A calotype negative was usually made on paper, so it was never as clear as a glass negative. The “translucent” part mainly came from using fairly thin writing paper and then sensitizing, developing, and fixing it so the image consisted of silver compounds embedded in the paper fibers. Light could still pass through the sheet, especially in the lighter areas of the negative, which was enough for contact printing.

So it was not that the paper became transparent in the modern sense. Rather, a suitably thin paper negative transmitted enough light to print another sheet placed directly beneath it in a contact frame. The darker image areas blocked more light; the lighter areas passed more light, creating the positive print.

In Talbot’s process, the paper was sensitized with silver nitrate and potassium iodide, developed with gallic acid and silver nitrate (“gallo-nitrate of silver”), then fixed with hypo (sodium thiosulfate). The resulting paper negative could then be pressed against silver-chloride printing paper under glass and exposed to sunlight for minutes to much longer, depending on conditions.