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Investment casting shells are manufactured by incorporating fumed metal oxide dispersions, or doped fumed metal oxides as a binder into the casting shell. The investment casting shells containing the fumed metal oxides have improved characteristics, such as increased strength and a reduced surface r

Investment casting shells are manufactured by incorporating fumed metal oxide dispersions, or doped fumed metal oxides as a binder into the casting shell. The investment casting shells containing the fumed metal oxides have improved characteristics, such as increased strength and a reduced surface roughness.

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1. An investment casting shell comprising refractory particles, colloidal silica and a fumed metal oxide, wherein the fumed metal oxide comprises a doped fumed silica or a combination of a doped fumed silica with one or more of fumed alumina, fumed titania and fumed zirconia, and wherein the fumed m

1. An investment casting shell comprising refractory particles, colloidal silica and a fumed metal oxide, wherein the fumed metal oxide comprises a doped fumed silica or a combination of a doped fumed silica with one or more of fumed alumina, fumed titania and fumed zirconia, and wherein the fumed metal oxide has a median secondary particle size of less than about 300 nm. 2. The investment casting shell of claim 1, wherein the fumed metal oxide is present throughout the shell. 3. The investment casting shell of claim 1, wherein the doped fumed silica comprises ions of cerium, caesium, rubidium, potassium, sodium, lithium, calcium, magnesium, beryllium, aluminum, titanium, iron, lead, fluorine, chlorine, bromine or a combination thereof. 4. The investment casting shell of claim 1, wherein the ratio of colloidal silica to fumed metal oxide is from about 20:1 to about 1:5. 5. The investment casting shell of claim 1, wherein the fumed metal oxide improves fired strength, green strength or wet strength of the shell by at least about 10% compared to a similar investment casting shell without the fumed metal oxide. 6. The investment casting shell of claim 1, wherein the fumed metal oxide decreases the surface roughness of the shell by at least about 5% compared to a similar investment casting shell without the fumed metal oxide. 7. The investment casting shell of claim 1, wherein the refractory particles comprise zircon or zirconia. 8. A method of improving the strength of an investment casting shell comprising: (a) incorporating an aqueous dispersion of fumed metal oxide into a refractory slurry, wherein the dispersion of fumed metal oxide comprises a dispersion of doped fumed silica and wherein the fumed metal oxide has a median secondary particle size of less than 300 nm;(b) depositing the refractory slurry and a refractory stucco in alternate layers over an investment casting shell mold, the aqueous dispersion of fumed metal oxide improving the strength of the casting. 9. The method of claim 8, wherein the fumed silica is doped with ions of cerium, caesium, rubidium, potassium, sodium, lithium, calcium, magnesium, beryllium, aluminum, titanium, iron, lead, fluorine, chlorine, bromine or a combination thereof. 10. The method of claim 8, wherein the dispersion of fumed metal oxide comprises a stable dispersion of fumed metal oxide. 11. A method of manufacturing an investment casting shell comprising: (a) incorporating a dispersion of fumed metal oxide into a refractory slurry, wherein the dispersion of fumed metal oxide comprises a dispersion of doped fumed silica and wherein the fumed metal oxide has a median secondary particle size of less than 300 nm;(b) depositing the refractory slurry and a refractory stucco in alternate layers over an investment casting mold. 12. The method of claim 11, wherein the fumed silica is doped with ions of cerium, caesium, rubidium, potassium, sodium, lithium, calcium, magnesium, beryllium, aluminum, titanium, iron, lead, fluorine, chlorine, bromine or a combination thereof. 13. The method of claim 11, wherein the dispersion of fumed metal oxide comprises a stable dispersion of fumed metal oxide.