The length of time they annealed for is staggering. I believe it was continuously rotated throughout that time, to ensure even thermal and mechanical load.

I also like that they pressed on with annealing the first cast even though it was a reject. Avoid the unknown when you repeat for the final one.

There's a blog post I can't find any more about a camera nut visiting one of the last Japanese refactories capable of making lenses and the excruciating processes they go through to make complex zonally variant glass. Mirrors seem almost simple by comparison.