Adam Clarke on Jeremiah 36:23

When Jehudi had read three or four leaves - Rather columns; for the law, and the sacred Hebrew Books, are written in columns of a certain breadth. דלתות delathoth, signifies gates or openings between column and column, or between section and section.

He cut it with the penknife - בתער הספר bethaar hassopher, "the knife of the scribe," properly enough penknife.

And cast it into the fire - To show his contempt for God's words.