Strongest substance?
Klaus went on to punch the air even harder than he was trying earlier at the sight of results. Cracking sounds and a visible white cleavage could be seen in the air. Sighting this, he swung harder than he did the previous time and while the punches hurt him in return, whatever he was hitting was not hitting back so it was safe to say he wasn't planning to yield just yet.
Moreover, his training background had once involved hitting targets that were solid as a wood so the actions he was performing now was simply like a callback gesture to the experiences he had in the dojo with his father, Mr Zen, the great Shiro Senshi.