Selenium toned silver gelatin print

Selenium occupies a strange place in the darkroom, and the reason is worth stating first: it is the only toner in common use that can be run for permanence alone, on a paper that will barely change colour, so the print looks untouched and is nonetheless a different substance. Silver selenide is what it makes, and the selenium page is where the historical preparation is recorded and refused.

The chemistry

What the toner makes is not in dispute. Kodak’s sheet states it in one sentence: its Rapid Selenium Toner converts the silver image to silver selenide.

What the toner is made of is less clear than that, and the course records the disagreement rather than flattening it.

⁇Four sheets, four different selenium species

• Kodak, on Rapid Selenium Toner: “the active ingredient is a sulfite salt that has a concentration of less than 2 percent”, and working solutions “generally contain less than ½ percent selenium sulfite”. Selenium sulfite is not a name with a CAS number behind it, and Kodak does not identify the species further. The course now holds the chemistry that name gestures at, and it is not Kodak’s: dissolving elemental selenium in a sulfite solution gives sodium selenosulfate, and Part XX traces that route to a published formula — Kodak Limited’s T-55 of 1949, whose mixing instruction is to boil selenium powder in sulphite until it dissolves. So the identification is verified for the historical bath, and reading it into a modern bottle remains unwarranted: no current sheet names a selenosulfate, and Kodak’s trade phrase names nothing at all.
• HARMAN, in the safety data sheet for HARMAN SELENIUM TONER: sodium selenite — selenium(IV) — at 1 to 5 per cent w/w, with ammonium thiosulphate and sodium sulphite each at 10 to 30 per cent.
• Moersch, in the safety data sheet for its MT1 toner: 2 to 3 per cent w/w sodium selenate, selenium(VI) rather than the selenium(IV) of a selenite, with 10 per cent ammonium sulfite and 20 per cent ammonium thiosulfate.
• Wall’s 1924 handbook, for the historical baths: sodium selenide, selenium(−II), made by dissolving the element in sodium sulfide or fusing it with caustic soda.

Four sources, four oxidation states of selenium. The finished print is silver selenide in every case; what went into the tray to make it is not one substance. Two of these are current products sold under the same shelf label, which is why the lesson insists on naming the salt rather than “the selenium toner”.

And the conversion is partial. Moersch’s own account of brown toning records that after six minutes in a selenium bath not all the silver has been transformed to silver selenide, and that the remainder can be re-halogenated and toned in thiourea afterwards. That is worth more than any general claim: a selenium-toned print is normally a mixture of silver selenide and untoned silver, in proportions the printer chose by time and dilution without necessarily knowing it. Protection is proportional to conversion, and conversion is partial.

Historical workflow

Fix, wash, tone, wash. Dilution is the control and it is not subtle: Harman gives 1+3 for a tone change and 1+20 where the purpose is protection of the image with minimal colour shift, complete in two to four minutes; Kodak gives 1+20 for print protection and 1+20 or 1+40 for more shadow contrast and maximum density with minimum tone change.

The historical route started from the element, and started from a problem. 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 solves that problem the same way — powdered selenium heated with a strong sodium sulfide solution until it dissolves, or granular selenium fused with caustic soda in a nickel or iron crucible and the cold melt dissolved in water. Kodak Limited’s own T-55 of 1949 boils six grams of selenium powder into a sulfite solution.

The course describes that preparation and does not give it, and Kodak T-55 is published in the formulary at Level D for exactly that reason. What replaced it changed nothing about the image chemistry — a print toned in a modern bath still ends as silver selenide — and everything about who does the dangerous part.

The image

Paper-dependent, which is unusual. Kodak: cool chocolate-browns on warm-tone papers, purplish browns on neutral ones, and very little or no change on cold-tone papers. The same bath on three papers gives three answers, and one of them is “nothing visible”.

Toning reaches the higher densities first, whatever the dilution or temperature. That is Moersch’s observation and it is the single most useful thing to know about selenium toning: a strong bath used briefly intensifies the shadows and leaves the highlights alone; a very dilute bath used longer will reach the highlights but risks losing shadow density first.

It intensifies. Kodak suggests shortening development slightly if a print is to be toned, which is the opposite of the sepia toner’s instruction to print darker — and a good reason to decide about toning before printing rather than after.

It leaves a signature. The Getty atlas records that the treatment can be detected by X-ray fluorescence long afterwards: the selenium is still there, and still where the image is. Compare sulfur toning, which hides inside the baryta layer’s own signal.

Permanence

The change of purpose is documented, and it is the interesting part of the history. The Getty atlas dates it: selenium was first used to give silver gelatin photographs a dark brown-orange tone, and only later recognised for its ability to increase the chemical and environmental stability of silver particles, after which it became a step in the archival processing developed for the art and museum market in the second half of the twentieth century.

!What the protection argument does and does not rest on

Kodak’s general claim is that toning converts the image to an inert compound, reducing the harmful effects of intense light, ultraviolet radiation, oxidising gases, extremes of temperature and humidity, and fumes. For silver sulfide the course can put a number behind that: a solubility product of 1.6 × 10⁻⁴⁹, forty-odd orders of magnitude below silver chloride.

For silver selenide it cannot. No source this course holds gives a solubility product, a solubility figure or a formation constant for Ag₂Se, and the reference table that supplies the rest of the encyclopaedia’s numbers has no entry for it. The mechanism of selenium’s protection is therefore stated here as the manufacturers and the conservators state it — an observed increase in chemical and environmental stability — and not as a calculation. Where a mechanism is not given by any source read, the course does not guess at one.

Set that beside the partial-conversion finding and the honest position is this: selenium toning is a well-attested protective treatment whose degree of protection depends on a conversion the printer controls by time and dilution and does not measure, and whose mechanism the course can name but not quantify. That is a weaker claim than the sepia toner’s and a much better documented one than the iron-blue toner’s, which offers no protection at all.

Hazards

Level B for the bought toner, which is what this entry describes: a liquid concentrate at a few per cent, diluted three- to twenty-fold, poured into a dish at room temperature. Nitrile gloves, eye protection, ventilation, and no weighing of a solid at any point.

Level D for the historical preparation, and the selenium page gives the argument. 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.

⚠The bath removes its own warning

Wall’s 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 bath and the sign of having absorbed it are the same smell. A person cannot use their nose to tell the two apart, which is why ventilation is an engineering control here rather than a comfort measure.

Two further points. A selenium toner contains thiosulfate — Moersch’s sheet gives 20 per cent w/w ammonium thiosulfate — so it is also a weak fixing bath, and a print that goes into it under-washed is not being rescued. And the spent bath carries selenium, silver and thiosulfate and is collected rather than discharged.

Where the course teaches it

Part XX, Toning Chemistry, in Selenium Toning: Selenosulfate, Silver Selenide and the Protection Claim and the Level B experiment that follows it.

The lesson settles the question this entry left open, and the answer is a distinction rather than a verdict. Selenosulfate is the historical route and, as far as the course’s sources reach, the only selenium toner whose composition anybody has actually published: Kodak Limited’s T-55 of 1949 is sulphite, selenium powder and ammonium chloride, boiled until the selenium dissolves, and the product of dissolving elemental selenium in sulfite is sodium selenosulfate. What the lesson forbids is reading that back into a modern bottle. HARMAN’s sheet names sodium selenite — selenium(IV) — at 1 to 5 per cent w/w; Moersch’s for MT1 names sodium selenate, selenium(VI), at 2 to 3 per cent; and Kodak’s “selenium sulfite” is a trade description that identifies no compound at all. Two different salts are sold under one shelf label for the same job, so any account that speaks of “the selenium toner” as one substance is wrong about at least one of them. This entry follows the lesson and says selenite and selenate where it means them.

The lesson also carries what this entry could only gesture at: why the shadows change first and how much of that is established, dilution as a rate control and never an endpoint control, colour by paper, the practice of toning inside the washing aid, and the rule about acid that the smell of the bath is a reminder of.

Experiment: Selenium Toning and the Print Curve is the one that puts numbers on the intensification: three papers of different image tone, each printed with the same step wedge twice on one sheet, one half toned and one half not, measured step by step — so that “increases shadow contrast and maximum density” becomes a number for the paper in your hand rather than a manufacturer’s phrase.

The protection claim is deferred by that lesson on purpose and settled in another. Toning, Permanence and What the Evidence Actually Supports sets selenium’s claim beside sulfide’s and gold’s and names the kind of statement each one is. The result is worth carrying away: not one of them is a controlled comparison of toned against untoned samples under a stability test, and Moersch — who sells a selenium toner — is on record arguing that more effective routes exist.

The substances are unchanged: the selenium, sodium selenite and silver selenide encyclopaedia entries, with Kodak T-55 at Level D.