Diazo Paper

paper covered on one or, less frequently, both sides with a thin layer of a photosensitive substance made from diazo compounds. The paper is used in photocopying in diazotype photocopiers.

The process of producing a visual image on diazo paper has two steps. In the first step—exposure—an unstable positive image is formed in the photosensitive layer; the image consists of sections of undecomposed diazo compounds remaining under opaque areas of the original document. In the second step—developing—the undecomposed diazo compounds are converted into light-stable azo dyes of black, brown, red, orange, blue, or violet color.

Diazo paper is classified according to the composition of the photosensitive layer as single-component, two-component, or hot-process. Single-component paper contains only a diazo compound, which is developed by the wet process in aqueous solutions of azo compounds. Two-component paper contains diazo and azo compounds and is developed by the dry process, usually in ammonia vapors. Hot-process paper contains, in addition to diazo and azo components, compounds that when heated release substances required for developing. Diazo paper is produced mainly in rolls 20 to 100 m long and 0.3 to 1.2 m wide. In addition to diazo paper, diazo tracing paper is produced on a translucent paper base for use in preparing duplicates and intermediate originals.