Melted is the past tense and past participle of the verb to melt. For example, we say something melted yesterday, that something has melted in the sun, and that the thing that was left in the sun is melted.
Molten is another participial adjective derived from melt, but in today’s English it is used primarily in reference to melted metals and minerals. And even in reference to these things, melted is often used as the past tense. For example, we might write the molten copper melted yesterday.
The piles of snow brought on by a bad winter storm a few weeks ago have melted due to unseasonably warm temperatures.[St. Louis Post-Dispatch]
The riot police have melted into the background and a number of prisoners have been released with a royal pardon. [letter to BBC News]
Place over hot, not simmering, water and stir constantly until 3/4 of the chocolate has melted. [Vancouver Sun]
Molten sulfur is used to make sulfuric acid and to bleach wood pulp for paper manufacturing. [Roanoke Times]
Indeed, the most tangible reminder of the molten metal that was the city’s lifeblood is the name of Pittsburgh’s indecently successful NFL team, the Steelers. [Independent]
The wax is then melted and drained away, leaving the mold open to be filled with molten sculpture material. [Stuff.co.nz]