Thermite, a mixture of powdered iron oxides and aluminium, can be burned to produce temperatures it would be hard to argue are truly safe outside of a meticulously controlled environment (over 4,000 degrees Fahrenheit / 2,200 degrees Celsius). It's used for a variety of purposes, from metal cutting and welding to military incendiaries.
It can also be used, apparently, to very quickly whip yourself up a sword (albeit not necessarily a very good one).
Kevin Kohler, the Backyard Scientist famous and slightly notorious on YouTube for experiments like building a molten-metal squirt gun and dropping cans of compressed air into boiling water, decided to use this extremely dangerous substance in conjunction with some molds to immediately melt steel into some form of scimitar. To Kohler's credit, after one failed round in which the thermite terrifyingly spurted out of the sides of his mould, and a subsequent round involving "cutting, grinding, welding and polishing" plus heat treatment, the resulting blade was capable of cutting through some cans.
As he also admitted, it was heavy, did not appear to have an ideal physical composition and its cutting edge was pitted through, so this is probably not something you would want to use on a battlefield.
Please don't try this at home, but if you do, don't wear shorts.