“He was such a whirlwind of energy and excitement and enthusiasm, he was like a magnet, drawing the most talented young people around him just to be in his presence to learn,” Mr. Moore said. “But he’s a shell of the man that he was — it breaks my heart.”

In the lawsuit, where pretrial motions are underway, Mr. Moi said the level of Mr. Chihuly’s disabilities were never disclosed to art buyers or the public and that Chihuly Studio often intimated that Mr. Chihuly’s paintings were entirely by his own hand. Other legal cases in recent years involving Mr. Chihuly and his former employees — him suing them or vice versa — were settled out of court, but those disputes could be dredged up again in depositions or testimony as the case goes forward.

“For years Leslie Chihuly and Chihuly Studio have undertaken efforts to hide Dale’s struggles with mental health and his inability to work on a daily basis, not to protect him, but to ensure that the cash cow known as ‘Chihuly’ continued to moo,” Mr. Moi’s suit says.

Mr. Chihuly, who said he now rarely paints for more than an hour or two at a time, perhaps three days a week, was working on a recent morning, surrounded by four assistants. One handed him a brush, then held the paint container at his elbow as he stood over a horizontal glass sheet, partly painted already with specially formulated enamel, composed of ground glass suspended in liquid.

“Do you want one over the other, or do you want it side by side?” Mr. Chihuly turned to ask an assistant, Jodie Nelson, referring to the blotched paint dobs that he was about to apply.

Ms. Nelson’s response was immediate: “I want what you want.”

Mr. Chihuly then proceeded to paint, in sweeping, fast brush strokes as a Bob Dylan song played in the background. The goal, he said, was to approximate, but not fully duplicate, two other glass painted images that would then be put together, fired and then lit for display, creating an illusion of three dimensions, called “Glass on Glass.” The design is still new — only displayed for the first time recently at Crystal Bridges Museum of American Art in Bentonville, Arkansas. During a pause, he gestured to one of the glass layer paintings hanging on back wall. “I rejected that one this morning,” he said. “I don’t like the way it looks.”