Aurotype Photography Guide — Iron-Gold Printing Process Explained

The aurotype is an iron-gold photographic printing process — a close relative of the chrysotype — that produces prints in rich, warm tones ranging from purple through brown to black, depending on the gold chemistry, paper, and development conditions. The name derives from the Latin aurum (gold), and like the chrysotype, the aurotype forms its image from colloidal metallic gold precipitated by the reduction of a gold salt through the action of light on an iron sensitiser. The aurotype designation has been applied to several variant iron-gold processes developed in the nineteenth and twentieth centuries, all based on the fundamental reaction: light reduces ferric iron to ferrous iron, which in turn reduces a gold(III) compound to metallic gold. The resulting gold image is among the most archivally permanent photographic images possible — gold is immune to oxidation, atmospheric pollution, and the chemical degradation that affects silver, platinum, and palladium prints over time. This guide covers the history, chemistry, technique, and aesthetics of the aurotype process and its place in the family of iron-gold printing methods.

History and Relationship to Other Gold Processes

The photographic use of gold salts dates to the very beginnings of the medium. John Herschel experimented with iron-gold processes in the early 1840s, producing what he called the chrysotype. Robert Hunt, another early experimenter, produced prints using gold chloride reduced by ferrous salts. The term "aurotype" has been applied to various refinements and variants of these early processes. In the broadest sense, any photographic process in which the final image consists of metallic gold reduced from a gold salt by an iron-based photosensitiser can be called an aurotype. The modern aurotype, like the modern chrysotype, benefits from the chemical research of Dr Mike Ware, who systematically investigated the iron-gold reaction and developed reliable, controllable formulations using well-characterised gold and iron compounds. Ware's research demonstrated that by varying the gold compound, the ligands, and the development conditions, a range of gold particle sizes — and therefore a range of image colours — could be achieved.

Chemistry of the Aurotype

The aurotype sensitiser consists of two components: a photosensitive iron(III) compound (typically ammonium iron(III) oxalate) and a gold(III) compound. The gold source may be gold(III) chloride (chloroauric acid), sodium gold(III) chloride, or — in some formulations — gold(III) thiosulphate or other gold complexes. UV light reduces the ferric iron to ferrous iron. The ferrous iron then reduces the gold(III) to metallic gold, which precipitates as colloidal particles within the paper fibres. The size and aggregation of these gold particles determine the colour of the image: very fine particles produce blue or purple tones; larger or more aggregated particles produce brown or reddish tones. By controlling the concentration of the gold salt, the pH, the development conditions, and the choice of paper, the printer can influence the particle size and thus the colour.

Sensitiser Preparation and Coating

Prepare the iron sensitiser and the gold salt solution separately. The iron solution is typically ammonium iron(III) oxalate dissolved in distilled water. The gold solution may be sodium gold(III) chloride or another gold(III) compound in distilled water. Mix the solutions in the required proportions immediately before coating. Coat high-quality, acid-free fine art paper (COT 320, Arches Platine, Hahnemühle Platinum Rag, or similar) under safelight conditions using a glass rod, foam brush, or hake brush. Apply the sensitiser evenly and allow the coated paper to dry in darkness. The coated paper should be used within a few hours — the working life of the aurotype sensitiser is limited.

Exposure and Development

Contact print under UV light through a suitable negative — a digital negative printed on transparency film, or a large-format film negative. Exposure times vary depending on the UV source and the specific formulation. The aurotype may be a printing-out or developing-out process depending on the formulation. In printing-out variants, the gold image appears during exposure; in developing-out variants, development in a suitable bath is required to complete the reduction of gold. After exposure (and development if required), the print is cleared in a dilute acid bath to remove residual iron, then washed thoroughly. The resulting image is composed of metallic gold embedded in the paper fibres — extraordinarily permanent and resistant to all forms of chemical degradation.

Colour and Tone

The colour range of the aurotype is one of its most distinctive features. Depending on the formulation and conditions, the image can range from cool blue-purple (very fine colloidal gold, similar to the "purple of Cassius" known to glass-makers and alchemists for centuries) through warm purple-brown to brownish-black. These colours are inherent to the colloidal gold itself — not applied toners or dyes. They arise from the surface plasmon resonance of gold nanoparticles, a quantum-mechanical effect in which the free electrons on the surface of nano-scale gold particles interact with light. The specific wavelength of light absorbed and scattered depends on the particle size, shape, and the refractive index of the surrounding medium (the paper and gelatin). This is the same physical phenomenon that gives stained glass its ruby-red colour when gold nanoparticles are incorporated into the glass.

Archival Permanence

Gold is the most chemically stable of all metals. It does not oxidise in air, does not react with hydrogen sulphide (the pollutant that tarnishes silver prints), and is unaffected by all common atmospheric conditions. An aurotype image, properly cleared of iron residues and washed, should outlast any silver, platinum, or palladium print — potentially surviving for millennia. This extraordinary permanence was recognised by Herschel in the 1840s and was a key motivation for his experiments with gold-based printing processes. For the modern printer, the aurotype represents the ultimate in archival photographic printing — an image literally made of the most permanent and stable substance available.

Practical Considerations

Gold is expensive, and the aurotype requires relatively high concentrations of gold salt. A single print may require several pounds' worth of gold. The process is technically demanding — the sensitiser is not stable for long, the exposure latitude is narrow, and the development conditions must be carefully controlled. For these reasons, the aurotype is not a process for casual experimentation. It is a process for the dedicated practitioner who values the unique colour, the historical resonance, and the extraordinary permanence that only gold can provide. Every aurotype print is a precious object — an image formed in the most noble of metals, by the oldest photographic chemistry, on fine paper.