Glissandi along string(s) without any corresponding key being pressed

In this technique, the fundamental of the string seems to be of little importance, because it is rather the friction from the object, depending on its material, pressure and speed, against a certain thickness and texture of the strings, that create the pitches (often microtonal).

• Superball mallet:
  The superball can be dragged on almost any piano surface, whether on the inside or the outside; some resulting in moaning-like bending tones, others with a higher noise content.
  

  

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  Sound sample and score sample:
  Øyvind Mæland: Boiling Web

  Dragging a superball along the string(s), creates uneven glissandi. With the sustain pedal depressed, it works very effectively, though it is always almost impossible to aim for specific pitches. The range changes with the size of the superball (and the surface), but also the speed and amount of pressure affects the sound. The superball friction itself creates the sound, so pressing a corresponding key is not necessary, unless looking for a quite different sound.

  The voice-like howl (as we can hear in the end of the sound sample to the right) is indeed highly microtonal through its unpredictable glissando

• Singing bowl:
  A singing bowl or temple bowl along the strings (it covers several strings) will create fascinating, yet relatively soft glissando sounds.
• Plastic box (or sim.) :
  In Juhani Vesikkala’s thesis on multiphonics^[1], he mentions the work 'one poetic switch' by Santiago Diez-Fischer^[2] where moving plastic tupperware boxes with thin edges create an effect reminding of a very dramatic and harsh version of the ‘seagull-glissando’ of the string instruments: Santiago Díez-Fischer — one poetic switch [w/ score]
  The plastic box is used both with and without a corresponding key.
• Bicycle inner tube:
  A bicycle inner tube glissando along a low string creates a fascinating voice-like sound with a very clear pitch, often microtonal. Allthough the movement feels like a glissando, the pitch remains the same, sounding approximately 3 octaves above the fundamental! If there is no stress bar hindering a longer movement (pulling the object towards the keyboard works best), a rather sustained and loud sound is possible. The sostenuto pedal may be used to let the string(s) of the desired pitch ring even more, without worrying too much about bumping into neighboring strings.
  
  I would advise composers to write the preferred approximate sounding pitch, perhaps specifying whether an octave transposition is allowed or not, and let the performer find out which low key to press, and take a note in the score.

For sure, there are other objects/materials that may result in interesting sounds. Several of the above-mentioned types of glissandi may of course also be combined (more or less effectively) with pressing corresponding keys.

Examples of works including this technique:

• Øyvind Mæland: Boiling Web

1. Juhani Vesikkala, “Multiphonics of the Grand Piano” (thesis, 2016)
2. Diez-Fischer, Santiago. “one poetic switch” for solo piano, 2014.