utilized to selectively increase fabric porosity the amount and character of such further processing will then determine the final permeability of the fabric the low reference permeability is preferably achieved by calendering as illustrated in fig4 in this regard a supply roll 38 provides uncoated fabric 40 of the type which is appropriate for use in an air bag a number of specifications for air bag fabric are well known including weight thickness and strength preferably the fabric of supply roll 38 will be either woven or warp knitted fabric constructed substantially entirely of synthetic fibers because nylon is somewhat hygroscopic i e water absorbing presently preferred embodiments may frequently utilize polyester yarn such yarn may have a size of 600 denier or other yarn size appropriate for the exigencies of a particular application fabric 40 is delivered from supply roll 38 into a calender device 42 which includes a relatively large center roller 44 and a pair of smaller rollers 46 and 48 as shown fabric 40 extends between rollers 44 and 46 and then around roller 48 after leaving calender device 42 fabric 40 is delivered to take up roll 50 as illustrated rollers 46 and 48 apply heat and pressure onto contiguous portions of center roller 44 as a result fabric 40 is calendered on.