Épaissir belong to the 2nd
group. Épaissir is a common french verb.
Épaissir is conjugated the same way that verbs that end in : -ir
Épaissir is conjugated with auxiliary avoir
Épaissir verb is
direct transitive, intransitive.
French verb épaissir can be conjugated in the reflexive form: S'épaissir
verb is a direct transitive verb, so passive voice can be used.