The colorant that made glass white in the 15th/16th Cs was tin oxide. Milk glass (lattimo), colored with tin oxide, was devised by Angelo Baroviero—who was (most likely) the inventor of rosetta (chevron) beads; and who devised the Venetian strategy of making molded canes for rosetta work (including millefiori beads). Lattimo was also used by Baroviero to create white vessels that were designed to resemble white Chinese porcelain—that was a fad among Venetians and other Europeans. Angelo was celebrated for his excellent chemical breakthroughs, and the color pallet he devised, that produced glasses of different colors that were compatible with each other—all of which are seen in early chevron beads and millefiori beads.

Note the reference to tin oxide under Colored Inclusions here:

https://en.wikipedia.org/wiki/Glass_coloring_and_color_marking

Fluorine-white glass is a rather more-modern composition.

Unless it is demonstrated to be otherwise, I will continue to understand that some early white glasses (such as those used to make white layers in rosetta beads) decay in the manner seen in the beads being discussed. This having become spotty and grayish. Or, presenting white spots on a more-gray ground. And, I have discussed most of these topics numerous times, here and elsewhere. I'm using the current links to support information I understand from research I began in 1980. At the moment, I don't have time to go through my copious library and cite old sources.

JDA.