"the mask was a thing on its own, behind which Jack hid"
Masks
GNU Free Documentation LicenseMasks - Credit: arts-primitive.com

Jack paints his face, which becomes like a mask.

People wear masks for many reasons. One reason may be to pretend to be someone or something else. The mask may also hide the wearer's real emotions, or it may allow him to do what he would never feel able to do.