No science degree, but.........I do a bit of woodworking. It's nearly as good as staying at a holiday inn express
Shellac is dissolved in alcohol, and some older stuff was DIY rather than sold premixed in cans. It still comes that way of you look for it.
So the amount of shellac mixed in and it's color will affect the end result. Anywhere from almost clear to dark brown.
Shellac is itself somewhat UV resistant, and supposedly doesn't darken with age. I'd almost bet that in some finishes it was mixed with other stuff that does, or that "shellac" has been used generically to cover any finish applied to a ball.
Being Alcohol soluble it generally doesn't like getting wet with water. Think of the cloudy rings you'd get on some furniture with a cold drink glass and no coaster, and you'll get an idea of the exact effect of water on shellac.
Fun trivia- most 78 rpm records are made of shellac.
Steve B