The wax is melted in the caldron you see above the vat, liquefied and brought to a boiling point so that it will distribute itself evenly over the body.
12 seconds sound clip from the House of Wax (1953) movie soundboard.
You can hear this line at 00:41:26 in the Blu-ray version of the movie.
Quote context
[...]
- Here we are. Interesting, isn't it?
- One of my pupils is about to dip a body into a bath of wax.
- Yes. The bodies are constructed separately to exact specifications, of reinforced plaster of Paris.
- Oh, uh, this is Leon, another one of my pupils. Mr. Wallace.
- I often wondered how that was done.
[...]