How to Compose for the Glass Armonica
My instrument has a range of C4 (middle C) to G7, which is 3 octaves and a fifth. This is on the large side as armonicas go these days. The range of armonicas varies widely — there really isn't a 'standard armonica' at present —so you'll need to consider who will be playing your music and the range of their instrument. (Meanwhile, the two Mozart pieces (K617 & K617a) require a 3-octave range from F to F, so that is a fair first-approximation of a 'minimum professional range'.)
The glasses are far enough apart that I can only reach a minor 6th in each hand at best. In the bass end (C5 and below) the glasses are a little farther apart so a perfect 5th is as wide as I can reach reliably. Some instruments have their glasses even farther apart, so again, you'll need to consider who will be playing your music and the characteristics of their instrument.
That said, multiple glasses (in each hand) are natural to the armonica and sound terrific.
Playing the piano involves pushing down a lever. Playing the glass armonica is more like bowing the glasses with your ten finger 'bows'. Consequently, piano technique doesn't map well to the glass armonica. 'Turning the thumb under', for example, doesn't work on the armonica, so quick scales in one hand are problematic. Consider using hand-over-hand if you want a scale.
Once a glass is singing, it's relatively easy to make it sing again, so trills and tremolos (of the piano variety) work well.
Notes can be sustained as long as the player can keep their fingers applied to the glasses. Sustained high notes (above C6 or so?) can sound eerily like feedback, so be cautious about sustained notes on high glasses.
The higher glasses (C5 and up?) speak relatively quickly. But the glasses in the bottom octave speak more slowly — quick passage work in the 'bass' bottom octave or so is a bad idea because it will be especially difficult to play, and sound muddy when successful. (Violins are more facile than double basses.)
The glasses continue to ring after you release them. This effect is most pronounced in the bass end, much less apparent in the high end. (Like the harp, where the high notes die away quickly, but the bass notes ring for a long time). There is no practical way to damp an armonica glass (how do you noiselessly damp a spinning glass?)
Vibrato works well, that is, moving one's finger from side to side on a sustained note. This does not change the pitch — the pitch of a glass (which is in essence a glass bell) is fixed; moving the finger from side to side works by manipulating the overtones.
The armonica definitely has dynamics, but the whole instrument is on the quiet side so its dynamic range is not large. Think of its dynamic range as ranging from ppp to p.
After a glass is singing, crescendi and decrescendi on sustained note(s) work well (within the armonica's ppp to p dynamic range).
I invariably amplify the armonica at performances — even just a little tasteful amplification makes a huge difference.
Write glass armonica music on a grand staff, or if the part is simple enough a single staff is fine.
This: and this: are great ways to reduce ledger lines.
'Pizzicato' of the glasses (flicking them with your fingernail like flicking a booger from your finger) works OK from about C6 up. Below that the result is more like a pitchless thud.
A 'glissando' of the glasses just doesn't work. (Unlike the lovely effect on the harp.) That's because making each glass sing requires a finger/bow 'tweak' on each glass — merely sliding your fingers over the glasses doesn't work. Think about doing the wet-finger-around-the-wineglass: that sound doesn't just happen; one has to get the speed/pressure/etc. right to make it sing at all. That's what the armonica is all about!
I simply won't play the glasses with any sort of mallet. The result is uninteresting (a barely audible pitched thud) and I dare not risk breaking any glasses. (Too damn expensive.)
Also, the glasses are too close together to bow them with a violin bow. (Ha, ha, that's what fingers are for!)