Diazotype

Diazotype is a historical photochemical printing process used to reproduce images, typically technical drawings. It relies on a light-sensitive coating containing a diazonium salt and a phenolic or naphthol coupling component. When the coated sheet is placed in contact with a transparency and exposed to ultraviolet light, the diazonium compound breaks down in the exposed areas and couples with the developer to form an insoluble azo dye. After a wash or development step, the unexposed areas are removed, leaving a visible image on the background. The resulting image is usually blue or blue-black, on a light background, and can be quite fine in detail.

The process was developed in the 19th century and became widely used for quick reproduction of architectural drawings, maps, and technical plans because it was simpler and cheaper than earlier photomechanical methods. It is sometimes referred to as the diazotype printing process or diazo copy, and it yielded images with relatively high contrast and sharp lines, though with limited tonal range.

With the advent of modern copying technologies such as xerography and digital printing, diazotype declined in mainstream use during the mid-20th century, although it persisted in some archival reproduction and specialty applications. Modern references typically treat it as a historical or niche technique. Safety concerns apply due to the handling of diazonium salts and coupling reagents, requiring proper ventilation and disposal.