  We investigate whether the recently suggested rotation and crystallization
driven dynamo can explain the apparent increase of magnetism in old metal
polluted white dwarfs. We find that the effective temperature distribution of
polluted magnetic white dwarfs is in agreement with most/all of them having a
crystallizing core and increased rotational velocities are expected due to
accretion of planetary material which is evidenced by the metal absorption
lines. We conclude that a rotation and crystallization driven dynamo offers not
only an explanation for the different occurrence rates of strongly magnetic
white dwarfs in close binaries, but also for the high incidence of weaker
magnetic fields in old metal polluted white dwarfs.
