1. A method for producing a ceramic core for investment casting of reactive
metal, said method comprising:
a. injecting a slurry comprising alumina particles and a siloxane binder
into a die; and
b. thermally converting said slurry into a ceramic core,
wherein said ceramic core comprises mullite, alumina, and up to 11 wt% free silica.
2. The method as claimed in claim 1, wherein said thermally converting occurs at a temperature of between 1400°C and 1700°C.
3. The method as claimed in claim 3, wherein said ceramic core contains between 0.1 wt% and 6 wt% free silica.
4. The method as claimed in claim 1, further comprising applying an oxide on substantially the entire surface of the ceramic core, wherein said oxide has a normalized Gibbs free energy of formation that is less than the normalized Gibbs free energy of formation for silica.
5. The method as claimed in claim 4, wherein said oxide comprises yttrium, zirconium, or aluminum.
6. The method as claimed in claim 5, wherein said oxide is selected from at least one of yttrium oxide, yttrium silicate, and yttrium aluminum oxide.
7. The method as claimed in claim 4, wherein said applying is accomplished by dip coating said ceramic core in a solution or suspension of an oxide comprising yttrium, zirconium or aluminum; or spraying or brushing a solution or suspension of an oxide comprising yttrium, zirconium or aluminum onto the surface of said ceramic core.
8. The method as claimed in claim 1, wherein said die is a disposable core die.
9. A method of making a cast metal article comprising bringing molten reactive metal into contact with a ceramic core comprising mullite, alumina, and up to 11 wt% free silica; and solidifying said reactive metal.
10. The method as claimed in claim 9, wherein said ceramic core comprises an oxide on substantially the entire surface of the ceramic core, wherein said oxide

has a normalized Gibbs free energy of formation that is less than the normalized Gibbs free energy of formation for silica.
11. The method as claimed in claim 9, wherein the reactive metal is selected from an alloy comprising nickel and yttrium.
12. The method as claimed in claim 10, wherein the reactive metal is selected from an alloy comprising nickel and yttrium.
13. The method as claimed in claim 10, wherein the oxide comprises yttrium, zirconium, or aluminum.
14. The method as claimed in claim 13, wherein said oxide is selected from at least one of yttrium oxide, yttrium silicate, and yttrium aluminum oxide
15. The method as claimed in claim 9, wherein said ceramic core is formed using a disposable core die.
16. A ceramic core comprising mullite, alumina, and up to 11 wt% free silica, and an oxide disposed on substantially the entire surface of the ceramic core.
17. The ceramic core as claimed in claim 16, wherein said oxide comprises yttrium.
18. The ceramic core as claimed in claim 17, wherein said oxide is Y2O3.