How create Thumbnails ?

Yup pretty much, it's not a pretty solution in any way, but so far I haven't found anything better.

I have to bypass the problem with the Render function?
I now try this solution.
Thanks for your help Marco :)

Well there's no "direct" way of doing it. And there is no access to the material previews in the Material Editor. Which means if you want previews you are going to have to render them yourself in the scene.

Yes Marco you are right ... I need to preview the material created in Material Editor as Thumbnails. It must be the finished material.
I need a small preview in a separate rollout.
Excuse me, next time I will be more precise in explaining my problem!

Hum I think you'll need to give a better description of what you want to do. Thumbnails of what? And for what?