Silver chloride

The first photographic substance. Kodak’s 1928 primer opens its chapter on photographic materials by saying the earliest photographs were made by coating paper with silver chloride and letting it darken — and adds, in the next sentence, that the sensitiveness was too slight to use in a camera.

In photography

It is made where it is used. Silver chloride is so nearly insoluble that it cannot be brushed on. Reilly states the consequence flatly: it must be formed in place, by treating the paper first with a soluble chloride and then with silver nitrate.

NaCl + AgNO₃ → AgCl + NaNO₃

Salting, then silvering: the salt print made in the fibre

The sodium nitrate stays in solution and washes out; the silver chloride is left precipitated inside the paper’s own fibres, which is why a salt print looks sunken into the sheet rather than sitting on it, and why the Getty atlas records untoned salt prints of 1835 to 1850 as running from light brown to reddish brown.

How much of each decides the print. Reilly gives 2 to 2.5 per cent chloride against a 10 to 12 per cent silver bath for salted paper. Lower the chloride and thin negatives print, but with less brilliance and a thinner silver deposit. The strength of the silver bath also sets the grain size of the precipitated chloride — stronger baths give larger grains, less likely to sink into the fibres — which is why matte and starch-sized papers want a strong bath and a short float.

⚗The chemistry

Light reduces silver(I) to silver and oxidises chloride to chlorine, and Ware stresses that the reaction is reversible: UV light + 2 AgCl ⇌ 2 Ag + Cl₂. With nothing to take up the chlorine, that powerful oxidiser attacks the fresh silver, re-forms the chloride, and printing out comes to a standstill. Something must accept the halogen. In salted paper it is water in the fibres together with the excess silver nitrate, which react with chlorine to make more silver chloride; in an albumen print the albumen scavenges it; in the commercial gelatin printing-out papers of about 1900 a halogen absorber such as citrate was added deliberately. That is the difference between a demonstration that stalls at a faint grey and a print with real shadows.

In developing-out papers it does a quieter job. Kodak’s 1928 primer notes that the less sensitive papers used for contact printing by artificial light contain silver chloride in place of silver bromide, and that a developing-out emulsion of either halide must carry excess soluble halide and no free silver or the whole surface fogs in the developer. Slowness is the feature.

Properties

A white solid, darkened by light, dissolving to 1.93 mg per litre at 25 °C — about one part in half a million, against silver nitrate’s 122 g per 100 cm³. It is anhydrous; there is no hydrate to convert.

Being white, it absorbs almost nothing in the visible. Ware puts the working figure at sensitive only below about 420 nm in the pure state, and takes the summation for its photosensitivity over 300 to 400 nm. Abney adds that when white light acts on chloride of silver long enough to blacken it, in the presence of moisture, chlorine is given off — a point he says is never disputed, its proof being as old as Scheele’s classical experiments.

🔬The rule is not a guarantee

Ware tested a photogenic-drawing paper behind a yellow filter removing everything below 470 nm and found the fogging rate only halved. Print-out silver absorbs visible light itself and passes the energy on, so a real coating sees further than the pure salt does. “Chloride paper cannot see green” is a working rule about fresh material, not a promise about a sheet that has already started to print.

What dissolves it. Aqueous ammonia does: Scheele’s third experiment poured caustic spirit of sal ammoniac on horn silver that had blackened for a fortnight in the sun, dissolved the unchanged chloride and left a black residue he then proved was silver — the first fixing, performed without being recognised. Thiosulfate does it far better, by complexation.

AgCl + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Cl⁻

Fixing: the halide leaves as a thiosulfate complex

Of the three halides this is the least insoluble by a wide margin and the one a fixer clears most readily; Kodak’s 1924 primer makes the contrast at the other end, noting that wet collodion plates, being made from silver iodide, are not easily fixed in hypo.

The colour of the finished image is a fact about particle size rather than composition. The Getty atlas records that photogenically formed silver particles are much smaller than developed ones, and that the colour of a printing-out silver chloride image follows that size, running from light yellow-brown through red to darker brown.

Handling

Read the aggregation before the label. The ECHA notifications held by PubChem give Warning with the corrosion, irritant and environment pictograms, but 50 of 398 reports (12.6 per cent) say the substance meets no GHS hazard criteria at all. Of the 348 that carry codes, 84.7 per cent give very toxic to aquatic life, 72.4 per cent the long-lasting form of it, 18.1 per cent may be corrosive to metals and 11.6 per cent may cause respiratory irritation. The environmental hazard is where the notifiers agree; the rest is a minority view.

Why Level B. Not because of the classification, which on its own would sit inside Level A of the course rubric, but because of how the course meets the substance. Every route to silver chloride here runs through silver nitrate, which the Level B criteria name explicitly, and a dry precipitate is a fine powder that must not be inhaled, which they also name. The controls are therefore the silver nitrate controls — splash goggles, gloves, apron, eyewash within reach — plus a particulate mask if a dry precipitate is ever moved. It stops short of Level C because no source the course holds names a fume cupboard as the recognised control and the waste is collectable for recovery.

⚠Safety

Two limits are worth stating precisely because they do not apply. The NIOSH Pocket Guide sets 0.01 mg/m³ as Ag for silver metal dust and soluble compounds, and EH40 Table 1 gives the same figure for silver as soluble compounds against 0.1 mg/m³ for the metal. Silver chloride is not a soluble compound, so neither number is written for it and the course quotes no limit of its own. EH40 states that absence from its list does not mean a substance is without hazard: control the dust because it is a dust carrying silver, not because a published figure covers it.

Waste

Silver chloride is silver, and the same fact makes it a pollutant and a resource. Everything carrying it is collected: spent sensitiser, the first two changes of wash water from a salt print, filter papers, spoiled sheets. Reilly’s workshop recovery method is the precipitation reaction used deliberately — add sodium chloride or sodium carbonate to saved wash water so the silver comes down as an insoluble salt, decant, and send the solid to a refiner — with the warning that smelting and re-nitrating are not home operations.

The coated sheet is a different question from the bottle. Kodak’s J-214 reports that representative samples of processed and unprocessed Kodak films and papers did not leach silver at 5 ppm or above under the Toxicity Characteristic Leaching Procedure, while noting that state and local agencies may be stricter. ILFORD tells United Kingdom domestic users to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern, and they differ.

History

Silver chloride occurs in the ground: Ware records it as the mineral chlorargyrite, the luna cornua or horn silver known to sixteenth-century alchemists, who were aware that it darkened after excavation. Everything in Part I of this course happened to this compound — Schulze’s stencilled letters in 1727, Scheele’s proof that the black product is metallic silver, Talbot’s photogenic drawing paper and his first fixing baths of strong salt or potassium iodide. Ware notes what went wrong with chloride fixation: in daylight the palest tones acquired a purplish-grey fog of photolytic silver that could eventually obliterate the image. That is why Herschel’s thiosulfate replaced it.

The compound’s commercial life outlasted the salt print. Printing-out paper — Kodak’s Solio, a chloride emulsion deliberately made with an excess of silver nitrate — was how ordinary photographs were printed into the twentieth century, and the slow chloride contact papers that replaced it are why a generation of family albums was printed by a bare bulb on a kitchen table.