Is there a kind of glue that would melt at water boiling point but would be water-resistant at room temp? I need to glue together some parts (glass and plastic) which I could later recycle by placing the piece in boiling water and recovering the parts.
While not technically a glue, you could use Friendly Plastic:
to hold the parts together. This stuff is similar to hot glue, but stiffer/harder at room temperature than hot glue. It softens at around
150F. One issue (which may be an advantage) is that it really sticks to almost any surface. You may have to scrape off small bits off the recovered parts, which will be an issue if you have very small, intricate or fragile details.
Another option might be a wax, like Flexwax (which is a reusable mold material that melts at 120F):