Before smartphones and DSLRs were everywhere, photography was a physical, hands-on process. I’ll give you a quick overview of the chemistry behind the first type of photograph, the daguerreotype!
Daguerreotypes were invented by Louis-Jacques-Mandé Daguerre in France in 1839. They consist of a picture developed on a silver-plated copper sheet. Each daguerreotype is a direct positive, meaning its image is created directly from the exposure without an intermediary negative. This contrasts with film, where your pictures are taken on a negative (a filmstrip) and then exposed as needed to create a positive. Because of this, the only way to replicate a daguerreotype is to rephotograph it, making each image incredibly unique!
When preparing to take a daguerreotype, you would need to first polish a silver sheet until it was reflective like a mirror. Traditionally, to save resources photographers would use copper sheets plated with silver. Next, they would expose the plate to halide vapors to make the surface photosensitive Ag halides.
When Louis Daguerre first invented the daguerreotype, he used iodine vapors. Iodine halides weren’t actually very photosensitive, and after a year of experimentation and innovation, people began to use more reactive halides such as chlorine and bromine as substitutes.
The chemical formula for this step can be written down as:
2Ag(s) + 1I2(g) à 2AgI
2Ag(s) + 1Cl2(g) à 2AgCl
2Ag(s) + 1Br2(g) à 2AgBr
Once the plate is prepared, the photographer would set up the camera, pose their subject, and focus the lens. After their shot was composed, they would insert the plate into the camera’s plateholder, and begin taking the photograph. Because exposure times were so slow, instead of having a mechanical shutter, they would simply take the lens cap off to start the exposure and put it back on to stop it. While the plate is exposing, it would react with light:
10AgX + 10hv à 2Ag5(s) + 5X2(g) (x being the halide)
After the exposure is taken, portions of the plate that reacted to light consist of just Ag, while nonreactive ones remain AgX. At this point, the picture would still be invisible, and needs to be developed to be seen.
Next, photographers would develop it using mercury vapor. This causes an amalgam (a mercury alloy) to form with the silver.
11Ag5 + 45Hg(g) à 5Ag11Hg9 & 3Ag5 + 20Hg(g) à 5Ag3Hg4
While this developed the image, it left the image in a highly reactive state. It could literally be wiped off by your hand! In order to fix the image to the plate, they would wash it with water and sodium thiosulfate. It would react with the thiosulfates to remove unreacted Ag halides and prevent the mercury amalgams from tarnishing.
2Na2S2O3 + AgX à Na3[Ag8S2O3] + NaX
This is where the traditional daguerreotype process finished! For display, they would be placed in a frame under glass to keep the picture safe. Further innovations to the technique included adding a layer of protective gilding & manually coloring the image.
What makes the daguerreotype so cool is that it’s accurate down to the molecular level. Unlike digital cameras limited by pixels and film stock limited by crystal grain, daguerreotypes are direct exposures to light. If you look at a daguerreotype, you’ll notice they’re incredibly sharp images.
Recently, there’s been a revival of traditional photographic techniques. Tons of practitioners have been experimenting with daguerreotype and other similar techniques such as wet-plate collodion, and many have been hosting workshops to introduce the public to them.
Hopefully, this has sparked an interest in the daguerreotype. I’d encourage you to explore it more! It’s a really cool form of photography, and learning about it helps you appreciate the point-and-click cameras we take for granted today!
Works Cited
https://www.metmuseum.org/essays/daguerre-1787-1851-and-the-invention-of-photography
https://daguerreiansociety.org/Making-a-Daguerreotype
https://doi.org/10.3390/s23094341
Picture credit Library of Congress
