4.2.1: Alchemy to Chemistry- Silver Salts and Sensitivity

Photography did not appear suddenly when Nicéphore Niépce exposed his first image or when Louis Daguerre announced the daguerreotype. Long before anyone successfully made a permanent photograph, scientists, alchemists, and natural philosophers were wrestling with a deceptively simple question:

Could light leave a lasting trace of itself?

The answer emerged slowly over centuries. What eventually became photography required more than a camera. It required a substance capable of responding to light and preserving the evidence of that encounter. The search for such a material led researchers to one of photography's most important ingredients: silver.

The story begins during the transition from alchemy to modern chemistry. While alchemy is often remembered today for its attempts to turn base metals into gold, many alchemists were careful observers of the natural world. Their experiments produced a growing body of knowledge about chemical reactions, materials, and transformation. Although they had no concept of photography, their investigations created the foundation upon which photography would eventually be built.

In 1614, the Italian physician and alchemist Angelo Sala made a curious observation. He noticed that silver nitrate darkened when exposed to sunlight. Even samples wrapped in paper changed color over time. Sala correctly suspected that light—not exposure to air—was responsible for the transformation.¹

At first glance, this might seem like a small discovery. Yet it revealed a revolutionary idea: light could physically alter matter.

Today we take that principle for granted. Every photograph, whether captured on film or recorded by a digital sensor, depends on light creating a measurable change. But in the seventeenth century, the notion that sunlight could actively transform a chemical substance was still remarkable.

A century later, German physician and chemist Johann Heinrich Schulze pushed the investigation further. Around 1717, Schulze mixed silver nitrate with chalk and placed the mixture inside a glass container. He then covered portions of the bottle with cut-paper stencils and exposed it to sunlight. The uncovered areas darkened while the protected areas remained light, creating temporary words and shapes.² More importantly, Schulze tested whether heat might be causing the reaction. When he exposed the mixture to warmth without light, nothing happened. This experiment established a critical principle that would guide every future photographic process: silver compounds react to light, not heat.

Schulze had essentially created the world's first light-made images.

The images, however, carried a fatal flaw. They could not be preserved. Continued exposure to light caused the entire surface to darken until the image disappeared. The picture existed only briefly before being consumed by the same force that created it. This challenge—the problem of permanence—would haunt photographic experimentation for more than a century.

From Chemistry to Images

By the late eighteenth century, inventors began wondering whether these light-sensitive materials could be used to create pictures directly from the world.

One of the most important figures in this transition was Thomas Wedgwood, the son of famed English potter Josiah Wedgwood. During the 1790s, Wedgwood coated paper and leather with silver nitrate and placed objects such as leaves, lace, and painted glass transparencies on top of the treated surfaces. When exposed to sunlight, the objects blocked portions of the light, creating shadow images.³

Today these images are known as photograms.

Photograms occupy an important place in photographic history because they eliminate both the camera and the artist's hand. Instead of drawing an object, the object itself creates the image through its direct interaction with light. For the first time, visual representation could be produced automatically.

This concept remains one of photography's defining characteristics. A photograph is not simply a picture of something. It is the result of a physical encounter between light reflected from a subject and a light-sensitive surface. The image is, in a sense, a trace.

Wedgwood's experiments demonstrated that this principle worked. Leaves, flowers, and other objects left recognizable impressions on treated paper. Yet the same problem that frustrated Schulze remained unsolved. The images could not be fixed. Exposure to ordinary daylight caused them to continue darkening until they vanished into blackness.³ The British chemist Humphry Davy documented Wedgwood's work in 1802, explaining both its promise and its limitations. Silver nitrate could successfully record shadow images, but no method had yet been discovered to make those images permanent.⁴ That missing piece proved crucial.

By the beginning of the nineteenth century, scientists had demonstrated that light could create images. They had identified silver salts as highly sensitive to light. They had even produced recognizable visual forms without drawing.

What they lacked was a way to stop the process.

Photography was trapped in a paradox. Light created the image, but the same light destroyed it. Solving that problem would require another generation of inventors. Yet the essential breakthrough had already occurred. Researchers now understood that nature itself could participate in image-making. If a method could be found to preserve these fragile traces, a completely new form of visual representation would become possible.

The camera obscura had shown people how to project an image. Silver salts revealed how to record one.

The final challenge was learning how to make that image last.

Innovator	Date	Contribution
Angelo Sala	1614	Observed that silver nitrate darkened when exposed to sunlight
Johann Heinrich Schulze	c. 1717	Demonstrated that silver compounds react to light rather than heat
Thomas Wedgwood and Humphry Davy	1790s–1802	Produced early photograms but could not permanently fix the images

The history of photography is often told as the story of inventors and cameras. Yet before photography could become a technology, it first had to become a chemistry. The discovery that silver salts could record the action of light transformed an alchemical curiosity into the foundation of one of the most influential visual media in human history.

Notes

1. Josef Maria Eder, History of Photography, trans. Edward Epstean (New York: Dover Publications, 1978), 9.
2. Helmut Gernsheim and Alison Gernsheim, The History of Photography: From the Camera Obscura to the Beginning of the Modern Era (London: Thames & Hudson, 1969), 12–14.
3. Larry J. Schaaf, Out of the Shadows: Herschel, Talbot & the Invention of Photography (New Haven: Yale University Press, 1992), 29–31.
4. Humphry Davy, “An Account of a Method of Copying Paintings upon Glass, and of Making Profiles, by the Agency of Light upon Nitrate of Silver,” Journals of the Royal Institution 1 (1802): 170–74.

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