Year	Milestone
1727	Johann Heinrich Schulze discovers photoreactive silver salts (AgNO₃ darkens under light).
1839	Daguerreotype announced (silver plate treated with iodine vapour) – first practical silver process.
1840	Hippolyte Fizeau formulates “Sel d’or” (gold chloride + thiosulfate) for intensifying daguerreotypes.
1842	Sir John Herschel introduces Chrysotype, printing images in pure gold.
1847	P.F. Mathieu publishes first albumen prints toned with gold chloride.
1851	Frederick Scott Archer invents wet collodion process (iodized silver on glass).
1855	William de W. Waterhouse develops alkaline gold toners for albumen prints.
1867	Charles Rousselin introduces gold-thiocyanate toners (warmer black tones).
1870s	Gelatin-silver emulsion plates emerge (Maddox, 1871), becoming dominant by 1895.
1895	W.H. Jackson uses gold toning for archival U.S. geological surveys.
1930s	Color films (Kodachrome etc.) use silver halides; non-silver dyes create color images.
1960s	Kodak Gold color negative film introduced (name only — actual layers use silver).
1980s	Alternative print revival (platinum, palladium, chrysotype).
2000s	Research into AuNP plasmonic color printing and sensor filters begins.
2010s	Gold nanoparticles used in photoacoustic and plasmonic imaging.
2015	Demonstrations of plasmonic color filters for CMOS sensors.
2026	Ongoing advances in nanophotography and conservation techniques.

 

Historical Use of Silver and Gold in Photography

Silver halides revolutionised photography because they uniquely combined sensitivity, reproducibility, and scalability. Daguerre’s process used silver-coated copper plates exposed to iodine vapour to create silver iodide, which darkened upon exposure to light. Around the same time, Fox Talbot demonstrated that silver chloride on paper could form a negative image capable of producing multiple positives. This was transformative. Unlike Niépce’s earlier bitumen heliograph, silver processes were sensitive enough to be practical.

By the 1850s, albumen prints, made using egg white and silver nitrate, were common throughout Europe and America. However, early silver images were chemically unstable. Without further treatment, silver prints were vulnerable to sulphur compounds, ozone, and atmospheric pollutants that caused fading or yellowing. Gold entered photography not as a competitor to silver, but as its ally. Fizeau’s early gold-thiosulfate bath intensified daguerreotypes, and later photographers refined gold toning to alter image tone and increase permanence.

Herschel’s chrysotype, meanwhile, proposed something radical: printing directly in gold. Using ultraviolet light to reduce gold salts into metallic gold nanoparticles, the process produced richly coloured images. Yet it never gained commercial traction because gold salts are less photosensitive than silver halides. Exposure times were long, and material costs were high. Silver remained the practical choice; gold became the enhancer.

Photochemical Principles: Why Silver and Gold Work

Silver halides, typically AgCl, AgBr, or AgI, are inherently light-sensitive because photons can eject electrons from the crystal lattice. When light strikes a silver halide crystal, an electron is liberated from a halide ion and migrates within the lattice. If it encounters a silver ion (Ag⁺), it reduces it to metallic silver (Ag⁰). A cluster of only a few silver atoms is sufficient to form a latent image centre. These centres catalyse chemical development, where reducing agents convert entire exposed grains into metallic silver.

The efficiency of this process depends on crystal structure and defect chemistry. Manufacturers deliberately introduce imperfections, such as sulfur sensitisation or trace gold doping, to create shallow electron traps. These traps stabilise the liberated electrons long enough for silver reduction to occur. Interestingly, gold itself plays a microscopic role here. In gold-sensitised emulsions, tiny gold clusters reduce the number of silver atoms required to trigger development, effectively increasing film speed.

Gold behaves differently because it does not form latent images through halide photoreduction in the same way. Instead, gold salts are reduced chemically or photochemically to metallic gold nanoparticles. In toning baths, gold ions replace surface silver atoms. In chrysotype printing, ultraviolet light reduces gold salts directly on paper fibres. Modern science adds another layer: gold nanoparticles exhibit surface plasmon resonance. Their conduction electrons oscillate in response to light, producing vivid, size-dependent colours. This optical behaviour now underpins cutting-edge imaging technologies.

Gold’s Roles in Photographic Processes

Gold Toning and Gilding

Gold toning emerged as one of the most important refinements in 19th-century photography. Early “sel d’or” baths combined gold chloride and thiosulfate, simultaneously fixing and toning images. In alkaline gold toners developed by Waterhouse, gold replaced silver more efficiently, producing denser blacks and improved stability. Chemically, gold ions are reduced while oxidising silver, depositing a thin layer of metallic gold on the image.

This thin gold layer profoundly alters the photograph. Gold is chemically inert, meaning it does not tarnish under typical atmospheric conditions. Silver, by contrast, reacts with sulfur compounds to form silver sulfide. By partially replacing silver with gold, photographers significantly slowed image degradation. Museums and archival institutions still use gold toning to preserve valuable prints, recognising that noble metal chemistry remains one of the most effective forms of conservation.

Gold Sensitisation

Beyond toning, gold has a subtle but powerful role in emulsion sensitisation. During film manufacture, trace amounts of gold salts can be introduced. Upon heating, these form microscopic gold nuclei within the silver halide crystals. These nuclei act as latent image centres, reducing the number of silver atoms required to trigger development.

The result is enhanced photographic speed and improved reciprocity characteristics. In practical terms, gold sensitisation allowed manufacturers to produce faster emulsions without sacrificing contrast. This represents a fascinating inversion: although gold cannot rival silver in light sensitivity, it can enhance silver’s performance at the atomic level.

Modern Imaging Applications

Silver and Gold in Digital Imaging

While consumer photography has largely shifted to silicon-based digital sensors, the influence of silver and gold persists. Silver remains essential in X-ray film, archival microfilm, and fine-art analogue photography. Its tonal richness and well-understood chemistry still attract artists and archivists.

Gold, meanwhile, has entered the nanotechnological domain. Plasmonic colour filters using gold nanodisks have been demonstrated for CMOS sensors. Instead of organic dyes, these filters rely on the resonant oscillation of electrons in nanostructured gold to absorb specific wavelengths. This approach promises higher resolution and improved light efficiency, particularly as pixel sizes shrink below one micron.

Nanotechnology and Biomedical Imaging

Gold nanoparticles have become indispensable in biomedical imaging. In photoacoustic imaging, gold nanorods and nanoshells absorb near-infrared light and convert it into acoustic signals. Their tunable plasmonic properties allow imaging deep within biological tissues, particularly within the therapeutic window of 650–900 nm.

In addition, gold nanoparticles enhance Raman spectroscopy, optical coherence tomography, and electron microscopy. Though far removed from the daguerreotype, these technologies rely on the same fundamental property that fascinated 19th-century chemists: gold’s stability and its extraordinary interaction with light.

Comparative Attributes: Silver vs. Gold