You can't reliably test the age of paint. Knowing that gives you the unfair advantage of being able to fake things at will.

Frankly, finding sketches at his house is less damaging than the texts. He sketched what he saw, and then tried to reproduce it on rock. That's a charitable view, though why he'd practice with rocks and not notebooks is a mystery. He'd have to know they'd be found. Notebooks can disappear in the fireplace, schist can't. Strange.