  We explore the effect of heat treatment in argon atmosphere under various
temperatures up to $500^\circ$C on single crystals of $\alpha$-RuCl$_3$ by
study of the mass loss, microprobe energy dispersive x-ray spectroscopy, powder
x-ray diffraction, electrical resistance as well as low-temperature magnetic
susceptibility and specific heat. Clear signatures of dechlorination and
oxidation of Ru appear for annealing temperatures beyond $300^\circ$C. Analysis
of the specific heat below 2~K reveals a RuO$_2$ mass fraction of order $1\%$
for pristine $\alpha$-RuCl$_3$ which increases up to $20\%$ after thermal
annealing, fully consistent with mass-loss analysis. The small RuO$_2$
inclusions drastically reduce the global electrical resistance and may thus
significantly affect low-temperature thermal transport and Hall effect.
