Other techniques

• Short strings
  For the string areas between the agraffes and tuning pins, I use Juhani Vesikkalas term ‘short strings’^[1]. There are also similar areas at the opposite end of the strings. (See 'Agraffes (and more)').
  The high pitches of the short strings have a narrow range, but every single string is slightly different. If looking for random microtones, this area may be of interest.
• Bowing
  Bowing the strings with a little bunch of fishing line with some rosin applied to it, can create beautiful and more or less sustained sounds. Hair from the bow of a string player may also be used. One should avoid rosin on the bass strings, if possible, and one should in any case wipe away rosin from the strings afterwards.
  The sound can be rather complex as it is, but it may of course also be used for detuned strings. Be aware that it takes some time to prepare the ‘bow’.
• Ebow
  An E-Bow (abbr. for ‘electric/energy bow’) is a handheld battery-powered elecromagnet originally designed for electric guitars^[2]. It can be placed along a string course of the piano (it only ‘speaks’ in the middle range), making an ‘infinite’ sustain from the strings, resembling a sine tone.
  I tried this on a 'non-unison string course' (with the leftmost string tuned differently), hoping for sounds full of ‘life’, but was quite disappointed: It may seem that the Ebow mainly chooses one of the strings, and certainly not the one to the left.
  
  In any case, the use of E-Bow is a rather unpredictable technique for the piano, and I must admit that I haven’t had too many experiences of an E-Bow contributing to a musically satisfying result.
  It is possible to let the ebow start ringing without an attack: One can simply depress the pedal or the corresponding key (the damper(s) needs to be lifted from the strings). One must then stop the sound by muting the strings either with the hand, or by releasing the key/pedal.
  A contemplative work for piano duo by Jonas Skaarud exclusively using E-bows, may be heard here: 'E oculta mão colora alguém em mim.')
  A door wedge (or sim.) that is carefully inserted right above the sustain pedal makes sure that all dampers are lifted from the strings.
• Piano resonance
  Singing (or playing with another instrument) a rather loud microtonal pitch into the piano with the sustain pedal depressed, will often result in a microtonal piano resonance^[3].
  Related to resonance, I want to mention a phenomenon in Lachenmanns Schattentanz from his Kinderspiel: After a while, when having listened to the highest possible minor 2nd of the piano being repeated over and over, one starts noticing a “microtonal shadow melody” from the resonance (of interferences or of wooden sounds). It moves up and down depending on how hard the hammers hit the very high and tight strings. One may listen to the composer himself playing, and the fascinating phenomenon described above is most clear at loud dynamics: Lachenmann: 7. Schattentanz from 'Kinderspiel'

1. Juhani Vesikkala, “Multiphonics of the Grand Piano” (thesis, 2016), p.90.
2. Alan Shockley, “The Contemporary Piano, A Performer and Composer’s Guide to Techniques and Resources” (Rowman and Littlefield Pub., 2018)
3. Composer Andreas Gundersen’s 'Microtonal Pieces’ is to some extent making use of this technique. His acoustic piano has no microtones at all, but the speakers from a detuned midi-piano faces the inside of the grand piano in order to let the sound ‘go through its acoustics’ and its vibrating strings.