This is a tough one because I can't figure out why it's doing it, and therefore I can't rule out my being an idiot.
I've made a normal "good" glass forge setup, using pre-space materials.
The forge is made out of ceramic, the pipe is insulated ceramic.
The problem is that sometimes the glass comes out cold and becomes solid in the first pipe segment right out of the forge. Usually it doesn't, sometimes it does.
That's an example of someone having this problem and "solving" it, typically by getting crap off the floor under the forge. There are other complaints which usually involve someone saying they never have a problem, and someone pointing to the wrong materials being used or there being something on the floor or the floor being made out of the wrong stuff.
Here's the deal.
When I make glass, I get three blobs of glass flowing out the liquid outflow pipe. The first one is hot, the second one is less hot, and the third one is cooler. Sometimes all three blobs are especially cold and kerblam.
First image is called "break". You can see the broken pipe, which is the pipe at the forge outflow, you can see that it's cold, and you can see that it's made out of ceramic. I made this and all other images after I paused maybe 1-2 seconds after this happened.
Second image is called "floor". You can see 5 kg of 500 degree C glass on the ground, kerplop.
Third image is called "wire". It's the tile with the conductive wire that feeds the forge. This wire tends to get hot when glass-making is done, but as you can see it is not super-hot relative to the surroundings. It's hotter but not insanely so.
Next one is called "under". It's the tile that Stinky is standing on. This tile tends to be affected by the temperature of the glass forge, and it contains a conductive wire, and this may be the problem. The glass in the forge may have stolen heat from this wire. If that's true it's weir enough that it's a bug, since people shouldn't expect that the forge is going to suck heat out of a pipe buried in the floor.
Next one is called "pipe". You can see the two glass blobs. I'm mousing over the top one and it is chilly related to what normally comes out of the glass forge. Something stole heat from the glass and I'm not sure what, but it happened to such extent that the trailing 5 kg froze.
Last one is called "next pipe". I made Stinky work overtime and create another bit of glass. I'm mousing over the center blob and as you can see it's much hotter.
The top one (5 kg) is 1328.2 degrees C. The middle one is 1593.1 degrees C. The bottom one is 1691.8 degrees C.
I'm guessing the 5 kg blob from the previous run was about 1000 degrees, which is way below freezing for molten glass, and I think that makes it less likely that the workaround/solution is to increase the temperature of the environment the forge is in.
I watched both of these take place. Power was continuous, they were sequential, and I didn't notice anything stupid happening. I had been making glass with this forge throughout the cycle and it hadn't broken previous to this time. But I have seen this problem in other cycles with this configuration and with various configurations I experimented with in order to try to solve this.
The problem is not stuff on the floor although it might be stuff *in* the floor. In any case the problem is intermittent and that suggests a bug or at least a behavior that is non-deterministic enough that it would frustrate players. If I engineer properly, things should not break, and I should be able to intuit what "properly" means.