After the first Age of Magic the disposal of grimoires began to become a severe problem on the discworld. A spell is still a spell even when imprisoned temporarily in parchment and ink. It has potency. This is not a problem while the book's owner still lives, but on his death the spell book becomes a source of uncontrolled power that cannot easily be defused. In short, spell books leak magic. Various solutions have been tried. Countries near the Rim simply loaded down the books of dead mages with leaden pentalphas and threw them over the Edge. Near the Hub less satisfactory alternatives were available. Inserting the offending books in canisters of negatively polarized octiron and sinking them in the fathomless depths of the sea was one (burial in deep caves on land was earlier ruled out after some districts complained of walking trees and five-headed cats) but before long the magic seeped out and eventually fishermen complained of shoals of invisible fish or psychic clams. A temporary solution was the construction, in various centres of magical lore, of large rooms made of denatured octiron, which is impervious to most forms of magic. Here the more critical grimoires could be stored until their potency had attenuated.

— The Colour of Magic by Terry Pratchett (Discworld #1) (Page 127 - 128)