Tip-3: Shell should have good permeability
This is a property of the shell as to how well it can vent, i.e., how well gases pass through the shell. And in other words, permeability is the property by which we can know the ability of the shell to transmit fluid/gases, like air and steam.
Good shell permeability allows steam to easily pass through it so that steam can quickly reach the wax surface and start to melt the wax. Good permeability also allows the molten wax to slightly pass through its layers. This reduces the wax pressure build-up, leading to lower incidents of shell cracking.
Other Advantages Of Using High Permeability Shells:-
Shell firing becomes more efficient as the hot gases can pass through the shell. During metal pouring the gasses escape easily through the shell. This helps very good detail reproduction, reduces problems of blowholes and airlock.
High permeability shells have considerable benefits over low permeability shells. It is generally good to build high permeability shells.