Ed Bush of Painted Post, NY adds the following information

After Marshall’s excellent organization scheme for Carder’s white glasses I hesitate to add to the discussion. However, there is a simple test that in some cases may be useful in identifying an unknown white glass. Carder’s white glasses are opacified either by a fluoride (alabaster, ivrene) or a phosphate (calcite, ivory, flint white).

If the filament of an incandescent unfrosted light bulb is viewed through thin parts of white glass articles, such as the foot of a goblet, the effect is different depending on the opacifier. With a glass opacified by phosphate the filament is red. With a glass opacified with fluoride the illuminated glass appears uniformly white, and the filament is difficult to see. When it is visible, the filament is white.

I have seen poorly opacified flint white articles that resembled alabaster, and the test described above would instantly tell which white glass it was. One should use a low-wattage bulb to avoid heating the article and to protect your eyes. Minimize the time looking at the filament.
P.S. – It is safer to wear dark glasses when conducting the test.

Jim Sutterfield of Corning responds to the use of his name in Rande Bly’s comments.
I see my name on several recent gazette. Let me add I have the common finger bowl and underplate in opalescent yellow. I think Marshall calls them also #2680.

Finally, please to hear from Michael Krumme of Los Angeles who reacts to the discussion thread on whites
I have REALLY been enjoying getting my Gazelle each day. I have learned so much, already. The discussion on the whites was especially illuminating.
Yet, more to come.