Selenium

Every selenium toner in a darkroom cupboard is a dilute solution of a selenium compound, sold ready-made, and that is not an accident of the market. Selenium metal is insoluble in water. To make a toner from the element you have to attack it with something that will dissolve it, and the things that will dissolve it are sodium sulfide, molten caustic soda and cyanide. This page exists to explain that chemistry and to say plainly why the course stops there.

In photography

What selenium does to a print is not in dispute. Kodak’s toner sheet states that its Rapid Selenium Toner converts the silver image to silver selenide, and the Getty Conservation Institute’s atlas of photographic processes records the two distinct reasons the trade used it. It was first used to give silver gelatin photographs a dark brown-orange tone. Later, selenium was recognised for its ability to increase the chemical and environmental stability of silver particles, and toning became a step in the archival processing that the art and museum market adopted in the second half of the twentieth century. Done for permanence rather than colour, the atlas notes, it causes no major colour shift — only a slight increase in image contrast and brilliance — and the treatment can still be detected by X-ray fluorescence a lifetime later.

The element is not what any of that uses. Read the sheets and the toners turn out to be solutions of selenium compounds, at low concentration, in a sulfite-bearing bath. Kodak states that the active ingredient in Rapid Selenium Toner is a sulfite salt at less than 2 per cent, and that working solutions generally contain less than half a per cent of what it calls selenium sulfite — bare percentages, with no basis stated, which is how the sheet prints them. Moersch’s safety data sheet for its MT1 toner is more specific and says which basis it means: 2 to 3 per cent w/w sodium selenate, with 10 per cent w/w ammonium sulfite and 20 per cent w/w ammonium thiosulfate. Harman’s sheet gives dilutions of 1+3 for a tone change and 1+20 where the purpose is protection of the image. By the time the bath reaches the print, selenium is present in fractions of a per cent.

The historical route started from the metal, and started from a problem. The solubility data explains everything about the old formulas: selenium does not dissolve in water, but it does dissolve in a sulfite solution, in dilute caustic alkali and in cyanide. Wall’s 1924 handbook prints half a dozen selenium toners and every one of them solves that problem the same way — powdered selenium heated with a strong sodium sulfide solution until it dissolves, or, in the version Wall credits to Namias, granular selenium fused with caustic soda in a nickel or iron crucible and the cold melt dissolved in water. The prints were bleached first and then toned, and the results ran from sepia to purplish-red. Wall records that the weaker the bath, the more sepia the colour; that diluting the bath throws some selenium back down, so it has to be filtered; that neutralising with boric acid prevents that precipitation; and that ammonia reduces the staining of the whites.

☠This is history, not a method

The course describes the preparation above and does not give it. Fusing an element classified toxic if swallowed and toxic if inhaled with molten caustic soda, in a metal crucible, to make a bath of sodium selenide in sodium sulfide, is not a home procedure at any level of care. It combines a hot strong alkali, a substance that liberates hydrogen sulfide on contact with acid, and a selenium compound, and it does so in the same room as photographic paper.

Wall’s own closing sentence on the family is the one to remember: all selenium salts have an intense garlic odour, which is very lasting. NIOSH’s list of symptoms of selenium exposure includes garlic breath. The smell of the historical bath and the sign of having absorbed it are the same smell, which means the bath removes its own warning.

What replaced it, and why. Nothing about the image chemistry changed — a print toned in a modern bath still ends as silver selenide. What changed is that the manufacturer does the dangerous part. A liquid concentrate at a few per cent, diluted twenty-fold, poured into a dish at room temperature, is a different exposure from a fused melt, and it removes the entire preparation stage from the darkroom. That is the whole argument for the modern product, and it is a good one.

Properties

An amorphous or crystalline solid, red to grey; CAMEO describes a reddish powder that may go black on exposure to air, and the ILO-WHO safety card simply calls it a grey solid in various forms. NIOSH gives a specific gravity of 4.28, a melting point of 392 °F (200 °C), a boiling point of 1,265 °F (685 °C), a vapour pressure of approximately 0 mmHg, and classes it a combustible solid. It occurs, NIOSH notes, as an impurity in most sulfide ores — which is where the photographic supply came from.

The solubility is the property that governs the chemistry on this page. Water does nothing: HSDB, NIOSH, CAMEO and the safety card all record it as insoluble, and the card puts it as bluntly as possible — solubility in water, none. What does dissolve it is a short and revealing list: aqueous potassium cyanide, potassium sulfite solution, dilute aqueous caustic alkali, concentrated nitric acid, and carbon disulfide at 2 mg per 100 mL. Three of those five are the reagents the historical toners used. The fourth, nitric acid, reacts vigorously with freshly reduced selenium according to CAMEO’s reactivity profile, and the fifth is a solvent no darkroom should hold.

Its reactivity is worth reading once. CAMEO records that selenium ignites in fluorine at ordinary temperatures, reacts violently with chromium trioxide, and reacts explosively with silver bromate and potassium bromate. None of that arises in photography; all of it explains why the element is stored away from oxidisers.

Handling

The aggregated ECHA notifications classify elemental selenium Danger, with the acute-toxicity and health-hazard pictograms: toxic if swallowed in every one of 1,063 reports, toxic if inhaled in 99.8 per cent, and capable of damaging organs through prolonged or repeated exposure in 99.7 per cent. NIOSH’s symptom list is the clinical version of that — irritation of eyes, skin, nose and throat; visual disturbance; headache; chills and fever; breathing difficulty and bronchitis; metallic taste, garlic breath and gastrointestinal disturbance; dermatitis; eye and skin burns — with the liver, kidneys, blood and spleen among the target organs.

The exposure limits are among the lowest a darkroom substance carries. NIOSH’s recommended limit and the OSHA permissible limit are both 0.2 mg/m³ as an eight-hour average, and both are stated to apply to selenium compounds as well, with the single exception of selenium hexafluoride. The IDLH — the concentration considered immediately dangerous to life or health — is 1 mg/m³. HSE’s EH40 is stricter still at 0.1 mg/m³ for selenium and compounds except hydrogen selenide, and lists hydrogen selenide itself separately at 0.02 ppm, one of the lowest limits in the table.

⚠Why Level D, and what that means here

Two criteria of the rubric put elemental selenium in the historical-study class. The first is that a safer modern route exists and is taught instead: a bought, dilute liquid toner does the same job to the print and never asks the user to handle the element. The second is the character of the original method — a fusion with caustic soda, or a boil with sodium sulfide, of a substance classified toxic by two routes, carried out beside photographic materials.

A Level D page describes chemistry, workflow in outline, image characteristics and hazards, and states explicitly that it is not a procedure. This one does. There is no protective equipment list that makes the historical preparation acceptable at home, and none is offered.

Handling the toner is a different question with a different answer. Harman calls its product a toxic chemical, states that it is toxic if swallowed and may cause sensitisation by skin contact, and requires rubber gloves in all use and disposal, eye or face protection, and work in a well-ventilated area, preferably with some form of air extraction. Kodak, for its own more dilute product, recommends print tongs and clean rubber gloves and immediate washing of any splash. Follow the sheet for the product in your hand, not this page.

Waste

No elemental selenium waste arises in this course, because the element is not used. The stream that does arise is a spent toner, and it is one of the most clearly regulated in photography. Kodak’s environmental guidelines for amateur photographers carry a table of waste routes for its own products, and Rapid Selenium Toner is the only entry with every route withheld except household hazardous waste collection: no sewer discharge, no discharge to a nearby treatment works, no trash disposal. The text is explicit that used Rapid Selenium Toner is regulated as hazardous waste for commercial users under the United States Resource Conservation and Recovery Act, and that domestic users should use a household hazardous waste collection facility or a licensed hazardous waste hauler.

Harman says the same in fewer words on its own label — do not empty into drains; dispose of this material and its container at a hazardous or special waste collection point. Moersch’s safety data sheet classifies its toner’s waste under the European hazard properties for specific target organ toxicity, acute toxicity and ecotoxicity, and directs that the material and its container be disposed of as hazardous waste. Three manufacturers on two continents agree. Check your local regulations; they govern, but there is no jurisdiction in which this goes down a sink.

History

Selenium arrived in the darkroom as a toner among dozens, and Wall’s 1924 handbook shows it in that company — sandwiched between platinum and molybdenum toners, credited to Mailiani, Valenta, Namias and the Mimosa works, each with a different trick for keeping the element in solution. Nothing there suggests anyone expected it to outlast the rest.

What made it last was permanence. The Getty atlas dates the change of purpose: first the dark brown-orange tone, then the recognition that selenium increases the chemical and environmental stability of silver, then its adoption as a standard step in archival processing for the art and museum market in the second half of the twentieth century. By the time Ilford was printing an optimum-permanence sequence with a selenium bath built into it, the colour had become the side effect and the protection the point.

The element itself dropped out somewhere in between, and the reason is on this page. Once a manufacturer could sell a stable, dilute liquid, nobody needed to fuse selenium with caustic soda in a crucible — and had a very good reason not to. That is the ordinary shape of progress in photographic chemistry: not a new reaction, but the same reaction moved out of the darkroom and into a plant equipped for it.