A method of manufacturing an investment casting shell mold comprising: providing a first refractory coat slurry formed by mixing a colloidal sol with a first dry blend including ceramic filler, and optionally fibers; providing a second refractory coat slurry formed by mixing a colloidal sol with a second dry blend including ceramic filler and fibers; applying said first refractory coat slurry over an expendable pattern to produce a coated preform; applying a stucco of refractory material onto the coated preform; drying the stuccoed, coated preform sufficiently to apply another of said first refractory coat slurry over the preform; repeating the application of first refractory slurry and stuccoing as many times as necessary to build a preform of desired thickness; applying said second refractory coat slurry over the stuccoed, coated perform to produce a coated preform; applying a stucco of refractory material onto the coated preform; drying the stuccoed, coated preform sufficiently to apply another of said second refractory coat slurry over the preform; repeating the application of second refractory slurry and stuccoing as many times as necessary to build a preform of desired thickness; drying the multi-layered preform to produce a green investment casting shell mold; and heating the green shell mold to a temperature sufficient to produce a fired investment casting shell mold, characterized in that the preparation of the first refractory coat slurry comprises the introduction of rice hull ash in the first dry blend, whereas the preparation of the second refractory coat slurry does not comprise the introduction of rice hull ash in the second dry blend. providing a first refractory coat slurry formed by mixing a colloidal sol with a first dry blend including ceramic filler, and optionally fibers; providing a second refractory coat slurry formed by mixing a colloidal sol with a second dry blend including ceramic filler and fibers; applying said first refractory coat slurry over an expendable pattern to produce a coated preform; applying a stucco of refractory material onto the coated preform; drying the stuccoed, coated preform sufficiently to apply another of said first refractory coat slurry over the preform; repeating the application of first refractory slurry and stuccoing as many times as necessary to build a preform of desired thickness; applying said second refractory coat slurry over the stuccoed, coated perform to produce a coated preform; applying a stucco of refractory material onto the coated preform; drying the stuccoed, coated preform sufficiently to apply another of said second refractory coat slurry over the preform; repeating the application of second refractory slurry and stuccoing as many times as necessary to build a preform of desired thickness; drying the multi-layered preform to produce a green investment casting shell mold; and heating the green shell mold to a temperature sufficient to produce a fired investment casting shell mold, characterized in that the preparation of the first refractory coat slurry comprises the introduction of rice hull ash in the first dry blend, whereas the preparation of the second refractory coat slurry does not comprise the introduction of rice hull ash in the second dry blend.