Quoted: Said Daniel Weix, a chemistry professor at the University of Wisconsin: “Butane lighters don’t burn very cleanly. Like a candle, they will deposit soot onto things.”
And the ice can melt without generating visible droplets.
“The ice can go directly to the vapor phase and so the person holding the flame does not see it,” he said. “Another explanation is that the melting part that becomes water is drawn into the porous snowball – much like the syrup that is poured onto a snow cone that one enjoys in the summer – so you don’t see it.”