  The effect of thermal surface cleaning on the Gilbert damping, $\alpha$, of
yttrium iron garnet (YIG), before capping with a metallic layer, has been
investigated. Our results show that $\alpha$ is strongly effected by relatively
mild annealing conditions (T = 300 C) when performed in a vacuum. This increase
needs to be taken into account when obtaining the spin mixing conductance from
spin pumping measurements. We measure an increase in $\alpha$ by a factor of 8
when the YIG is vacuum annealed at 300 C. No such changes in $\alpha$ are
observed when annealed at the same temperature in 0.1 mbar of oxygen.
