try thin Walled off or on.
I found this.
Thin Walled - This property sets the volumetric effects of the surface. Compare a bubble (on) to a marble (off) for example.
On - When the property is on, the surface takes on the characteristics of a thin walled surface.
Off - When this property is off, the surface takes on the characteristics of a thicker or solid surface item. Thin Walled must be off to get volumetric effects such as Sub Surface Scattering.