The status of clean or unclean may be imparted to any object. A classic
example of this is a couch that a woman in her period or a man with a discharge
sits on. Anything, in fact, that she/he sits on will become unclean. A given
object may become unclean in any one of the following ways:
1) It comes
in contact with an unclean substance,
2) It is touched by an unclean person,
3) It comes in contact with a dead person,
4) It is in the same room with
a person who dies.
Given the different ways in which an item may become
unclean, there are different ways in which that item is to be cleansed.
For an object that has become unclean through a person who is unclean that
has touched it: if it is an earthen vessel it must be broken, and if it
is a wooden implement it must be cleansed with water (Wayyiqra' 15.12). If
a vessel is in the same room with a person who dies, it must be cleansed
with the ashes of the red heifer (Bemidbar 19.14-18). If an object is a
spoil from war, it must both be cleansed by the waters of the red heifer
and passed through fire - or, if it will be consumed or ruined by the fire,
it must be cleansed by water (Bemidbar 31.19).