History of Alum
In the mid-15th century, the first European alum mines were exploited at Tolfa, a volcanic area north of Rome in central Italy.
This particular
site is of interest to us as the source of what appears to have been one of the best varieties of alum in papermaking:
the so-called Roman
alum. Known for its high quality until the 19th century, Roman alum was recognizable by its distinct reddish appearance, which was a result of dusting the
alum crystals with a
pigment--probably iron oxide. The pigmentation served as a trademark of Roman alum.