The glass forge produces 25kg of molten glass at a time, which means that 3 packets of molten glass go through its output pipe. Unfortunately, the glass that's still inside the glass forge is exchanging heat with the environment and/or the forge itself, and cools off very rapidly. I tested with a ceramic glass forge and a very short output pipe made of ceramic insulated pipe. By the time the second packet leaves the forge, there are 5kg of 900C glass remaining in the forge. Since this is below the condensation point of molten glass, when the next packet enters the pipe, it instantly solidifies, damaging the pipe.
The way the glass forge is set up now, the only way to not damage the output pipe would be to keep the forge itself at a temperature near molten glass. (But then it would overheat.)
Steps to Reproduce
try to make glass in the glass forge. watch the temperature of the molten glass contained in the forge.
This issue has not been confirmed by a developer yet.