Mungo Ponton (1801–1880): The Edinburgh Lawyer Who Found Light in Dichromate

established claims rest on Ponton's 1839 paper, the records of the Society of Arts for Scotland, the National Bank of Scotland's records now held by NatWest Group, and histories of photographic processes. shared claims involve interpretation of priority and influence. traditional claims are repeated but not documented.

The weak points are these. Others had noticed the light sensitivity of chromium salts before 1839 in a purely chemical context; the Austrian historian Josef Maria Eder credited the German chemist Gustav Suckow with observing it in 1832. Ponton's contribution was to propose it as a photographic process. One account says he used sodium dichromate; his paper refers to the potassium salt.

A banker with a laboratory

established Ponton trained as a lawyer and became a Writer to the Signet in 1825. In 1838 he joined the National Bank of Scotland as its salaried solicitor, and in 1840 he also became its secretary. Science was his passion. He was elected to the Royal Society of Edinburgh in 1834, and in 1838 the Society of Arts for Scotland gave him a silver medal for his work on the electric telegraph.

The 1839 discovery

established Photography burst on the world in January 1839, with the announcements of Daguerre's and Talbot's processes. Both relied on expensive silver salts. Ponton, experimenting with Talbot's method, found that paper soaked in a dichromate solution turned from yellow to brown where light struck it, and that the unexposed salt could be washed away to fix the image. On 29 May 1839 he presented this to the Society of Arts for Scotland as a cheap alternative to silver-based photographic paper, and he published the method that year. He chose not to patent it.

shared Ponton's images were not permanent in the way later processes would be, and he did not realise why his paper worked. The photographic historian Michael Gray explains that the dichromate was reacting with the gelatine size in the paper. When that was understood, the discovery became immensely useful: gelatine, gum or albumen mixed with a dichromate hardens in proportion to the light it receives, while unexposed parts wash away. Gray adds, quoting J. M. Barrie, that the discovery confirmed "there are few more impressive sights in the world than a Scotsman on the make".

From Ponton's paper to the printing press

established Talbot repeated Ponton's experiment in 1842 and used dichromated gelatine in his photographic engraving patents of 1852 and 1858. Alphonse Poitevin in France and others developed carbon printing, collotype and photolithography from it, and Joseph Swan's carbon tissue of 1864 made permanent carbon prints practical. Karel Klíč's photogravure of 1879, which the Glasgow firm of T & R Annan brought to Britain, also used a dichromated gelatine resist. For a century, dichromated colloids underpinned the reproduction of photographs in print, and related light-hardening resists are still used in printing and electronics.

Later life

established Ill health from the mid-1840s forced Ponton to take leave from the bank, and he resigned in 1846. He moved to Clifton, near Bristol, for its milder climate, and continued to write on science and religion. He died there in 1880 and was buried in Edinburgh.

• Ponton announced his silver-free photographic paper in May 1839, only months after photography itself was made public.
• He never patented the discovery, while others built patented processes on it.
• The same principle that Ponton noticed lies behind photogravure, the process James Craig Annan made famous as an art.
• Ponton was a banker by profession: secretary of the National Bank of Scotland.

• Myth: Ponton invented carbon printing and photogravure. He noticed the light sensitivity that later workers such as Talbot, Poitevin, Swan and Klíč built on, and he did not fully understand why his paper worked.
• Myth: Ponton was the first to see chromium salts react to light. Others noticed it earlier in a purely chemical context. Josef Maria Eder credited the German chemist Gustav Suckow with observing it in 1832. Ponton's contribution was to propose it as a photographic process.
• Disputed: which dichromate salt he used. One account says sodium dichromate, but his paper refers to the potassium salt, bichromate of potash.