Change a shutter's core material to glass
Set a shutter panel's core material source to custom and pick a glass material.
To change a shutter's core material, select the shutter, go to the panel level, set the material to custom, open the library and pick the desired material such as plain glass, then remove the existing profile.
The core material is then changed at the panel level by setting its source to custom and choosing the glass material.
Changing a shutter panel's core material source to Custom and selecting a different glass core material is also the way to change a glass shutter's panel thickness — the thickness comes from the chosen glass core material (for example a 4 mm or 6 mm glass variant from the catalogue), not from a separate thickness setting on the default panel.