I think the 'art' aspect is a red herring. What if we substitute this scenario, taking the 'art' aspect out of it?
My nephew got one of these when he started college: http://www.livescribe.com/en-us/
It's a really cool pen thingie that has a voice recorder built in. You use special paper and if you go to a certain section of your notes, the voice recorder will play what the instructor was saying in class at the time you wrote that section of the notes.
These things aren't cheap, and there are other consumables that you have to buy, like the paper. But he got it b/c he and his parents thought it would help him in his college courses, so they thought the investment was worth it.
So let's say he's taking a class with an old friend from high school. Friend sees this funky pen and asks him about it. Nephew tells him about how it works, etc. They continue to hang out and attend class for a couple of months. Then come midterms, and the friend just assumes that Nephew will hand over his notes and voice recordings of the lectures so friend can study from them. See, friend still attended class, but never took his own notes after he saw Nephew's cool new tool; he just assumed that Nephew would 'share' his notes because, you know, they're friends. The classmate was still at the lectures, but Nephew had notes PLUS the voice recordings. And it's not like it would cost Nephew any money to share, so why shouldn't Nephew share?
Well, to me, I wouldn't want to give my notes and recordings to the friend. Friend had ample opportunity to take his own notes. Sure, he didn't have the cool tool with the voice recorder, but he could have had it if he'd bought one for himself. Nephew made the monetary investment in this tool, and he invested the time to learn how to use it effectively. Just because Nephew has "better" notes, does that mean that he's obligated to give everything to the friend, just because he has the cool tool?
I think not. The keys, I think, are the expectations by the friend that he could use Nephew's stuff and never even asked, and because of that assumption, the fact that the friend did nothing to help himself. No, it costs nothing for Nephew to let the friend use his notes and tool. But I don't think he has any obligation to do that whatsoever.
Art or not, just because one person might invest the time and money in some equipment, it doesn't make the output from all of that into community property for all and sundry, even if they may have happened to have shared an event (vacation, class, etc.) with that person when he/she used the equipment.