how to get the pitch of a triangle higher or lower?
I know this might be stupid, but check this out: https://en.wikipedia.org/wiki/Harmonic
"On stringed instruments, harmonics are played by touching (but not fully pressing down the string) at an exact point on the string while sounding the string (plucking, bowing, etc.); this allows the harmonic to sound, a pitch which is always higher than the fundamental frequency of the string."
I haven't found anything on how idiophones vibrate, but i believe for a triangle it would be a transversal wave like a guitar string. With luck you may be able to hold it at any harmonic node to make a higher tone. Note that it will in that case sound muted and weak.
Also, to tune a triangle higher just remove material from the ends. Dunno why you would want this though.
Interesting question! There are actually a few ways you can do so, both permanent and non permanent. The easiest would be to play the triangle in various positions. Try playing in the center of one side and compare to the sound of playing in the corner. You can also try playing in the center of the whole bar verses near one of the ends. The next easiest is changing your striking stick. A heavier, thicker rod will sound a bit lower than a thinner, shorter rod. Different materials sound slightly different as well. Note that both of these methods won't produce a huge difference in sound. The only way to radically change its pitch is to alter the length or thickness of the body, and at that point you're better off investing in another triangle.
All sources I checked say that the triangle is of indefinite pitch. For example, the Percussive Arts Society has a fact sheet: https://www.pas.org/docs/default-source/pasic-archives/triangle.
You can get different effects from different size triangles, and with different playing techniques, but apparently it is not possible to change the pitch as such, given that there is no pitch as such.
(This is very different from another percussion instrument, the chimes. How many times have we heard the chimes come in sounding completely out of tune in relation to the rest of the ensemble.)
Here's another attempt:
The triangle doesn't have a pure pitch. It's a bit like banging a pot or a piece of scrap metal. But some triangles are higher and some are lower, and once it's been produced, the highness or lowness can't be changed.
Also some sound more beautiful than others. Generally the more beautiful sounding ones cost a lot more.