Diazo reproduction processes are based on light sensitive aromatic diazonium compounds and their ability to react with aromatic coupling components to form azo dyes under conditions of high pH. In one version of the diazotype process, the paper is coated with a solution containing the light sensitive aromatic diazonium compound and an acid, along with such other auxiliary components as may be deemed advantageous. After image-wise exposure of the sensitized paper, the coupling agent is supplied from a developer solution containing, in addition to the coupler, an alkaline-reacting substance.
In another version of the diazotype process, i.e. the two-component process, both the light sensitive aromatic diazonium compound and the coupling agent are coated on the paper along with an acid to prevent premature coupling. After image-wise exposure to ultraviolet light, during which the diazonium compound accessible to the light is decomposed, the dye image is formed by passing the print through an atmosphere of warm ammonia and water vapor. The acid of the coating is thus neutralized, creating the conditions for the coupling reaction to occur in those areas of the print where the diazo compound has not been decomposed by the action of the ultraviolet light during exposure.
Since it is frequently desirable to make a diazo print from which additional prints can be made at a later time, it is common practice to apply the sensitizing coating to a transparent base and to utilize diazo compounds and couplers which will yield a dye with strong absorption in the ultraviolet regions of the spectrum where aromatic diazonium compounds have their principle sensitivity. This type of diazo material is known as an "intermediate paper". When an ordinary diazo sensitized base is exposed to ultraviolet light through a print present on this intermediate paper, a contrasting image results since the dye forming the intermediate image has high "blocking power" to ultraviolet light while the paper base transmits these rays quite readily.
It is a common practice for draftsmen and designers to use diazotype intermediates for making changes and modifications in original designs. Accordingly, it is essential to be able to eradicate portions of the developed dye image. Prior art techniques for erasing such images have involved applying an excess of a liquid eradicator to the area to be bleached, followed by blotting. This method for eradication has obvious disadvantages in that it is inconvenient, time consuming, tends to weaken or even destroy the base, and leaves a poor surface for subsequent pencil or ink redrawing.
In addition, it is the usual practice in the diazotype coating industry to apply the solution to a moving web of paper by means of an applicator roller turning in a tray of the solution. The excess is then "doctored off" by means of a jet of air. Such a technique, however, has the tendency to cause the formation of streaks in the final coating originating at the applicator roller. Various adjustments made in the coating operation have not been completely successful in eliminating these streaks.