The discovery of the light sensitivity of silver compounds marked the scientific beginning of photography, a technology that fundamentally changed how people record and share visual information. This phenomenon, where exposure to light causes a chemical change in silver-based materials, was the foundational element for capturing images. The earliest experiments demonstrated the possibility of drawing with light, though the initial results were temporary and faded quickly. Understanding this light-activated reaction provided the scientific context necessary for the eventual creation of permanent photographic images.
The Initial Observation: Wedgwood and Davy
The first individuals to systematically document and attempt to apply the light-sensitive properties of silver compounds for image capture were Thomas Wedgwood and Sir Humphry Davy around the turn of the 19th century. Wedgwood conducted a series of experiments in the late 1790s focused on using light to create profiles and copy paintings on glass. He utilized paper or white leather that had been sensitized with a solution of silver nitrate.
Davy, a prominent chemist, assisted Wedgwood and later published their findings in 1802 in a paper for the Royal Institution of Great Britain. Their method involved placing objects directly onto the sensitized material and exposing it to sunlight to create a silhouette, a process known as a photogram. They also experimented with the camera obscura, but the projected images were too faint to produce a strong enough reaction on the silver nitrate paper.
The images they successfully created were temporary. The unexposed parts of the silver nitrate material remained sensitive to light, causing the entire image to darken completely upon subsequent exposure to ambient light. Davy explicitly noted this limitation, stating that the pictures had to be kept in an “obscure place” and could only be examined briefly by candlelight.
The Chemical Reaction: Photoreduction of Silver Salts
The light-induced darkening that Wedgwood and Davy observed is a phenomenon known as photoreduction, the core chemical process of black-and-white photography. This reaction involves silver compounds, typically silver halides like silver chloride (\(AgCl\)) or silver bromide (\(AgBr\)), which are sensitive to light. When a silver halide crystal absorbs photons, an electron is released.
This free electron is then captured by a neighboring silver ion (\(Ag^+\)) within the crystal lattice. The gain of an electron reduces the silver ion to metallic silver (\(Ag^0\)). This metallic silver is black, or very dark, and it forms the visible image.
The amount of light exposure directly correlates to the amount of metallic silver created, producing the varying tones of a photographic image. Areas receiving more light generate more metallic silver and appear darker, while areas receiving less light remain largely unchanged. The initial image formed is a latent image, which is then made visible by a developer, a chemical process that amplifies this initial light-induced reduction.
Achieving Permanence: The Fixation Breakthrough
Despite the successful initial creation of images, the lack of permanence remained the primary obstacle to practical photography for decades. The residual, unexposed silver salts would continue to darken when exposed to light, eventually destroying the picture. A solution was needed to remove this light-sensitive material without harming the newly formed silver image.
The breakthrough came with the application of a chemical that could dissolve silver halides, but not the metallic silver atoms forming the image. Sir John Herschel, the renowned astronomer and chemist, provided this solution. He had previously discovered in 1819, during unrelated chemical studies, that sodium thiosulfate had the power to dissolve silver salts.
Herschel realized the significance of this property and, in 1839, introduced the use of sodium thiosulfate as a “fixer” to the photographic community, initially calling the compound “hypo.” This chemical formed a water-soluble complex with the unexposed silver halides, allowing them to be washed away completely with water. The process of fixation permanently stabilized the image, leaving behind only the light-insensitive metallic silver that comprised the photograph.
This step transformed the ephemeral observations of Wedgwood and Davy into a practical reality, enabling the subsequent commercial success of photographic processes like the Daguerreotype and William Henry Fox Talbot’s Calotype. Herschel’s contribution completed the chemical process necessary to move photography from a scientific curiosity to a medium for permanent image-making.