Abstract: Compositions hardenable by exposure to heat or electromagnetic radiation are provided by dissolving together to form a blend from about 0% to about 50% by weight of an uncrosslinked polymer, from about 2% to about 30% of a polymerizable monomer, from about 10% to about 70% of a crosslinked polymer in the form of discrete particles having average diameters of from 0.001 micron to about 500 microns and being swollen in said solution and from about 20% to about 70% of a crosslinking agent for said monomer. Such compositions exhibit superior chemical and physical characteristics when hardened and are suitable in a wide variety of applications as construction media. Such compositions are especially suitable in the production of prosthetic dental appliances, such as, for example, prosthetic teeth. It is believed that the hardening of such compositions as, for example, during the preparation of dental appliances forms a polymer system known as an interpenetrating polymer network.