Chrysotype

The chrysotype is the siderotype whose ending is gold. Herschel named it in imitation of Talbot’s calotype and said so in a letter, calling it “one of the most striking and magical effects which has yet turned up in photography” — and it was a commercial failure for a century and a half. The Part I lesson establishes the 1842 work; chloroauric acid owns the substance, and the distinction that matters most on this page.

!A chrysotype is not a gold-toned print

Ware is emphatic about it and the difference is not pedantic. A toner modifies an image that already exists; a chrysotype sensitiser makes the image out of gold in the first place. A gold-toned silver gelatin print and a gold-toned salt print both start as silver. This does not. There is no silver anywhere in a finished chrysotype.

The chemistry

The first half is the siderotype’s own and is shared with the cyanotype: ultraviolet light acting on an iron(III) salt of an organic acid transfers an electron from the acid’s anion to the metal, giving iron(II) and carrying the acid away as carbon dioxide. The second half is where the metal is chosen. Iron(II) is a reducing agent strong enough to throw a noble metal out of solution, and Ware’s table of couples gives gold the most positive value in it: E(AuCl₄⁻/Au, 4Cl⁻) = +1.00 V, against +0.80 V for silver, +0.73 V for platinum and +0.62 V for palladium.

That is why Herschel could print in gold with the citrate sensitiser where platinum was out of reach — ammonium iron(III) citrate records the argument, and the number that goes with it, the citrate couple at +0.372 V against the oxalate’s +0.02 V.

And it is why the process was uncontrollable for a hundred and fifty years. Ware’s account is that the untamed reaction is too vigorous to control, and his solution is not a new gold salt but a ligand: 3,3′-thiodipropanoic acid, which binds the gold and moderates its reactivity. The modern sensitiser is three stocks — the ligand, the gold as either the sodium salt or chloroauric acid, and ammonium iron(III) oxalate as the light-sensitive component. Ware spent six years and, by his own account, many thousands of experiments applying twentieth-century coordination chemistry to the problem, and published a working process in 1987.

⚗Why the colours are a particle-size effect and not a pigment

Colloidal gold is not one colour. Ware’s account of the chrysotype’s range — pink, magenta, purple, violet, blue, green — attributes it to the size of the metal particles, and it is the same particle-size argument that governs silver: what a process makes finely divided looks warm, and what it makes coarse looks cold.

Two levers reach that size, and both are process variables rather than formula variables. The first bath is one: Ware lists tartaric acid at 1 to 2 per cent as giving redder tones but still fairly subdued, citric acid as producing rose-pink hues crossing into blue highlights, and oxalic acid as giving the most intense red-blue colour splits and the longest tonal range — with a warning that applies to all three, that because they are acids, overlong treatment tends to block up the shadows by coagulating the gold. Humidity is the other, and it is why the practice carries a table of saturated salt solutions for constant-humidity enclosures at 20 °C, with lithium chloride at the dry end at 15 per cent RH.

Historical workflow

Herschel’s chrysotype belongs to a fortnight of 1842 in which he produced four processes from one idea. Ware’s table of the siderotypes is the family tree of half the alternative-process world: the cyanotype, whose image substance is Prussian blue; the argentotype, silver; the chrysotype, gold; and the kelainotype, mercury, which defeated him. His paper “On the Action of the Rays of the Solar Spectrum on Vegetable Colours, and on some new Photographic Processes” was accepted on 15 June 1842 and partly read to the Royal Society on 16 June, with 43 specimen sun-prints attached.

The one he thought least of is the one that changed drawing offices worldwide, and the one he called magical went nowhere.

The modern workflow is a coated sheet, a contact exposure under ultraviolet, and wet processing that begins with a clearing bath rather than a developer, because the modern sensitiser prints out. Ware’s instruction for that first bath carries a trap worth repeating: 1 per cent w/v disodium EDTA, and do not use tetrasodium EDTA, which is alkaline and will cause iron stains. The disodium EDTA page works through why four substances sold as “EDTA” are not interchangeable and why the difference between two of them is six units of pH.

The image

Colour is the reason to make one, and it is a range rather than a hue. Nothing else in this atlas offers pink through green from one metal, and the colour is chosen at the bench by the first bath and the humidity rather than by a toner afterwards.

Surface is the siderotype family’s: metal among the fibres with no binder over it, so the paper is part of the look and the sheet is matte.

Scale follows the first bath, oxalic giving Ware the longest. This atlas does not print a figure for the exposure scale, because the course has not read one.

Permanence

Elemental gold is the most inert image substance in this atlas. The argument the chloroauric acid page makes for gold toning — that gold coats the silver particles and protects them from the sulfur that would otherwise convert them — applies here a fortiori, because there is no silver to protect. Complete conversion of image silver to silver sulfide is what gives so many early photographs their putty-coloured, faded look, and a chrysotype has nothing that reaction can act on.

What remains is the failure the whole siderotype family shares: residual iron. Iron left in the paper after inadequate clearing stains it yellow and goes on doing chemistry there, which is why the process is cleared rather than fixed and why the clearing bath’s composition is worth as much attention as the sensitiser’s. Residual iron is the diagnostic term, and the troubleshooting atlas carries the fault.

Hazards

Gold(III) as chloroauric acid is deliquescent and corrosive, and its own encyclopaedia entry classifies it Level B; the same page records that nineteenth-century photographers made their own by dissolving gold coins in mixed nitric and hydrochloric acids and evaporating, and Reilly’s comment that because of the fumes and the concentrated acids this is not a procedure for a home laboratory. The course agrees, and no route to the salt is given here.

Ammonium iron(III) oxalate is a Level B material — soluble oxalates are systemic toxins — and the ligand is a thio-acid whose own classification the course has not read. The waste carries iron, oxalate and gold; gold is worth recovering and oxalate does not belong in a domestic drain.

The cost is a control in its own right. Gold is the most expensive consumable in this atlas, and a process where every drop is counted is a process that gets coated carefully.

Where the course teaches it

Nowhere as a lab, and the reason is scope rather than hazard. The register assigns this row to Part XXI, and it is here at all because Herschel’s page already names chrysotype in its processes: list — a reader following that reference landed on nothing until this page existed. Part XXI’s written plan covers the cyanotype and its variants; no lesson in the current curriculum coats a gold sensitiser.

What the course does hold, and what this entry is built from, is the chemistry: the gold and iron encyclopaedia entries, the redox table that explains which metals the citrate can and cannot reduce, and the siderotype principle that Part XXIV and Part XXV are organised around. If a chrysotype lab is ever written it belongs beside the palladiotype, on the same iron photochemistry and at the same level.