Abstract: An improved core material is used in an improved method of forming cores for investment casting of articles. The core material comprises fused silica, synthetic amorphous silica, a refractory material, and a binder. The synthetic amorphous silica has a surface area of at least 200 square meters per gram. Even though the synthetic amorphous silica has a relatively large surface area, it has a particle size of less than 10 microns and is 5% or less by weight of the total weight of the solid constituents of the core material slurry. After a green core has been formed in a core mold it is fired to set the core material. During firing, the green core shrinks in size. To compensate for deviations in the size of a finished core from a desired size, the percentage of synthetic amorphous silica in the core material slurry is varied.