# Like the glaze overlaying an earthen vessel, so are burning lips and an evil heart
"People who have burning lips and an evil heart are like the shiny glaze that covers a clay pot." A clay pot is cheap and common. So people covered it was a glaze to make it shiny and appear more expensive.
# so are burning lips and an evil heart
"so is a person who says nice things but whose heart is evil"
# burning lips
"emotional speech" or "saying nice things"
# an evil heart
"evil thoughts" or "evil desires"