To a Verdigris

From Publishers Weekly's interview with Frances Hardinge, author of Fly by Night:

I read that when you were six, you wrote a short story that included an attempted poisoning, a faked death, and a villain being thrown off a cliff--all in just one page! Would you say that your style has changed since then?

Yes. I have clearly become less concise.

You’ve also written a book called Verdigris Deep that was published in the U.K. last year. Is this a different book than Well Witched, or the same? If the same, why the title change?

Yes, Well Witched and Verdigris Deep are indeed the same book. My U.S. publisher changed the title because they felt that “verdigris” was too difficult and unfamiliar a word for many American readers, including adults.

If only Americans had continued spelling the word "verdigrease," as I've seen it rendered in eighteenth-century newspapers, we might be able to puzzle it out. "Verd" as in "verdant"; "grease" as in slimy stuff.

Then again, we Americans may not have to feel inferior to the Brits. All the copy I've seen from the UK publisher, such as this webpage, includes a helpful definition of "verdigris."