I watched the video with Adam and Michael where they built a Kendama. Adam started talking about hardness and ductility and explained it, but it wasn't exactly right. Since I am a materials science PhD candidate I thought I'd elaborate on the topic, so I posted it on reddit: https://www.reddit.com/r/tested/comments/6t7e6x/kendama_build_on_adams_explanation_of_soft_and/. And then I remembered the Forums and thought I'd also post it here.
Adam got parts of it right, but missed some steps on the way. As he said, metallurgy is a deep and complex topic. With my post, I just wanted to shine some light on that stuff if anyone is interested.
(I loved the video by the way, what a great way to use a lathe! That radius tool is stunning.)