Calotype

Calotype, also known as talbotype, is an early photographic process introduced by William Henry Fox Talbot in 1841. It produced a negative image on paper that could be used to make multiple positive prints, making it one of the first practical negative–positive photographic systems.

The process begins with sensitizing paper by dipping it in a solution of silver nitrate and potassium iodide to form a thin layer of silver iodide. After drying, the paper is exposed in a camera, forming a latent image. The exposed sheet is then developed with a solution containing gallic acid (often with added silver nitrate or other developers) to bring out the image. The negative is fixed with a soluble salt such as sodium thiosulfate and thoroughly washed.

A key advantage of calotype was the ability to produce numerous positives by contact printing the negative onto sensitized paper, typically salted paper or later albumen-based papers. This negative–positive system contrasted with the daguerreotype, which produced a single direct positive image. However, calotypes generally produced softer images with less crisp definition due to the texture of paper and the development process.

Despite its limitations, the calotype was influential because it established the principle of reproducible negatives, a cornerstone of modern photography. It remained in use through the mid-19th century and influenced later processes, including more durable negative formats such as wet-collodion glass plate negatives introduced in the 1850s.