Glass beads for prepared piano

Question

Music for Piano (Frangis Ali-Sade) notes:

Prepare the piano by laying a glass-bead necklace tautly along the strings between the above mentioned notes [C#-G#: 9.5 whole steps].

What type of glass necklace? True glass necklaces (as opposed to resin beads) are hard to find in the US so I have to get it right on the first attempt.

Three strings form a note and strings of the same note are closer together. Do these instructions mean:

• Beads between notes
• Beads between all strings
• It doesn't matter

I assume the beads should be large enough to rest between the strings without support from the necklace.

Answer 1

I find it hard to imagine that the exact material matters so long as each bead is a rigid crystalline substance. (Yeah, I know glass is usually noncrystalline. Go argue with Penrose). He's presumably looking for the beads to rebound elastically, so anything with beads whose size is on the order of the string spacings should work out fine.

Answer 2

Given my piano: strings of the same note are separated by 5mm and string between notes are separated by 7mm. I managed to test:

• 8mm diameter (advertised as 10mm) glass beads with nylon (fishing line) core
• 12mm diameter (advertised as 14mm) glass beads with nylon (fishing line) core
• 8mm diameter (advertised as 10mm) amethyst (quartz) with doubled cotton string core

Buyer beware: Despite the label most "glass" beads are actually glazed ceramic or acrylic or some other dried resin. I can only hope my glass beads are actual glass, though since they have the look and feel of glass marbles I'm pretty confident. Also that my quartz beads aren't man-made (amorphous) fused silica, though I think that's pricier than natural quartz.

The glass is amorphous, the quartz crystalline. My results:

• 8mm glass beads: A good first impression but a little "twangy". The additional vibrations producing the zither effect sometime fade in advance of the natural piano sound.
• 12mm glass beads: Dull sound, probably from the excess weight. The beads are also too large to fit beneath the metal crossbars.
• 8mm amethyst beads: Similar to the 8mm glass beads. Produces a slightly less "twangy" sound even though the beads seem to hop off the strings to greater degree. Might be an issue in long sessions. Possibly the cloth core is muting the excess vibrations rather than a fundamental difference between glass and quartz. The sound is also more homogeneous with less separation between the zither and piano sounds.

I can't emphasize enough how subtle is the difference between glass and quartz, even though I prefer the quartz. Judging by the degree of bounce I suspect any lightweight bead, even inelastic plastic beads, would vibrate sufficiently to produce a zither effect.

Also note that if the necklace is strung too tightly you lose most of the effect.