ArtMetal

I no longer do ceramic shell casting so this draws from what was used about 35 years ago when I did. Then we would use a sand / resin binding material that would not need any burnout.

That means no drying, and no baking for it to work. In this class comes pre made cores of sand and sodium silica hardened with an injection of CO2, or sand and one of the oil / acid hardening binders.

Also be sure to put some vent holes in the actual metal parts being cast so the core can still vent out somewhere. You can not trap it in totally but can almost. Those vents can be small copper or bronze or aluminum tubes or pipes (depending on what metal you are casting, or just core extensions that go through the new cast walls and will need to be removed and welded shut later. For most of our larger works we would have some exit for the core at a later time and if this means doing it in two, sometimes very unequal sections, that was always a preferred good trade off. bpfink