Would love to see a picture of the window.

Linen does stand up pretty well to sun rot if it's good-quality, long-strand linen, but adding in polyester or acrylic content helps even more. I like all-natural fibers, too, but I also like long-wearing drapes. :-)

Years & years ago PB sold a line of cotton sheers that were called "scrimm panels." I bought them and loved them -- a very open weave, but not gauzy, sheer cotton. If you picture a breeze on a warm country afternoon, this is THE panel you will envision billowing near the window. :)

Time went by, they discontinued them, and I needed more so I went to the local fabric store and asked for scrimm. What I was shown was not MY scrimm, but something used in theater (apparently true scrimm has some kind of special effects value, like you can only see through one side of it or something).

After 6 stores and hours of looking I finally found something that was almost am exact match - cotton voile (rhymes with boil).

More to the point -- I made 8 panels for different rooms, and have had both white and off-white voile panels used as sheers in east and west windows for years, and not a single one of them has shown any signs of fading, anywhere.

JP Penny sells a pinch pleated sheer and semi sheer drape that looks good and is durable. It's probably poly, though.

West Elm has a Linen Cotton Drape. I just bought some and love 'em! A bit more weight than Silk but not heavy weight either. Decent price and lots of lengths!

Here is a link that might be useful: West Elm Drape

Thanks for your thoughts thus far. Here are some pics of the window. You can see the cove we have there (6" hanging down). the track for the curtains is to be located on the ceiling behind the cove.

this is before the floor was done. The floor restorer had to intersperse some of his stock of old flooring because we did not have enough to salvage after the old floor was taken up.

Wow! Great room. Even with light-weight fabric, drapes for that expanse will be heavy. You might want to consider a motorized track.

Ballards Designs has sheer indoor/outdoor panels made from sunbrella fabric-- search for outdoor sheer panels online and you will find some suitable for indoor or outdoor that will hold up to sun

If you really want a lightweight silky drapery look, you might also think about pairing it with a professionally-installed window film. There are many types -- privacy, sun-blocking, etc. (And the sun will eventually effect any upholstered furniture near the windows, framed art and the flooring, too.)

Here's a link to a very old GW thread about the stuff. I notice an impt caveat, tho: Read your warranty info on your new windows first.

Here is a link that might be useful: GW

(make that, "affect any upholstered furniture" etc etc.)