What was this "homemade firework?"

So, when I was a Cub Scout, we once made “firestarters” (newspaper dipped in wax) to use on a camping trip. One particular kid waited until the fire was roaring before he tossed his in. Somewhere between two and ten minutes later, a column of fire blasted out of the fire.

It wasn’t particularly dangerous looking – basically a column of sparks that shot up about ten to fifteen feet. I don’t remember any particular noise.

He told us later he’d “doctored” his firestarter with some household additives. He mentioned, if I recall, dish soap or detergent and baking soda specifically. He insisted he wasn’t sure anything would happen which (to my mind) means he didn’t just stick gunpowder or firecrackers in there. It also didn’t behave the way I’d assume a packet of gunpowder or premade fireworks or firecrackers to work.

Taking him at his word that it wasn’t just a “normal” explosive wrapped in newspaper, anybody have any guess what he could have stumbled upon? Like it said, I don’t recall it being a particularly forceful explosion, just enough to scatter a bunch of sparks about ten feet.