Rendering techniques that increase realism in volume visualization help enhance perception of the 3D features in the volume data. While techniques focusing on high-quality global illumination have been extensively studied, few works handle the interaction of light with materials in the volume. Existing techniques for light-material interaction are limited in their ability to handle high-frequency real-world material data, and the current treatment of volume data poorly supports the correct integration of surface materials. In this paper, we introduce an alternative definition for the transfer function which supports surface-like behavior at the boundaries between volume components and volume-like behavior within. We show that this definition enables multi-material rendering with high-quality, real-world material data. We also show that this approach offers an efficient alternative to pre-integrated rendering through isosurface techniques. We introduce arbitrary spatially-varying materials to achieve better multi-material support for scanned volume data. Finally, we show that it is possible to map an arbitrary set of parameters directly to a material representation for the more intuitive creation of novel materials.