Burnt pages are a possible reward from supply crates, which are obtained upon subduing the Wintertodt. They are used to charge the tome of fire, which gives it additional effects.
Twenty charges are added to the tome per burnt page, and one charge is consumed when an elemental fire spell is cast with the tome equipped. Players may remove the pages from the tome should they wish to do so.
As of an update on 11 January 2018, empty tomes of fire can be traded in for 100 burnt pages per tome by speaking to Ignisia.