We often get questions about paint on glass, and here's a bit of info for the curious.
Glass paint comes in powder form until diluted with water or glycerin. At that point it becomes ink-like in consistency until it dries back into a powder. It's painted on when wet, and then fired in a kiln after it's dry. While it's dry, it's easily smudged and wiped off of the glass, so it's imperative that it's handled carefully so as to not ruin the work.
And while working with it, one needs to wear a respirator as the paint powder is a hazardous material! Here's Vasile demonstrating proper safety protocol and mastery.