Menu  Volume attributes

Volume section of the ifmTransmittance material
All attributes create an effect which appears inside the material. When turning on "Thin Walled" some effects are no longer available because these expect a closed object.
Adjusts the absorption and scattering cross section to the scale of the object. Lower values create stronger absorption; higher values create less absorption.
Sets the color of the material and implicitly defines, together with the Scale attribute, the absorption cross section of the material. Thus the amount determines how much of the light is transmitted and absorbed by the volume. Use this attribute to create colored glass.
Simulates the scattering inside the object the material is assigned to and implicitly defines, together with the Scale attribute, the scattering cross section. Higher values need a higher value for the maximum path length (Render Settings) to create correct results.
Scattering Direction
Specifies how light is scattered inside the object, where the default value of 0 defines isotropic scattering – light is equally scattered in all directions. Use positive values for forward scattering; use negative values for backward scattering.