  A wide range of thermoreversible gels are prepared by cooling down to ambient
temperature hot aqueous polymer solutions. During the sol-gel transition, such
materials may experience a volume contraction which is traditionally overlooked
as rheological measurements are usually performed in geometries of constant
volume. In this article, we revisit the formation of 1.5\% wt. agar gels
through a series of benchmark rheological experiments performed with a
plate-plate geometry. We demonstrate on that particular gel of polysaccharides
that the contraction associated with the sol/gel transition cannot be
neglected. Indeed, imposing a constant gap width during the gelation results in
the strain hardening of the sample, as evidenced by the large negative normal
force that develops. Such hardening leads to the slow drift in time of the gel
elastic modulus $G'$ towards ever larger values, and thus to an erroneous
estimate of $G'$. As an alternative, we show that imposing a constant normal
force equals to zero during the gelation, instead of a constant gap width,
suppresses the hardening as the decrease of the gap compensates for the sample
contraction. Using normal force controlled rheology, we then investigate the
impact of thermal history on 1.5\% wt. agar gels. We show that neither the
value of the cooling rate, nor the introduction of a constant temperature stage
during the cooling process influence the gel elastic properties. Instead, $G'$
only depends on the terminal temperature reached at the end of the cooling
ramp, as confirmed by direct imaging of the gel microstructure by cryoelectron
microscopy. The present work offers an extensive review of the technical
difficulties associated with the rheology of hydrogels and paves the way for a
systematic use of normal force controlled rheology to monitor non-isochoric
processes.
