any ideas?

Can we see a fully rendered image ? For the one above, it looks like you've over smoothed the glass mesh. Sometimes meshs surfaces will separate when smoothing at over 90 degrees. In Rhino, convert the glass to a mesh at greater resolution, then use the Tools / Polygon mesh / Weld function (welding perhaps not necesary, depends how you made it). In Bryce, smooth at 45 degrees first, if that still leaves facets, smooth with higher degree.