Passage Summary: There's a painting that looks exactly like a famous artist's work. Experts say it's either by him or his students. Because it uses a paint color he's never been known to use, the author concludes he couldn't have painted it.

Conclusion: The painting Nightbird was definitely created by one of Larocque's students rather than Larocque himself.

Reasoning: The painting contains a specific pigment called orpiment, which has never been seen in any of Larocque's known works.

Analysis: The argument relies on the assumption that because Larocque hasn't been caught using orpiment before, he never used it at all. To weaken this, we should look for evidence that Larocque might have actually used orpiment, perhaps in works that haven't been analyzed yet or during a specific period of his career. The author treats a lack of evidence (we haven't found it in his work yet) as evidence of absence (he never used it). Any information suggesting the artist's palette was broader than currently documented would undermine the conclusion.