The author, a paper restorer, questions why alum has fallen into disuse since World War II, while it has been in constant use during more than three centuries for the manufacture of books. Alum has been proven to cause browning and embrittlement of paper. This was particularly noticed on 19th-century papers made mechanically from a paste that contained large amounts of aluminum sulfate. On the other hand, it has been observed that alum-tawed leather skins do not damage the paper they touch even after many years, whereas alum-tanned skins do. Consequently, the tanning process must be held responsible for the deterioration, and not the presence of alum. Marbled papers made in the 19th century rarely show signs of degradation attributable to the use of alum as a color fixative. The author argues that alum should no longer be totally rejected and that more research is needed on the matter.