The premise is simple in that a sheet of material has the gasket shape "punched out" of it. This leads to a very crude, fast and cheap gasket. In previous times the material was compressed asbestos, but in modern times a fibrous material such as high temp graphite is used. These gaskets can fill many chemical requirements based on the inertness of the material used and fit many budgetary restraints. Common practice prevents these gaskets from being used in many industrial processes based on temperature and pressure concerns.