This was an a/b test to see if a potential alternative oxidizer (Na₂S₂O₅) would work as well or better.

We made two copies of the ball ground mixture (test 3), one the same as before and the other with Na₂S₂O₅. They were randomly labeled "A" and "B" and the people watching were asked to rate them without knowing which was which.

This turned out to be pretty easy since the KNO₃ mixture worked as well as it had in test 4, and the Na₂S₂O₅ mixture didn't even light.

So what went wrong? It turns out that, while both sodium metabisulfite and potassium nitrate are used as stump removers, the mechanism is totally different. Sodium metabisulfite doesn't promote decay / oxidation, it breaks down the protein that holds wood together. It's actually an antioxidant. That will teach us to read more than the material safety data sheets when reasoning by analogy!