Photographic Printing in Colloidal Gold

The photochemical formation of images in colloidal gold has not previously been perfected as a printing process. The present method employs a sensitizer composed of ammonium trisoxalatoferrate(III) and a gold(I) complex of 3,3' thiodipropanoic acid, imbibed into cellulose paper. Contact printing from monochrome negatives under UV radiation yields well-graduated, continuous tone images consisting of colloidal gold embedded in the surface fibres of the paper. Wet-processing removes all excess gold and iron salts. The state of dispersion of the gold colloid, which has nanometer dimensions, is controlled by the composition and pH of the sensitizer, the humidity of the paper and the choice of the developer. The hydrophobic gold sol is stabilized by hydrophilic colloids such as the gelatin used as a paper sizing agent. This gold printing process (“New Chrysotype”) offers archivally permanent, Ughtfast prints in a wide range of image colours, with a plain paper surface. Colloidal gold has a very high covering power, which mitigates the expense of using it for enduring photographs.