There's a millennia-old urban legend that applying alum mixtures to the vagina is effective at tightening the cunt, since it is an astringent; and in primitive tribes it has been said to restore virginity.
In fact there are many drawbacks:
The constriction does not last long
Sometimes the vagina does not constrict at all
The process increases risk of vaginitis
The process kills vaginal bacteria that are a defense against venereal disease
It increases the risk of vaginal tearing during childbirth