Covering the face is not permitted for women in the state of Ihram; even it is with a mask, fan or any other thing. The obligatory precaution is not to cover the face with mud or such like that.
Covering a part of the face in the way that is not called mask is not Haraam, also covering the face during sleep or putting the face on the pillow or covering it by hands are permitted.
It is permitted that a woman pulls her veil down in the way that it shades half or all of her face, even if it sticks to the face or not. But the obligatory precaution is to perform this act only when she wants to cover herself from men and keeps her face open in other conditions (the round of the face can be shown in other than in the state of Ihram).
The atonement of covering the face is one sheep for women as obligatory precaution.