Another point about shaders in general. The preset shaders that come with Carrara are really just jumping off points to show you how to create shaders. I can't think of one default shader that I would use in one of my scenes. Even my own shaders have to be adjusted for every scene depending on scale, lighting, etc. You really need to experiment with the shader setups and see what changes in values and type of channels that are used can do to the look of a shader. When it comes to 3D, shaders are a huge part of the "art" of a scene. You will never create anything unique or original by using presets. So have some fun and just try making all kinds of crazy shaders until you start to get an intuitive feel for how shaders are created. Carrara's shader abilities are almost unlimited and very powerful.