Figure 19. Diazotype with a blue azo dye (top), which takes on a reddish-brown hue at a lower pH value (outgassing of ammonia) (motif: Kaiser Wilhelm Bridge, Wilhelmshaven/Germany)

From

Diazotype – A Historical Copying Process

Matthias Ducci

World Journal of Chemical Education. 2021, 9(4), 136-143 doi:10.12691/wjce-9-4-6