Silver sulfamate

Almost every other silver compound in this encyclopaedia is either something you can buy in a jar or something a print makes for itself. This one is neither: it has no supplier, no CAS number and no entry in any regulatory inventory the course can find, and it is the whole reason one iron-silver process behaves differently from all the others. Mike Ware chose it in 1990 not for anything it does, but for what its anion refuses to do.

In photography

One process uses it, and the process exists because of it. Ware sets the problem out in a single paragraph of his own account of the argyrotype, and it is worth following in order because it is a piece of design reasoning rather than a recipe.

Every iron-silver process before 1990 — argentotype, Van Dyke, kallitype, brownprint — used silver nitrate, because it is the soluble silver salt everybody has. But nitrate is an oxidising anion, and it tends to dissolve the colloidal image silver during wet processing, particularly under acid conditions. The kallitype’s answer to that is alkaline-buffered developers of high pH, borax among them; and those create a second problem, because a high pH hydrolyses the excess iron(III) in the sensitiser and deposits insoluble ferric hydroxide in the image, which is what ultimately makes a kallitype fade.

So the cure is to change the salt rather than the process. Ware’s sentence is “replace silver nitrate with a soluble salt of silver having a non-oxidising anion”. Few such salts are known; most — silver fluoride is his example — have properties or a toxicity that “debar them from ‘home chemistry’”. Silver sulphamate is the one that fits, and his description of it is the one this page would keep if it could keep only one: little-known and relatively innocuous.

Ag₂O + 2 NH₂SO₃H → 2 NH₂SO₃Ag + H₂O

Made where it is used, and the counter-ion leaves as water

Why the oxide and not a salt. Read the right-hand side for what is absent. Start from silver nitrate and every nitrate ion you brought in is still in the bottle at the end. Start from silver carbonate — which Ware allows as a substitute, at 8.4 g, dissolved cold in a tall vessel because it foams — and the carbonate leaves as carbon dioxide. Start from the oxide and the counter-ion is O²⁻, which takes two protons from the acid and becomes water. The finished argyrotype sensitiser therefore contains silver, sulfamate, citrate, ammonium and iron, and no oxidising anion at all.

What the bottle holds, and how it compares

Ware’s sensitiser is 7 g of silver(I) oxide, 7 g of sulfamic acid, 22 g of green ammonium iron(III) citrate and 1 cc of glycerol, made to 100 cc, with a wetting agent whose placement he changed between accounts — the formulary page sets that out. The silver arithmetic falls out of the first figure alone.

2 × (7 ÷ 231.74) = 0.0604 mol Ag in 100 cc = 0.604 mol/L; 0.604 × 203.96 = 123 g/L = 12.3 g per 100 cc

Silver, and silver sulfamate, in 100 cc of Ware's sensitiser

Ag₂O has a relative molecular mass of 231.74 and gives two silver ions; each ends up as one formula unit of silver sulfamate at 203.96. So the bottle is 0.604 mol/L in silver, about 12.3 per cent w/v as the sulfamate, and it is a single bottle that keeps for a year or more.

Set that beside the two neighbours and the point lands.

Sensitiser	Silver salt	Silver in the coating solution	Bottles	Keeping time
Van Dyke	Silver nitrate	3.8 g in the roughly 100 mL of mixed sensitiser = 0.224 mol/L	One, with citrate and tartaric acid holding it	“Months”; a supplier’s premixed version, at least a year
Kallitype	Silver nitrate	Two solutions, mixed at the bench	Two, because one bottle throws silver oxalate	The mixed sensitiser has no published keeping time at all
Argyrotype	Silver sulfamate	0.604 mol/L	One	A year at least; the workshop handout says several years

The argyrotype carries 2.7 times the silver concentration of a Van Dyke, in one bottle, and keeps longer. That is what a non-oxidising, non-complexing anion buys.

⚗The pH is chosen, not accepted

The sensitiser is made with a deliberate excess of acid. Ware states that 5.87 g of sulfamic acid is the stoicheiometric quantity for 7 g of silver oxide and specifies 7 g — an excess of 1.13 g, which is 19.3 per cent, his “about 20 per cent” — and that this brings the solution to about pH 3.5. He then gives three reasons in one sentence, and they are three different pieces of chemistry:

1. It suppresses hydrolysis of the iron(III), so no ferric hydroxide forms in the image. This is the failure the kallitype’s alkaline developer causes.
2. It keeps the silver in solution, “otherwise silver citrate may precipitate out”. The sulfamate is not the only anion in the bottle, and citrate would take the silver if the pH let it.
3. This pH is also the optimum for the photosensitivity. The siderotype lesson gives the photochemical reason from Ware’s Cyanomicon — in the iron citrate system the monomeric complex below pH 1.5 is photo-inactive and the dimer above pH 2 is the photoactive species.

And the acid is the contrast control. An extra 1 g of sulfamic acid per 100 cc gives a more contrasty sensitiser, because the added acid tends to dissolve silver in the highlights; mixing the two formulations tunes the contrast between them. Ware allows the figure to be dropped to about 6 g for less contrast and warns that it may then precipitate.

What this salt does not do is take the picture. The photochemistry is the iron’s, exactly as in a cyanotype or a platinum print: light reduces iron(III) to iron(II), and the iron(II) then reduces silver(I) to colloidal metallic silver. The sulfamate’s job is to be present, soluble and inert — to deliver silver ions to the paper and then take no further part. Compare that with a silver halide, where the anion is the photochemistry. It is unusual, in the history of this course, to find a salt chosen entirely for what its anion will not do.

Through the wet processing the same inertness shows up as a simplification. The print washes for five minutes in water — a static bath with about 2.5 g/L of citric acid in the workshop version — which carries out the unexposed iron and silver, and then fixes for three minutes in 2 to 2.5 per cent sodium thiosulfate, which removes traces of insoluble silver salts and intensifies the image, the colour shifting from red to brown. There is no alkaline developer anywhere in it, and therefore no hydrolysed iron left behind in the paper.

Properties

NH₂SO₃Ag, as Ware writes it; PubChem computes the molecular formula as AgH₂NO₃S and the molecular weight as 203.96. One silver(I) ion, one sulfamate ion.

The anion is the conjugate base of a strong acid, and that is the property the whole design rests on. Sulfamic acid is a solid, odourless, non-volatile strong acid, which is why the argyrotype can be made from weighed powders on a kitchen balance rather than from a fuming liquid. Its anion carries a sulfur–nitrogen and three sulfur–oxygen bonds and, as Ware states directly, it is not an oxidiser — there is nothing in it waiting to take electrons back from silver metal, as a nitrate ion will do to a 20 nm particle of colloidal silver in an acid bath.

Its regulatory footprint is essentially nil, and that is a fact about the compound rather than a gap in the course’s reading. PubChem’s record carries two synonyms — its own name and a patent-database identifier — and no CAS number that any depositor has supplied. ECHA CHEM returns nothing for the name in either spelling. There is no EC Inventory entry and no CLP notification, so there is no aggregated classification of the kind the other silver salts on these pages carry. Do not read that as reassurance. It means only that nobody sells it, which is exactly what Ware said when he published it.

What the course could not find, and states rather than guesses: no melting point, no decomposition temperature, no density, no crystal description, no solubility limit at a stated temperature, no ecotoxicological datum, and no published account of anyone isolating the solid for photographic use. Everything on this page about its behaviour is behaviour in solution, in the one formulation that uses it.

Handling

There is no classification to quote and the page says so plainly. The searches are listed in the hazards note above. What governs the bench instead is a stack of three things, each of which does have a source.

The silver. HSE’s EH40 gives silver as soluble compounds, as Ag, a long-term limit of 0.01 mg/m³ against 0.1 mg/m³ for the metal, and the NIOSH Pocket Guide gives the same 0.01 mg/m³, an IDLH of 10 mg/m³, and symptoms of blue-grey eyes, nasal septum, throat and skin — argyria, which is permanent. Neither names this salt. Since it is made and used as a solution the airborne route is not the live one; the live one is the splash, and the stain, which is metallic silver bound to skin protein.

The acid, and the oxide, at the preparation step. Sulfamic acid’s harmonised European entry classifies it for skin irritation, serious eye irritation and long-lasting aquatic harm; silver(I) oxide is a classified oxidiser at category 1 on most notifications, with serious eye damage the most-agreed statement on its record. Those two are on the bench together, in hot water, for up to an hour.

The originator’s own warning, which is the most specific statement anyone has published about the finished solution: it “is irritant and toxic, and will stain skin and fabrics”, and spillages are washed away with plenty of cold water.

Why Level B. The course rubric puts a page at Level B for handling concentrated sensitisers and fine powders that must not be inhaled, for holding a solution above 50 °C, and for a failure mode that is a splash or a burn rather than a spoiled print. All three apply to the preparation: 70 °C for as much as an hour, a weighed oxidising powder, a strong acid, and 0.6 mol/L of silver in the beaker at the end of it. It stops short of Level C because no source read for this page names a fume cupboard or a licensed disposal route as the recognised control — the controls are goggles, gloves, an apron, dust discipline, an open window and silver recovery.

⚠Safety

Goggles before the acid jar is opened, and a dedicated spatula. Add the silver oxide to the hot acid in small portions with the stirrer going, never the acid to a heap of the oxide, and do not hurry the hour it may take. Nothing metal touches any of it. Weigh over a tray. And treat the finished olive-green bottle as a silver nitrate bottle in every respect except the one that matters chemically: it will stain everything it reaches, it goes in a brown bottle, and it is labelled with the date and the strength.

Waste

Silver-bearing, and unusually recoverable, because the argyrotype’s own washing carries most of the unexposed sensitiser off the sheet within five minutes. That first wash, the clearing bath and the thiosulfate fixer all go into the labelled silver container, and none of them goes to the drain. Kodak’s J-52 lists silver among the effluent parameters municipalities most often regulate, at a mean limit of 1.2 mg/L, and J-214 sets the United States federal line at 5 ppm of silver in a liquid waste, EPA number D011. ILFORD tells domestic users to bottle wastes separately, label them and take them to a household waste and recycling centre.

The honest limit of this section is the anion. No ecotoxicological data for sulfamate was found in any document read for this page, and none is invented; the silver is what the published limits describe and the silver is what is recovered. Check your local regulations; they govern.

History

Herschel made the first iron-silver process, the argentotype, in 1842, and every descendant of it for a century and a half used silver nitrate. Ware’s account says he “re-visited the underlying chemistry in 1990”, and the result was published in the British photographic press in June 1991; his own workshop handout dates the version to 1991. The name argyrotype is his.

What makes the episode worth a page is the kind of move it was. Photographic chemistry is full of people improving a formula by changing a concentration, a temperature or a development time. This is one of very few cases where somebody identified the culprit as an ion that everybody had stopped seeing — the nitrate that comes along free with the only silver salt anyone stocks — and then went looking through the soluble silver salts for one whose anion had no redox chemistry and no toxicity worth the name. The salt he arrived at is so obscure that thirty-five years later it still has no CAS number and no supplier, and it is made in the beaker every time anybody wants it.