Expansion of granular or particulate material is effected by supplying such material, for instance slate chips, at a charging zone CZ of a kiln 10, to the upper surface of a rotating annular hearth 12 so as to lie thereon in a uniform layer up to about three chips in thickness. The hearth 12 is rotating in the direction of the arrow 96, and the chips face thereon just after a preheating zone PHZ. The chips are carried successively through a treatment zone TZ wherein they are subjected to heat from regenerative burner arrangements 68, 70 and 64, 66 beneath hood 44, to a discharge zone DZ whereat they are loosened by a reciprocating rake 80, 82 and are blown off the hearth 12 and into a receiver 88 by air jets from radially directed nozzles 78 supplied by way of a manifold 76. The kiln, and its manner of use, enables expansible material, such as slate chips, to be substantially fully expanded and thereby achieve a product, suitable for use as a lightweight aggregate, of minimum possible density.