Sounds like the mess you might make on a table top when playing with dangerous chemicals...(caustic)... but its not. Its a modern take on the ancient art of painting with melted, coloured wax. Hundreds of years ago icons and important or political figures figures were painted with pigmented beeswax heated over a flame or in a little 'kettle'. This one is from the Roman era.
Nowadays its much easier and more high tech, using a little iron, similar to a travel iron but staying at a constant temperature rather than going up and down, or with a heated stylus. The artworks are rarely as detailed and perfect as they were then, but some people get remarkable pictures from it.
I saw it at a craft show over 20 years ago and, like most craft, had to have a go and got hooked for a while, buying all the kit and taking a day course in different techniques.
These are some of the pictures I managed to do on the training day:
Japanese water, grasses and rocks,
a landscape with sky, hills, lake and foliage at the front,
and a waterfall.
There are loads of examples on Google images and the like. I'll show you some of the others I did another time.