If I cool it slowly, I get:
You pull the lever up, and pistons suddenly descend from the ceiling to pound at the conveyor. Its surface becomes more even under the assault, the small lumps and irregularities in the metal smoothing out. When they're finished, you wait as the metal slowly cools. It rings out a beautiful melody as it does, the crystals crinkling as they settle. But it's not over yet. The monsters keep running their fingers over the metal.
With a viciousness that startles you, one grabs a small hammer and slams it down hard. The arm-monster recoils from the impact, and the new conveyor appears unblemished.
"Oh, I see!" Pearl says. "Adding vanadium increased the hardness of the metal. How fascinating!"
The monsters take a small piece out of the stream and begin doing… something with it. First one squeezes it between its thumb and forefinger, and after a few seconds of that, two grasp one end each and try to pull it apart.
It breaks with an audible snap. The rest of the monsters don't even hesitate before flinging the whole thing into the lava.
You wail and tug at your hair as you watch the new conveyor melt into slag. What was wrong with it? You'll just have to try again!