  The theory of substellar evolution predicts that there is a sharp mass
boundary between lithium and non-lithium brown dwarfs, not far below the
substellar-mass limit. The imprint of thermonuclear burning is carved on the
surface lithium abundance of substellar-mass objects during the first few
hundred million years of their evolution, leading to a sharp boundary between
lithium and non-lithium brown dwarfs, so-called, the lithium test. New optical
spectroscopic observations of the binaries DENIS+J063001.4-184014 and
DENIS+J225210.7-173013 obtained using the 10.4-m Gran Telescopio de Canarias
are reported here. They allow us to re-determine their combined optical
spectral types (M9.5 and L6.5, respectively) and to search for the presence of
the LiI resonance doublet. The non detection of the LiI feature in the combined
spectrum of DENIS\,J063001.4$-$184014AB is converted into estimates for the
depletion of lithium in the individual components of this binary system. In
DENIS\,J225210.7$-$173013AB we report the detection of a weak LiI feature which
we tentatively ascribe as arising from the contribution of the T3.5-type
secondary. Combining our results with data for seven other brown dwarf binaries
in the literature treated in a self-consistent way, we confirm that there is
indeed a sharp transition in mass for lithium depletion in brown dwarfs, as
expected from theoretical calculations. We estimate such mass boundary is
observationally located at 51.48$^{+0.22}_{-4.00}$ $M_\mathrm{Jup}$, which is
lower than the theoretical determinations.
