In the paper, presented is a research on effectiveness of absorbing electromagnetic waves at frequency 2.45 GHz by unhardened moulding
sands prepared of three kinds of high-silica base and a selected grade of sodium silicate. Measurements of power loss of microwave
radiation (Pin) expressed by a total of absorbed power (Pabs), output power (Pout) and reflected power (Pref) were carried-out on a stand of
semiautomatic microwave slot line. Values of microwave power loss in the rectangular waveguide filled with unhardened moulding sands
served for determining effectiveness of microwave heating. Balance of microwave power loss is of technological and economical
importance for manufacture of high-quality casting moulds and cores of various shapes and sizes. It was found that relative density
influences parameters of power output and power reflected from samples of moulding sand placed in a waveguide. Absorption expressed
by the parameter Pabs is not related to granularity of high-silica base: fine, medium and coarse. It was found that the semiautomatic
microwave slot line supports evaluation of effectiveness of microwave absorption on the grounds of power loss measurements and enables
statistic description of influence of relative density of the sandmix on penetration of electromagnetic waves in unhardened moulding sands.