  The use of transparent test/source masses can benefit future measurements of
Newton's gravitational constant $G$. Such transparent test mass materials can
enable nondestructive, quantitative internal density gradient measurements
using optical interferometry and allow in-situ optical metrology methods to be
realized for the critical distance measurements often needed in a $G$
apparatus. To confirm the sensitivity of such optical interferometry
measurements to internal density gradients it is desirable to conduct a check
with a totally independent technique. We present an upper bound on possible
internal density gradients in lead tungstate (PbWO$_4$) crystals using a
Talbot-Lau neutron interferometer on the Cold Neutron Imaging Facility (CNIF)
at NIST. We placed an upper bound on a fractional atomic density gradient in
two PbWO$_{4}$ test crystals of ${1 \over N}{dN \over dx}<0.5 \times 10^{-6}$
cm$^{-1}$. This value is about two orders of magnitude smaller than required
for $G$ measurements. We discuss the implications of this result and of other
nondestructive methods for characterization of internal density inhomogeneties
which can be applied to test masses in $G$ experiments.
