When a rock is heated and cooled rapidly, it will shatter. In nature, this phenomenon is called weathering. Alex was going to simulate it to weaken the thick body of the stone giant. Thermal expansion and contraction was the best way to do it.
When a stone is heated, the different materials present inside it will expand at different rates. For example, if a stone contains air pockets and water, it will expand before the solid materials inside the stone. This will create pressure inside the stone which leads to tiny fractures on it.
Now, if the stone is cooled rapidly after that, the different rates at which the material inside the stone shrink will make it even more brittle. And that's exactly what Alex was aiming for. It was the best way for him to get Thee out of his turtle shell.
