Wait a period of time after pouring then the shall could be cleared.
The length of this period of time is measured by the range of casting temperature. In general, carbon steel castings could hulling and clearing in 600-650 ℃, Alloy steel shall generally be dropped below 500 ℃，For complex alloy steel it should cleaning when the temperature dropped below 200 ℃.
Late clear could make some thicker decarburization layer for the high carbon content steel.
Strictly speaking, the time choice for castings from finished pouring to shell clear also depends on the casting material, shape, size and production conditions. The cooling time should be longer for the structure of complex alloy casting or prone to deformation, easy cracks casting；For thin wall and small size casting, the cooling time can be short.
The general rule, no matter what kind of casting is, all should ensure the casting shell temperature must be lower than the phase transition temperature of the casting alloy.