inner shaping device 52 and an outer shaping device 54 which when brought together as suggested by the arrows in fig4 may heat and compress the web to produce a denser web upon heat shrinking with any method of making the rigidized fiber filter element of the present invention surface treatments and finishes may be applied to the filter element after the rigidification process the fibers used to produce the rigidized and densified fiber filter element of the present invention may be of more than one variety for example a rigid permeable structure may be prepared from a mixture of high shrink polyester fiber and a polyester binder fiber the rigidity of the structure is due not only to the high shrink fiber but also to the binder fiber heating and cooling of the precursor fabric structure results in shrinkage and bonding at fiber crossover points this further rigidifies the structure compared to that composed solely of high shrink fiber the density and rigidity of the fiber structure can also be increased by compression during heating the rigidized and densified fiber filter elements of the present invention have been made into rigid permeable structures of a range of densities and void volumes the low density precursor webs could include particulate matter within their web structure a precursor web containing such particles could be rigidified and densified in accordance with any one of the process described above and the resulting reduction in void size would confine the particles within the rigidized and densified fiber filter element for example particles of activated carbon could be incorporated within the filter element to remove toxic gases alternatively the incorporation of catalyst particles could permit desired chemical reactions to occur in the fluids passing through.