machine by rotating rubber friction wheels. There are two chambers inside the machine. 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. Ultraviolet light penetrates the original and neutralizes the light sensitive diazonium salt wherever there was no image on the master. These areas become the white areas on the copy. Once this process is complete, the undeveloped image at the locations where the UV light could not penetrate can often be seen as very light yellow or white marks/lines on the diazo sheet. This completes the exposure phase. Next, the original is peeled from the diazo paper as the sandwich of master and diazo exits the machine, and the diazo sheet alone is fed into the developing chamber. Here, fumes of ammonium hydroxide 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 unexposed lines changing color from invisible (or yellow) to a visible dark color. The range of colors for these lines is usually blue or black, but sepia (a brownish hue) is also quite popular. When making multiple copies of an original no more than four or five copies can typically be made at a time, due to the build-up of ammonia fumes, even with ventilation fans in the duplication room. A slight delay of perhaps five minutes is often required for the fumes to subside enough to permit making additional copies if no ventilation exists. Many blueprint shops ran ventilation ducts from the machines to outside. Smaller and mid-size blueprint machines were often outfitted with neutralizers which absorbed some of the ammonia for a period of time. If the lines are too light, it is also possible to run the blue-line through the developing chamber once more, which often increases the contrast of the lines relative to the base media. Repeated lack of contrast and light prints is also a tip-off that the operator needs to adjust the speed or amount of ammonia. Sometimes both the master and the diazo print are inadvertently fed through the developing chamber together. If this occurs, one simply peels the master from the diazo paper and runs the diazo sheet through the developer once more to more fully develop the lines. Diazo printing was one of the