This fabric is actually a vintage curtain (my guess would be the 70's. It would go well with brown wallpaper and orange carpeting) that I bought from a thrift store.
I'm not sure about the material. It doesn't feel synthetic and it doesn't look synthetic (it doesn't look as shiny as in the pictures). I'm no expert by any means but my guess would've been some sort of cotton damask. Or something. But it didn't do very well on the burn test (recently explained by Fuchsia here) so I don't know.
I'm thinking I'm not too bothered. I'm no purist anyway, and if it looks good, it's good enough for me.
What's bothering me more is the pattern. Not that it's not an 18th century pattern - who'd be so lucky that they found a Period Correct patterned fabric at a price they could afford? Certainly not me! - no, I'm more worried about the size of the pattern. Methinks it's too large. Here I've placed a medium-sized cat on the fabric for comparison:
If I'm going to use this fabric for an 18th century project (and in that case I see a jacket, and I see a black petticoat to go with it!) I'll have to do a bit of research on jackets made of damask-type fabrics first, if any should exist, and see what the fabrics look like. Any input would be appreciated of course!