Argentotype

The argentotype is the member of Herschel’s 1842 family in which the iron(II) that light makes reduces silver. It matters less as a process anyone practises than as the point the whole iron-silver branch of the alternative-process world descends from: Van Dyke Brown, the kallitype and Ware’s argyrotype are all its children. The Part I lesson establishes the family; this entry exists in part because that page already names argentotype in its processes: list, and a reader following the reference landed on nothing until it did.

The chemistry

Siderotype is Ware’s umbrella term, from the Greek for iron, and it earns its place by saying where the light acts. Ultraviolet reduces iron(III) to iron(II) in an iron(III) salt of an organic acid; the iron(II) then makes the image. In a cyanotype it makes Prussian blue; here it reduces silver(I) to metallic silver.

The consequence a printer has to keep hold of is a negative one. The silver in an iron-silver print is not light-sensitive at all. Forgetting that leads to the wrong diagnosis every time, because a fault that looks like a speed problem is an iron problem and a fault that looks like a fixing problem is a clearing one.

Ware’s redox table sets the boundary of what the citrate sensitiser can do: the iron(III)/iron(II) citrate couple sits at +0.372 V, which reduces gold at +1.00 V and silver at +0.80 V but not platinum at +0.73 V or palladium at +0.62 V under printing conditions. Herschel could therefore print in gold, silver, mercury and Prussian blue with the salt he had, and platinum printing had to wait for the more energetic ferric oxalate. Those numbers do not on their own explain the outcome — palladium sits below the citrate couple’s own value and is still not reduced — and the citrate page records that the course reports the observed division rather than offering the potentials as a complete account of it.

Historical workflow

Herschel produced four siderotypes in one summer and published them together in the paper accepted on 15 June 1842. Ware’s table of them names the image substance in each case: cyanotype, Prussian blue; argentotype, silver; chrysotype, gold; kelainotype, mercury — the last of which defeated him.

The argentotype itself did not become a trade. What became a trade was its descendants, and the line is short enough to give whole. The brownprint or Van Dyke Brown is the same idea with tartaric acid added and the sensitiser sold as three solutions; the kallitype is the same idea developed out rather than printed out; and Ware’s argyrotype of 1991 is a deliberate redesign of the chemistry, described below.

The image

Warm browns, and the colour is a particle-size effect rather than an added substance. Ware’s argyrotype notes state it directly: the colour of a silver print is determined by the size of the metal particles, which is affected by the humidity of the coating during the exposure. That is the silver page’s argument arriving from the weather, and it is why the modern practice controls humidity with a saturated-salt enclosure rather than leaving it to the room.

The surface is the family’s: metal among the fibres, no binder, matte, the paper visible in the picture.

Permanence

The iron-silver processes have a reputation for impermanence, and the course’s position — the same one it takes about the salted paper print — is that the reputation belongs mostly to short clearing and washing rather than to the chemistry. Every one of the failures happens after the exposure.

Ware’s argyrotype is the strongest evidence for that reading, because it is a redesign aimed at the two mechanisms he identifies.

⚗Two design faults, and what replacing them fixes

The anion. Ware replaces silver nitrate with silver sulfamate, made in situ, on the grounds that nitrate is an oxidising anion which tends to dissolve the colloidal image silver during wet processing, especially under acid conditions. An image substance that its own processing bath is attacking is not a permanence problem you can wash your way out of.

The developer’s pH. The kallitype’s usual answer to contrast is an alkaline developer, and Ware’s objection is that alkali causes the excess iron(III) to hydrolyse and deposit ferric hydroxide in the image. A non-oxidising anion lets the whole process run at pH 3.5, in a single-bottle sensitiser with a long shelf life, whose contrast is controlled by added acid instead.

Sulfamic acid’s page carries the reasoning and the ranking of clearing acids that goes with it.

The residual-iron problem is the family’s other permanence question and it is a clearing problem rather than a fixing one. Residual iron stains the paper yellow and goes on reacting; the troubleshooting atlas carries the fault and its confirming test.

Hazards

Silver nitrate, which the rubric names explicitly under Level B, and which stains everything it touches; ammonium iron(III) citrate, a Level A irritant; and, in the modern version, sulfamic acid, a strong acid you can weigh — odourless, non-volatile and Level A, which is exactly why Ware ranks it above hydrochloric and nitric where a safety objection exists.

The level for the process is set by the silver nitrate. The wash and the dilute thiosulfate bath are silver-bearing and belong to recovery.

Where the course teaches it

Not directly, and its descendants carry it. No lesson in the curriculum coats an argentotype sensitiser. Part XXIV, Iron-Silver Processes: Van Dyke Brown and Kallitype, teaches the siderotype principle and two of its children at Level B, and the argyrotype follows the Van Dyke instructions closely enough that the names are used loosely in the field.

This entry is therefore the ancestor page rather than a procedure page, and its job is to hold the one idea the whole branch runs on: light acts on iron, and the metal that ends up in the paper is whichever one the iron(II) was able to reduce. Read it beside the chrysotype, which is the same sentence with gold, and the platinotype, which is the same sentence with a metal the citrate could not reach.