I consulted Museums Research Scientist Emeritus regarding your interest in the refractive index of Tiffany Favrile glass.Dr. Robert Brill, Research Scientist Emeritus, said:
We analyzed a fragment of Tiffany Favrile glass. It turned out to be a high-lead glass. The composition was approximately 57.3% SiO2, 38.7% PbO, 3.1% K2O, + some trace elements. I would estimate that the index should be ~1.61.
Favrile Paperweight Vase with Morning Glories,
Tiffany Furnaces, Louis Comfort Tiffany, Corona, NY,
about 1913. Gift of Mr. and Mrs. Howard Stein. 97.4.125