Silver

Every photograph in this course is a picture made of this metal. Not of a compound of it: the halides, the nitrate and the complexes exist to deliver silver atoms to a place and leave them there. What arrives is element 47, and what it looks like depends on nothing but how finely it was divided.

In photography

The image is metallic silver in one of two shapes. Ware’s account is the clearest: silver made directly by light — photolytic silver, the substance of a printing-out paper — has a particle size in the 10 to 100 nanometre region, while silver made by a developing agent is a micron-sized bundle of filaments. Reilly says the same from the print side, calling the print-out aggregates colloidal and much smaller than the filaments a developer generates. The Getty Conservation Institute’s atlas records the consequence as an identification feature: photogenically formed particles are much smaller than chemically developed ones, and their colour follows their size.

⚗Why size is colour

A particle smaller than the wavelength of light does not simply absorb everything and look black. Ware gives the physics: light excites collective oscillations of the conduction electrons, called plasmons, and for copper, silver and gold — and, on Creighton and Eadon’s calculations, almost no other metal — those resonances fall in the visible. The absorption band moves with the particle, and Ware tabulates Wiegel’s calculated and confirmed colours for silver hydrosols: 10 to 20 nm transmits yellow, 25 to 35 nm red, 35 to 45 nm purplish-red, 50 to 60 nm violet, 70 to 80 nm dark blue, 120 to 130 nm grey-green.

Three other things move the same band. Departure from a sphere splits it. Aggregation shifts a yellow 10 to 20 nm sol to red-brown. And the refractive index of the surroundings matters: Ware attributes the lilac highlight of a Photogenic Drawing to 10 nm particles inside silver chloride, whose refractive index of 2.071 drags an absorption normally near 390 to 400 nm out to 550 nm, where the eye is most sensitive. Dissolve that chloride away in hypo and the colour collapses to pale yellow, which is why Ware says re-fixing an original cannot be recommended.

Reilly draws the practical conclusion. The binder changes the colour, because albumen, gelatin and starch have different refractive indices; fixing changes it again, because dissolving the unreduced chloride alters the index of the whole system and packs the particles closer; drying changes it a third time, colder and darker. A print-out image that looks purple in the frame and red in the fixer has not gone wrong — it has changed its optical environment twice.

Fineness is also fragility. Ware puts it plainly: nanoparticle silver has, for its mass, a very high surface area exposed to attack, so a print-out image is far more vulnerable than a developed one and is rapidly dissolved by reagents that etch or bleach silver. That sentence also explains toning — convert the surface to silver sulfide or silver selenide and there is nothing left to attack.

How much is actually in the picture is small, and the sources disagree. Ware cites analyses of contemporary salt prints made in 1855 which found only about 2 per cent of the silver taken up by the sensitised paper remaining in the final image. Reilly, on the same class of material, gives 6 to 8 per cent, with most of the rest in the first two changes of wash water. The course quotes both and reconciles neither — they are different measurements of different papers a century apart — and notes that they agree on the direction. That is the whole argument for recovery. Where the silver comes from is silver nitrate, the only soluble silver salt this course uses.

Properties

Wall’s 1912 dictionary is still the most useful single paragraph on the metal. It is the best conductor of heat and electricity of all the metals, extremely malleable and ductile — 400 feet of silver wire weighing one grain. It melts at about 1,832 °F. Molten caustic alkalis and alkaline nitrates have no effect on it, and it is unaffected by air.

Two exceptions follow immediately, and both matter here. It is oxidised by ozone. And sulphurous vapours act upon it at once, forming sulphides. That second sentence is the entire chemistry of tarnish, and it is not a different reaction from toning — it is sepia toning, run by accident, on a spoon.

Nobility, in practice, is the solubility list on this page. Water does nothing, hot or cold; alkali does nothing. What dissolves silver is an oxidising acid — nitric, hot sulfuric — or a ligand strong enough to hold Ag⁺ once something has oxidised it, which is what cyanide does in the presence of oxygen. That is why a silver image survives a century in a drawer, and also why the historical cyanide fixers ate the image while they cleared the halide.

Handling

The classification of this element is itself organised by particle size — the argument of this page arriving from an unexpected direction. Regulation (EC) No 1272/2008 carries three records: massive at 1 mm and above, powder between 100 nm and 1 mm, nano below that. All three carry Warning, suspected damage to fertility and possible organ damage on prolonged exposure; only the two finer ones are also very toxic to aquatic life.

The airborne limits disagree by a factor of ten and the course does not hide it. HSE’s EH40 Table 1 lists silver, metallic, at 0.1 mg/m³ and silver as soluble compounds at 0.01 mg/m³. The NIOSH Pocket Guide’s entry npgd0557 covers “silver (metal dust and soluble compounds, as Ag)” in one line at 0.01 mg/m³ for both the recommended and the OSHA permissible limit, with an IDLH of 10 mg/m³. HSE distinguishes the metal from its soluble salts; NIOSH does not. Where dust could be raised the course works to the lower figure, because the cost of being wrong is argyria, which Reilly records as permanent.

⚠Why Level A, and the one operation that is not

Against the rubric, the metal as this course handles it is Level A: a substance classified Warning rather than corrosive or acutely toxic, in a form — an image in gelatin, or wet sludge in a jar — that raises no dust and needs no extraction. Gloves and eye protection are the controls, and they are the controls for the rest of the tray sequence anyway.

One operation takes it straight out of Level A: dissolving it in nitric acid. Re-nitrating reclaimed silver is how the sensitiser is made, it evolves nitrogen oxides, and Reilly warns in terms that smelting and re-nitrating are not home operations. The course studies that route and gives no procedure for it. Sending sludge to a refiner is the whole of the domestic answer.

Waste

The waste stream here is the valuable one. A print holds a few per cent of the silver that was coated; the rest is in the fixer and the first wash, dissolved and mobile. Kodak’s J-52 gives 1.2 mg/L as the mean of the limits municipalities set for silver in effluent, and its J-214 sets the United States federal threshold at 5 ppm in a liquid waste, EPA number D011, while reporting that films and papers themselves did not leach silver at that level. The Environment Agency’s WM3 sorts British photographic waste the same way: silver-bearing liquids from on-site treatment and reclamation are hazardous entries, film and paper containing silver are not. So collect the fixer and the first wash, labelled, and take them where your authority says they go — ILFORD tells United Kingdom domestic users a household waste and recycling centre. Check your local regulations.

History

Wall’s entry lists three ways the metal was won from its ores — cupelling a lead alloy, amalgamating with mercury and distilling it off, or dissolving it and precipitating it with copper — and a fourth for the laboratory, reducing the chloride with zinc and hydrochloric acid to a spongy metal fused under sodium carbonate to keep air away. Photography arrived in the middle of that industry and took a startling share of it: Kodak’s 1928 primer states that the company was using about one eleventh of all the silver mined in the United States, second only to the Mint.