Silver and Sunlight: The Science of Early Photography

Daguerreotype of the 1843 Nantes flood

Daguerreotype of the 1843 Nantes flood by an unknown photographer. Courtesy of Charles Isaacs Photographs, inc., New York.

Shadows in Silver

Chemistry, it seemed, could provide an answer to this dilemma. Scholars had known for centuries that certain compounds, most notably natural silver salts, darkened when exposed to the sun. In the 1720s German physicist and medical professor Johann Heinrich Schulze baked silver nitrate in an oven to show that light, not heat, caused this discoloration. Schulze also put stencils of texts on glass jars filled with a mixture of chalk and silver nitrate. As he explained, “The sun’s rays, where they hit the glass through the cut-out parts of the paper, wrote each word or sentence on the chalk precipitate so exactly and distinctly that many who were curious about the experiment but ignorant of its nature took occasion to attribute the thing to some sort of trick.”

As the study of chemistry developed throughout the 18th century, more pieces of the puzzle came together. Swedish pharmaceutical chemist Carl Wilhelm Scheele, now well-known as one of the codiscoverers of oxygen, examined the properties of silver chloride in the 1770s, recognizing that the reaction taking place in sunlight was a reduction to metallic silver. He also noted that ammonia stabilized darkened silver chloride—an important step toward solving the problem of “fixing” the camera-obscura image. At roughly the same time, Jean Senebier, a Swiss pastor and botanist, experimented widely with the effects of light on living plants, plant products, and other materials. One of these substances was fused silver chloride, called lune cornée, or horn silver. Senebier found that focusing sunlight through a lens sped up the darkening reaction, while filtering it through various materials slowed it down, and that different colored rays of the spectrum worked at different rates.

Schulze had made silver nitrate “write” messages, but decades passed before anyone tried to create an actual picture using light-sensitive chemicals. The English physicist Thomas Wedgwood of the famed ceramics family and the rising young chemist Humphry Davy moved in the same advanced scientific circles. Wedgwood experimented with silver nitrate in the 1790s and perhaps even earlier; Davy published his friend’s results in 1802, adding some of his own observations. As he reported, the first attempts with a camera obscura failed, for the images were too faint to affect the silver nitrate. The two then took another approach, covering a white surface with the compound and placing it behind a painting on glass. Sunlight passing through the glass produced “distinct tints of brown or black, sensibly differing in intensity according to the shades of the picture, and where the light is unaltered, the colour of the nitrate becomes deepest.” Such contact printing of pictures or silhouettes on treated leather or paper was, however, only partially successful. Though an image did form, Wedgwood and Davy were unable to arrest the process, and the surface soon turned dark all over. Varnishing the picture did not help, nor did “repeated washings.” Davy concluded with the hope that further chemical experimentation could make the image permanent, but it would be another quarter-century before this dream was realized.

The First Photographs

Early Daguerrotype

Daguerreotype of Boulevard du Temple, Paris taken by Louis-Jacques-Mandé Daguerre in 1838 or 1839. 
Wikimedia Commons.

In 1807 Frenchman Joseph Nicéphore Niépce, a modest amateur scientist and inventor from an old aristocratic family, devised an early combustion engine called the Pyréolophore with his brother Claude. Around 1816 Niépce began experimenting with printmaking techniques, and by the 1820s he was creating reproductions of prints with a contact printing process he called heliography, or sun writing. Through trial and error Niépce determined that bitumen of Judea, a resinous substance that hardened upon exposure to light, worked best: the bitumen not affected by light was washed away with oil of lavender, and the metal thus exposed was then etched to form a new printing plate. He also used this bitumen on pewter plates, sensitized with iodine gas, to take images using a camera obscura. The world’s first surviving photograph, an indistinct eight-hour exposure of the view from the window of Niépce’s attic workroom, is a heliograph from 1826.