Finely-divided graphite flake is heated to a high temperature in the presence of boron or a boron-containing compound to cause the boron to permeate the crystal structure of the flake, which is then subjected to an intercalating agent and rapidly heated, resulting in an expanded or vermicular graphite with a bulk density substantially less than what might be expected if the flake were not pretreated with boron. The expanded graphite then is compressed to form a preform that is reduced in thickness by rolling or molding to form a higher-strength flexible graphite foil than heretofore.