I think since the third to fourth century AH, the ruling of all four Madhabs has been that the Niqaab is obligatory, based on the moral decline in most societies. Sisters who wear the Hijaab, but not the Niqaab, should be commended and encouraged to adopt the Sunnah of the wives of the Prophet (Allah bless him and grant him peace) who wore the Niqaab in an era during which the hearts of men and women were incomparably more pure.