  Using images from the Hubble Space Telescope Advanced Camera for Surveys, we
measure the rate of cooling of white dwarfs in the globular cluster 47 Tucanae
and compare it to modelled cooling curves. We examine the effects of the outer
convective envelope reaching the nearly isothermal degenerate core and the
release of latent heat during core crystallisation on the white dwarf cooling
rates. For white dwarfs typical of 47 Tuc, the onset of these effects occur at
similar times. The latent heat released during crystallisation is a small heat
source. In contrast, the heat reservoir of the degenerate core is substantially
larger. When the convective envelope reaches the nearly isothermal interior of
the white dwarf, the star becomes brighter than it would be in the absence of
this effect. Our modelled cooling curves that include this convective coupling
closely match the observed luminosity function of the white dwarfs in 47 Tuc.
