Whiteprint

Whiteprint is the commercial terminology to describe document reproduction using the diazo chemical process.

There are two components in this process:

1. the diazonium salt - a light sensitive chemical
2. the azodye (aka coupler) - a colourless chemical that combines with diazo to produce color

In a variety of combinations and strengths, these two chemicals are mixed together in water and coated onto paper. The resulting coating is then dried yielding the specially treated paper commercially sold as Diazo paper. It is important to note that this solution can also be applied to polyester film or to vellum.

The Diazo Printing Process

The process starts with original documents (those that need to be reproduced) that have been created on a translucent media. Such media would include polyester films, vellums, or translucent bonds. Any media that allows some quantity of light to pass through.

The original document is laid on top of a sheet of the diazo paper and the two sheets are fed into a diazo duplicator. There are two chambers inside the machine. The first is the exposure area where the two sheet pass in front of an ultraviolet lamp. Ultraviolet light penetrates the through the original and neutralizes the light sensitive diazonium salt in the areas that it can penetrate (ie. where there is no drawing or image. Once this process is complete, the undeveloped image can be seen on the diazo sheet. This completes the exposure phase.

In the next phase, the diazo paper only (not the original) is introduced to the developing chamber. In the developing chamber, fumes of ammonium hydroxide are introduced to create an extremely alkaline environment. Under these conditions, the azodyes (Couplers) react with the remaining diazonium salt and undergo a chemical reaction that results in the coupler changing colour from invisible to a visible colour. The range of colours was usually blue or black but sepia (a brownish hue) was also quite popular.

Diazo printing was considered to be one of the most economically viable methods of document reproduction until environmental and health safety aspects of working in close proximity to ammonia over long periods of time permitted the migration to xerography.

See also

• Azo
• Azo compound