![]() ![]() Ultraviolet light penetrates through the original and neutralizes the light sensitive diazonium salt in the areas on the sheet that soon become the white areas on the copy where it penetrated (where there was no image on the master.) Once this process is complete, the undeveloped image can often be seen as very light yellow or white marks/lines on the diazo sheet. The first is the exposure area, where the sandwich of the two sheets (the master and the diazo paper) pass in front of an ultraviolet lamp. There are two chambers inside the machine. The original document is laid on top of the chemically-coated side of a sheet of the diazo paper, which is retrieved from a light-protected flat file, and the two sheets are fed into the diazo duplicator, being pulled into the machine by rotating rubber friction wheels. Similarly, the speed control (for setting the speed at which the sheets would be pulled through the machine) are likewise typically pre-marked in any particular shop, having been optimized based on trial runs. Depending on the thickness of the master and the media type, the intensity of the UV exposure light is adjusted through an intensity knob that had typically been pre-marked for all the typical media types that are used for masters in any particular shop. The desired durability of the master determines the choice of media material. Any media that allows some light to pass through typically works as a master. Such media include polyester films, vellums, linens, and translucent bond papers (bonds). The process starts with original documents that have been created on a translucent medium. This solution can also be applied to polyester film or to vellum. The resulting coating is then dried yielding the specially treated paper commercially sold as Diazo paper. In a variety of combinations and strengths, these two chemicals are mixed together in water and coated onto paper. The azo dye (also known as coupler): a colorless chemical that combines with diazo to produce color.The diazonium salt: a light sensitive chemical.Compared to the cost of creating a large-format copying machine, a diazotype machine is a great bargain.There are two components in this process: The reason people still use blueprints is because it is an inexpensive process. This diazotype method produces dark lines on a white background, and is the popular method used today for reproduction of large-format drawings. The chemicals on the paper acquire color only in the areas not exposed to light. Ammonia gas or solution is used as a developer after exposure - it neutralizes the acid and allows the remaining diazonium salt to combine with the reactant to create a blue dye. The semi-transparent original is placed on top of the sensitized paper, and a copy of the same size as the original is made by direct contact. In the diazotype method, the paper is light-sensitized with a mixture of a diazonium salt (used in the manufacture of dyes), a reactant, and an acid that keeps the diazonium salt and the reactant from reacting with each other. This produces a negative image, with the drawing appearing in white against a dark blue background. The exposed paper is then washed in water. Where the areas of the sensitized paper are not obscured by the drawing, the light makes the two chemicals react to form blue. The sensitized paper is then exposed to light. The drawing to be copied, drawn on translucent paper, is placed against paper sensitized with a mixture of ferric ammonium citrate and potassium ferricyanide. ![]() Blueprinting is the older method, invented in 1842. ![]()
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