What is the density of modelling clay?

There is a wide variety of moDELLing clays: Mineral clays, are refined forms of the clay you may find in the ground, and are dense over two kg/l

Polymer clays are much lighter, about 1 kg/l

and paper based clays can be very light, depending on their dryness.

If you have an actual sample the best thing to do would be to measure it.