Gold Chloride

Gold chloride (chloroauric acid, HAuCl₄·xH₂O or gold(III) chloride, AuCl₃; CAS 16903-35-8), sometimes called gold salt, is the gold source for gold toning of silver prints and for a handful of specialty alt processes. Gold toning is the oldest surviving photographic permanence treatment — known since the 1850s^[1] — and remains the gold-standard archival protection for fibre-based silver gelatin prints. Gold deposits preferentially onto the silver image, shifting tones from warm-black-to-brown toward cooler blue-black or blue, and replaces susceptible silver with inert gold that resists environmental attack for centuries.

Photographic uses

• Gold toning (Nelson gold toner): Dilute solution (~0.2–0.5 g/L gold chloride) with thiocyanate or thiourea as auxiliary reducer. Tones silver gelatin prints from warm-brown toward cool blue-black, simultaneously converting roughly the top layer of silver to gold metal.^[2]
• Split-toning protocols: Brief gold toning after sepia toner or copper toner produces the classical split-tone looks (warm highlights with cool shadows) much favoured in fine-art printing.
• Archival protection ("gold protective"): Some workers use very dilute gold toner as a permanence-only treatment that adds gold without substantially shifting print tone.
• Chrysotype: Mike Ware's gold-based alt-process print, analogous to cyanotype but producing metallic gold images directly on paper. Uses gold chloride as the primary noble-metal sensitizer.

Practical notes

Supplied as dark yellow-to-brown crystalline solid or as a pre-mixed 1% or 5% aqueous solution in small ampoules. The hydrated chloroauric acid (HAuCl₄·3H₂O) is the commercial form; the anhydrous AuCl₃ is less common.

Very expensive — roughly $200–300 per gram in 2026 prices, reflecting gold's commodity value plus purification cost. Sold in small quantities (typically 1 g ampoules) to match realistic darkroom use.

Stability: Solutions stored in amber glass keep for years. Do not store in contact with aluminum, zinc, or other reactive metals — the gold plates out instantly onto the metal surface, destroying both the reagent and often the container.

Related compounds

Potassium chloroplatinite is the platinum analog for Pt printing (not toning). Palladium chloride is the palladium analog, commonly paired with platinum in Pt/Pd. Sodium chloroaurate is the sodium form sometimes substituted; essentially interchangeable chemically.