Carrara treats all reflective objects as "solid" like  a marble unless you engage the Fresnel effects. Fresnel is a real-world physical phenomenon where the more perpendicular an object is to the viewer, the less the properties of reflection and refraction have an effect on the object. However, the greater the angle is away from perpendicular, the more significant those effects become. 

Carrara has a good, (albiet somewhat slow) calculation scheme for real world Fresnel effects. It does take longer to render than the the "fake" fresnel of ShaderOps by DCG and Shoestring Shaders. Although Shoestring Shaders has a "real" Fresnel version as well, it was created before Carrara had the Fresnel effect  as part of the software and is redundant. 

Why use real Fresnel over fake Fresnel effects? The fake fresnel effects only present an approximation to the camera of what Fresnel would look like and not what is really happening optically. So, if you have multiple lights hitting the ojbect, the fake  fresnel solutions will make the other shadows from the glass object behave in a very weird fashion, rendering as a solid instead of a transparent object. 

Hope this helps. Click on the picture if you want to see the shader tree more clearly.

Mark