Longman tells me that melt is a regular verb, and molten is simply an adjective. But in the irregular verb appendix of the Shanghai High School English Exam Vocabulary word book, molten is the past participle of melt, which leads me into confusion.
How would you explain this? Are there differences between molten and melted?
Molten was formerly a past participle of melt: (the OED says "Middle English– molten (now archaic)".
In modern use it is only an adjective.
'Molten' is not the past participle of 'melt' in modern English, although that may be the etymology of the word. It now describes a state of matter, namely that it has been heated to very high temperatures to turn a solid into liquid state.
It is only really noteworthy when a substance is not in its usual state. For example, people tend to expect nitrogen to be a gas, so when it is in liquid state we nearly always refer to it as 'liquid nitrogen'. Likewise, 'molten metal' or 'molten rock' are noteworthy because they are normally solid.
This is somewhat in contrast with the way 'melted' is used. When you note that something has melted, it is usually because it was solid when frozen or refrigerated, but subsequently thawed into a liquid in warmer temperatures. 'Molten' is more likely to refer to something that remains a solid even in warm temperatures and must be deliberately subjected to very intense heat to change its state of matter to a liquid. Nobody would ever say 'molten ice' - they would say it had melted. In cooking, we do refer to butter melted in a pan as 'melted butter' but this isn't really an exception - it doesn't require extreme temperatures to cause butter to melt; in fact it can often melt itself at some room temperatures.
Normally, only viscous liquids with a very high melting-point are molten, especially when something that’s normally very solid gets so hot that it changes and becomes hazardous, like molten rock, molten iron or molten glass. Discussions of nuclear accidents often talk about “the molten core.” We never say *molten water, *molten ice, or even *molten butter. (Edit: Michael Harvey found a news story from British Columbia that referred to “molten butter used as a weapon” in Canadian prisons. I stand corrected, but I think this illustrates the point that something molten is dangerous.)
One important way that molten is like an adjective and melted is a past participle: something that melts and then cools and becomes solid again still is/has been melted, but is no longer molten.
The word is sometimes used in literary or poetic ways. For example, English translations of the Bible call a bronze basin filled with water that once stood in a temple in Jerusalem “the Molten Sea” (translating a Hebrew term that means something like, “sea of cast metal”).
