The average grand piano has 230 strings for the 88 keys of the keyboard. They range from 3 strings per note in the treble, to 2 strings further down the bass, until the lowest bass notes have but one string per note. The modern piano has seen an increase of the tension on the strings to improve the tone and volume of the instrument, and the tension on each string ranges from 160 to 200 pounds. That gives a total inner tension exerted on the frame of over 18 tons. That's the reason why pianos are so heavy. There is an inner cast iron frame to deal with such tremendous forces, otherwise the instrument would collapse under the tension.The preceding information is by way of introducing unconventional music for the piano. Imagine using fists or forearms to play clusters of keys, or playing directly on the strings by plucking, rubbing, or scraping with a finger nail. Or maybe inserting various things between the strings such as screws, bolts, washers, etc. Maybe tapping the strings with a finger, or bowing the strings.
All of this has been done. Some of it quite a while ago. And I can't help but think some of these techniques may not be all that good for piano actions or strings. With the cost of a modern 9 foot Steinway Model D being around $171,000, I would think that sticking nuts and bolts and fingers inside and using the piano in ways it was not designed for may not be too wise, despite the interesting sounds achieved.
Henry Cowell composed pieces that consisted of tone clusters, groups of notes played by the fist or forearm, such as his piece The Tiger of 1928. And pieces to be played on the strings directly, such as the Aeolian Harp of 1923 where the piano keys are silently depressed and the opens strings are played by a finger gliding over them, and The Banshee of 1925, where the damper pedal of the piano is kept down while fingers pluck, glide and scratch over the strings.
The notorious bad-boy of music John Cage developed the Prepared Piano where he put all manner and sizes of nails, screws, nut, bolts, and other paraphernalia between the strings. The result was sounds that were completely different than a piano, such as the Sonata V For Prepared Piano of 1946.
Add to all that the idea of the bowed piano, first suggested by composer Curtis Curtis-Smith in 1972, where pieces of mono-filament fishing line and other items that are rosined and positioned under the strings so that when the performer pulls back and forth on the fishing line a tone is produced. Enter Stephen Scott, a composer who took the idea and created a group of players known as The Bowed Piano Ensemble. Scott composes for this group, and there are many techniques besides bowed piano that he uses to create an interesting sound palette. One of the pieces he has composed for the instrument is titled Entrada. The various techniques can be seen in the video below of The Bowed Piano Ensemble playing the piece.
By my reckoning, there are 10 performers crowded in and around the piano, so this piece takes much coordination and choreography to play successfully.