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. 
Oleg B. Zaslavskii
gr-qc astro-ph.CO hep-th