Just listened perhaps on of those:
SOPHIE AGNEL "capsizing moments" CD
came out last year. Got couple of her discs. This is CD divided into 3 parts, two c. 20 min pieces and one little less than 10. It's all free improvisation with grand piano, recorded live in concert without overdubs/electronics. All acoustic preparations with objects and materials placed between the strings in order to alter its sound. There is no melody, no harmony really. It is digitally clean, and in few loud moments you realize that yeah, it isn't some PROPER classical music recording, when mics suddenly override by distortion. But overall very clean, very accurate sound how it probably sounded in performance. Liner notes lists things like aluminium ashtrays, nylon fishing lines, disposable watercups, rubber, rocks,..... result is obscure sounds of creaking, banging, screeching, etc. From minimal and quiet to full of thunderous cacophony. I appreciate the wide range of dynamics and sound, although occasionally I'm also feeling the usual dissatisfaction of mere object/instrument improvisation. I start to think what could be achieved if there was a bit more. Perhaps vocals with lyrics? Perhaps few other elements. But then again, the clarity of sound, very natural reverbs of this instruments own nature, and subtle textures would be lost if it was reduced to be some backing noise.
If comparing to something like classic John Cage "music for prepared piano" type of pieces, Sophie does it much better. It is also far superior to many contomporary piano improvisation LP's I've heard. Be it Stockhausen or Xenakis, they lose. This I'd recommend to check out. I'm sure there are others that are good.
(edit: typo corrected)