to 70 percent by weight of inorganic fibers the inorganic particles have an average particle size smaller than 600 mesh and the inorganic fibers have an average length to diameter ratio of more than 30 a preferred embodiment the invention provides a method of forming a shell mold comprising the steps of providing an investment casting shell mold composition comprising a binder composition comprising a mixture of 20 98 percent by weight of a colloidal sol comprising inorganic particles 1 20 percent by weight organic polymer and 0 1 to 70 percent by weight of inorganic fibers the inorganic particles have an average particle size smaller than 600 mesh and the inorganic fibers have an average length to diameter ratio of more than 30 refractory flour is mixed with the binder to form a slurry a primary coated disposable pattern is coated with the slurry composition to form a slurry coated pattern the slurry coated pattern is formed by repeatedly dipping the primary coated disposable pattern in the slurry composition draining and drying a stucco coated pattern is formed by stuccoing applying dry coarse refractory layer and drying the slurry coated pattern in accordance with a preferred embodiment of the invention is provided an investment casting shell mold composition comprising a binder composition comprising a mixture of 20 98 percent by weight of a colloidal sol.