a light coating of binder is applied in the first stage followed by an air dried gellation to dimensionally stabilize the fiber mat thereafter the steps of impregnating gelling and curing are repeated one or more times until the total desired amount of binder has been added typically about 15 to 300 wt of binder is used based on the initial weight of fibers and microballoons in the mat the impregnating step may be accomplished by wicking spraying vacuum infiltrating and the like after impregnation the binder is converted to a rigid gel usually by air drying or by subjecting the binder impregnated mat to an atmosphere of ammonia gas since the ammonia sol reaction is exothermic the tendency of bubbles to form in the mat can be avoided by allowing the first batch of binder to gel in air after gelling the binder the mat is cured preferably by heating to about 200 u00b0 f for several hours about four hours are preferred then by slowly increasing the temperature to about 600 u00b0 f for a longer period of time usually about five hours and finally by reducing the temperature to ambient temperature in addition to the ceramic fibers and microballoons and or diatoms.