“the null hypothesis for a boiling water test would be that a copper disc or clad vessel would bring water to a boil faster.”
To put us on the same page, the null hypothesis always start with the two things/conditions have no real difference or no real relationship, and it is up to the experiment to reject the null hypothesis and establish a statistical difference. In other words, the default position is nothing, and let the data show otherwise.
“In inferential statistics, the term “null hypothesis” usually refers to a general statement or default position that there is no relationship between two measured phenomena, or no association among groups.”
"Null hypothesis. The null hypothesis, denoted by H0, is usually the hypothesis that sample observations result purely from chance. "
In the pure sense, what Franz really showed isn’t that “copper and aluminum pots are the same”, rather that he couldn’t establish a statistical difference. His data were not strong enough to disprove his null hypothesis (two pots being same).