5.2.4: Mungo Ponton

The Chemist Who Introduced Dichromate Sensitivity to Photography

Mungo Ponton (1801–1880) was a Scottish inventor and amateur chemist whose discovery in 1839 laid the groundwork for an entirely new branch of photographic and photomechanical processes. Though often overshadowed by the better-known figures of Daguerre and Talbot, Ponton’s contribution—identifying the light sensitivity of potassium bichromate (now more commonly called potassium dichromate)—became a pivotal turning point in the expansion of photographic methods beyond silver-based chemistry.

A solicitor by profession and a Fellow of the Royal Scottish Society of Arts, Ponton had a strong interest in scientific experimentation. In May 1839, shortly after news of Daguerre’s invention swept across Europe, Ponton presented a paper to the Society describing how paper coated with potassium bichromate would darken upon exposure to sunlight.¹ This reaction demonstrated that light could alter the chemical composition of dichromate salts—specifically, by reducing the compound and rendering it insoluble in water. Although Ponton did not use this principle to produce images with a camera, his findings suggested a new avenue for photographic reproduction.

Ponton’s shadowgraphs—made by placing objects directly onto sensitized paper and exposing them to light—were an early form of photogram. When the paper was washed after exposure, the areas that had received light remained stained, while the protected areas washed clean, leaving behind a silhouette image.² Though rudimentary, these experiments proved that bichromates could be used to create fixed, light-stable images without the use of silver salts. This was a crucial development, since silver was expensive and often chemically unstable unless carefully fixed.

More importantly, Ponton’s work would later become the foundation of bichromate-based processes, such as the carbon print, gum bichromate, and photogravure techniques that flourished in the late 19th century. These methods enabled photographic images to be printed in permanent pigments or inks—an essential leap in making photographic prints archival and reproducible in ink, bridging the gap between photography and the printing press.

Though Ponton did not continue to develop photographic methods himself, he was acutely aware of the significance of his discovery. He predicted that bichromates would find use in printing technologies, a foresight that proved accurate.³ His work demonstrated that the invention of photography was not a single event but a constellation of breakthroughs, each adding new tools and techniques to the expanding medium.

Today, Mungo Ponton is remembered as one of photography’s unsung heroes—an innovator whose scientific curiosity quietly transformed photographic chemistry and made possible some of the most artistically expressive processes in the history of the medium.