a method of decreasing the expansion temperature and improving the expansion characteristics of vermiculite ore comprising subjecting vermiculite ore containing interlamellar cations to a potassium nitrate solution for a time interval sufficient to ion - exchange interlamellar cations within said ore with potassium ions . subsequent to this ion exchange the low temperature expandable vermiculite may be expanded by heating in an oven , or incorporated in an intumescent material and heat expanded in situ , or expanded by other heating means . an intumescent sheet is provided which may be used for mounting automotive catalytic converter monoliths comprising an unexpanded vermiculite produced by the method of the invention ; an inorganic fibrous material ; and a binder .