Follow these steps from Chris Tyler's Art & Science of Strata Design 3D to create a glass material which properly coordinates reflection, transparency (opacity) and refraction channels.
Create a new material then follow these steps:
1. Set the Diffuse/Ambient amount to 0. A lock on these two channels causes them to act as a single variable.
2. Set the Opacity interpolator to Fresnel.
3. Set the secondary Opacity value to 0. This tells the material to be mostly transparent ‘face on’ (incidence zero).
4. Leave the primary Opacity value at 100. When an interpolator is turned on, this value governs what happens toward the edge. This causes the surface to become less transparent (more opaque) at the edge where the material will be most reflective.
The more light reflecting at the edge can’t be available to transmit through the object.
5. While holding the Option(Alt) key, move the Reflectivity slider to 100. The option key will cause the channel’s secondary parameter to auto-configure to zero. This reduces the number of clicks necessary to set up the channel.
6. Disable the ‘Blurred Reflections’ option in the Reflectivity channel. This was turned on automatically by the Option(Alt) key modifier in the previous step. It’s not really necessary to have blurry reflections for glass.