About 10 minutes in, he used some pretty blue liquid to "blacken" some bolts he made. What would that be? Something like bluing you might use on a gun?
"Stump remover. You can use this to refinish firearms in a process called nitre bluing (hot bluing). The main ingredient in this particular product is potassium nitrate (KNO₃) which you melt and submerge the parts you want to blue."...
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