Strange Soldering

I had an electrician come to the house here in Colombia to run some new lines. Among other things he needed to solder a #8 ground cable to a bronze post. He wrapped the stripped end of the cable around the post then pulled out a small hinged box made of I don´t know what and configured it around the joint stopping up gaps with mud. Into the top of the box he poured a vial full of some kind of powder. This he ignighted with a fireworks sparkler (it was the only available heat source hot enough to ignite the power). The powder burned furiously for some seconds and when he opened the box the cable looked like it had been brazed to the post with brazing rod. Anyone seen this?