  Physical head phantoms allow assessing source reconstruction procedures in
electroencephalography and electrical stimulation profiles during transcranial
electric stimulation. Volume conduction in the head is strongly influenced by
the skull representing the main conductivity barrier. Realistic modeling of its
characteristics is thus important for phantom development. In the present
study, we proposed plastic clay as a material for modeling the skull in
phantoms. We analyzed five clay types varying in granularity and fractions of
fireclay, each with firing temperatures from 550 {\deg}C to 950 {\deg}C. We
investigated the conductivity of standardized clay samples when immersed in a
0.9% sodium chloride solution with time-resolved four-point impedance
measurements. To test the reusability of the clay model, these measurements
were repeated after cleaning the samples by rinsing in deionized water for 5 h.
We found time-dependent impedance changes for approximately 5 min after
immersion in the solution. Thereafter, the conductivities stabilized between
0.0716 S/m and 0.0224 S/m depending on clay type and firing temperatures. The
reproducibility of the measurement results proved the effectiveness of the
rinsing procedure. Clay provides formability, is permeable for ions, can be
adjusted in conductivity value and is thus suitable for the skull modeling in
phantoms.
