Heat causes the metal to expand, thus breaking the seal.

You put the seal on the glass then put the window in the frame. This eliminates the pressure on the glass. IT is easier to run a glazing tool around the metal frame without damage than it is to run it around the glass.

It works best with a metal lid on glass. When the metal lid gets hot from the hot water it expands. The glass doesn't expand as much as metal so the tight seal that the lid had on the glass gets loosened as the lid gets slightly larger.

You have to get the whole back glass, the seal is attached to the glass and that is the way it comes.

any metal or glass