  Cassiopeia A is one of the best-studied supernova remnants. Its shocked
ejecta emits brightly in radio and X-rays. Its unshocked ejecta can be studied
through infrared emission, the radio-active decay of $^{44}$Ti, and low
frequency free-free absorption due to cold gas internal to the shell. Free-free
absorption is affected by the mass, geometry, temperature, and ionisation
conditions in the absorbing gas. Observations at the lowest radio frequencies
constrain a combination of these properties. We use LOFAR LBA observations at
30-77 MHz and L-band VLA observations to compare $u-v$-matched images with a
common resolution of 17". We simultaneously fit, per pixel, for the emission
measure and the ratio of the emission from the unabsorbed front of the shell
versus the absorbed back of the shell. We explore the effects that low
temperatures and a high degree of clumping can have on the derived physical
properties, such as mass and density. We also compile published radio flux
measurements, fit for the absorption processes that occur in the radio band,
and consider how they affect the secular decline of the source. We find a mass
in the unshocked ejecta of $M = 2.95 \pm {0.48} \,M_{\odot}$ for an assumed gas
temperature of $T=100$ K. This estimate is reduced for colder gas temperatures
and if the ejecta are clumped. We measure the reverse shock to have a radius of
$114$" $\pm $6". We also find that a decrease in the amount of mass in the
unshocked ejecta (as more and more material meets the reverse shock and heats
up) cannot account for the observed low frequency behaviour of the secular
decline rate. To reconcile our low frequency absorption measurements with
models that predict little mass in the unshocked ejecta we need the ejecta to
be very clumped, or the temperature in the cold gas to be low ($\sim10$ K).
Both conditions can jointly contribute to the high absorption.
