The famous ones that go for millions are, yes. But not all. Sorry this is off topic and has nothing to do with Star Wars but perhaps some here who are interested in art will find this story interesting....Rothko's originals are mammoths...
When Mr Rothkowitz, the name he originally went by, was a young struggling artist, he needed a place to stay. My grandfather's family took him in. Rothko couldn't always come up with cash for rent, so sometimes paid my grandfather's family in artwork. My grandfather kept these pieces and a few of them were passed down to me. I actually own three very early original charcoal Rothkos. About a year ago I had a research associate from the National Gallery of Art come to my house with a professional photographer to take shots of these charcoals for inclusion in an upcoming Mark Rothko catalogue raisonné. Here are some pics I took of them myself. As you see, a far cry from the monstrous, abstract, color blocks he became known for later in his career.
They have been given the titles: Harlem Street from Morningside Heights, Old Elevated Arch, 125th Street, and Central Park Scene
I've since taken the drawings out of the ancient frames (which were probably speeding up the yellowing of the artwork) and put them in archival paper folders. I don't think these will fetch big bucks when I eventually sell but they are an interesting piece of art history.