What medium is "Roundel with the Crucifixion, the Virgin, and Saint John"?

Roundel with the Crucifixion, the Virgin, and Saint John is Colorless glass, silver stain, vitreous paint.

Is "Roundel with the Crucifixion, the Virgin, and Saint John" in the public domain?

Yes, Roundel with the Crucifixion, the Virgin, and Saint John is in the public domain and available for free download under CC0 license.

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This artwork was created ca. 1525.

Unknown Artist, ca. 1525