Which early photographic process involves creating a negative on a silver-coated copper plate using mercury vapor?

• A. Daguerreotype
• B. Calotype
• C. Ambrotype
• D. Collodion

Answer: (A)

Mercury vapor development on a polished silver-coated copper plate is the hallmark of the daguerreotype. In this process, the plate is prepared and exposed to light to form a latent image, and then mercury vapor is introduced to chemically reveal that image by forming a silver-mercury amalgam where the light struck the plate. The result is a highly detailed, direct-positive image on a metal surface (often viewed as positive rather than a negative in the usual sense).

Other early methods differ in materials and development steps: the calotype uses a paper negative coated with silver iodide, the ambrotype uses a glass plate with a collodion emulsion and a dark backing to appear positive, and the collodion process refers to the wet-plate technique using collodion on glass (producing negatives or other formats). The combination of a silver-coated copper plate and mercury vapor development is distinctive of the daguerreotype.