Black hole masking and black hole thermodynamics
  Masking of black holes means that, for given total mass and Hawking
temperatures, these data may correspond to either "pure" black hole or a black
hole of a lesser mass surrounded by a massive shell. It is shown that there is
one-to one correspondence between this phenomenon and thermodynamics of a black
hole in a finite size cavity: masking of black holes is possible if and only if
there exists at least one locally unstable black hole solution in the
corresponding canonical ensemble.
