from the article, it seems to be some sort of adhesive tape over a painted surface.

> Part of the problem is that unlike most of Mondrian’s earlier works, New York City I does not bear the artist’s signature, possibly because he hadn’t deemed it finished.

> In spite of all the evidence pointing to the work being currently displayed upside down, the work will be shown the way it has hung for 75 years in the new Mondrian. Evolution show that opens in Düsseldorf on Saturday.

> “The adhesive tapes are already extremely loose and hanging by a thread,” Meyer-Büser said. “If you were to turn it upside down now, gravity would pull it into another direction. And it’s now part of the work’s story