salt and you have another.

Its solubility is 36.0 g per 100 g of water at 25 °C, or 1 g in 2.8 mL. The number that matters is the next one: 1 g in 2.6 mL of boiling water. Kodak’s 1928 primer states it plainly — solubility increases very little on heating — so there is no making a hot concentrate and letting it cool. The solution is neutral, pH 6.7 to 7.3.

The salt is somewhat hygroscopic, with a critical relative humidity of 75.3 per cent at 20 °C, above which it takes up water from the air. Kodak calls the pure salt stable and not deliquescent; Wall notes that ordinary salt usually carries traces of sodium sulfate and magnesium chloride, which make it hygroscopic. On grade, Reilly is specific: technical or ACS grade is sufficiently pure for albumen and salted paper, and the highly purified grades are needed only for emulsion making. What the iodide and anti-caking additives of retail table salt do to a salting solution is not something this course has found a source for, and it does not guess.

Handling

The aggregated ECHA notifications held by PubChem classify sodium chloride Warning, irritant, with one statement above the display threshold: serious eye irritation at 10.8 per cent. The striking figure is the other one: 1,840 of 2,063 reports, 89.2 per cent, state that it meets no GHS hazard criterion at all — a majority position reached for none of the other three halide salts on these pages. PubChem’s Haz-Map summary, citing CAMEO, reads “An eye irritant; Toxic after ingestion of large amounts”.

Why Level A. Against the course rubric this is Level A on every criterion: the classification does not reach beyond irritant at the concentrations handled — a 2 to 4 per cent salting bath; nothing is heated above 50 °C; the waste needs no specialist neutralisation. A permissive classification is not a reason to drop the ordinary controls, so the salt used in this course is weighed on the laboratory balance and kept in a labelled container never used for food. Neither EH40 nor the NIOSH pocket guide sets an exposure limit for it, and EH40 states that absence from its list does not indicate that a substance is without hazard.

⚠Safety

The hazard of a salting session is not the salt. It is the step that follows: a silver nitrate sensitiser at around 12 per cent — a substance Kodak’s 1928 primer describes as attacking organic material and blackening skin, wood and cloth on exposure to light — and it is that solution, not the salt, that sets the protective equipment and the safety level for the whole procedure. PubChem carries no reactivity profile or chemical-dangers entry for sodium chloride, so the only incompatibility this course states for it is the one its own process depends on — silver nitrate, precipitated at once as silver chloride. Reilly’s practical consequence: make the sensitiser with distilled or deionised water, because chloride and hard-water ions in tap water throw the same cloudy precipitate in the bottle.

Waste

A salting solution is dilute salt and gelatine. What comes off a salted-paper bench and needs thought is the sensitiser and the first rinses: the Getty atlas has the exposed paper washed in several changes of water to remove excess silver nitrate, and that silver is both the valuable part and the regulated part, so those rinses are collected rather than run away. Wall’s 1912 dictionary records the same chemistry turned deliberately on the waste — salt used to precipitate silver from print washings.

Kodak’s J-300 guidance for amateur photographers is explicit that septic systems are not designed to treat photographic processing solutions, and ILFORD tells domestic users in the United Kingdom to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard. Check your local regulations; they govern.

History

Photography on paper begins here. Eder records that Talbot’s first experiments coated the paper with the moist precipitate of silver chloride, and that he then found the better way: saturate the paper with a strong solution of common salt, dry it, and immerse it in silver nitrate. In 1835 he made that paper more sensitive still by repeated baths of salt and of silver nitrate, and with it produced the small camera picture of Lacock Abbey. Eder credits him with stating precisely the relationship this page turns on — silver chloride with an excess of silver nitrate is more light-sensitive than with an excess of sodium chloride.

Then the same substance, at a different strength, became his fixer. In the letter Eder dates 30 January 1839 Talbot described his images on silver chloride paper and “an approximate fixation with an excess of a strong solution of common salt”; the results, Eder adds, were imperfect and of doubtful permanency, and Talbot adopted Herschel’s hypo as soon as he learned of it. One salt, two concentrations, two opposite jobs — weak, it makes paper sensitive; strong, it very nearly stops the paper responding to light at all, which is the desensitising by excess halide that the bromide restrainer would later put to work in a developer.