i have some of these yellow glass round cushion-shaped beads that i got in the 1970s and their most distinctive feature is their heavy weight, compared with other opaque amber yellow coloured glass beads...

I saved this image from BCN about 6 years ago. My notes say that this is from the Levin & Co.(barely visible under the title on the card), and dates from 1850s - 1870s. You can see one large yellow disk bead with rounded edges on the bottom strand.

The Picard exhibit on Pre-1900 Sample Cards show a good number of monochrome beads but only the more common tapered-edge yellow disk beads.

Because of the discussion about the use of uranium glass - I think I'll check my similar "greasy yellow" large disks to see if they were colored with uranium salts, since as was pointed out, the density on these is very high.

None of these beads show any readings of radioactivity above background.

Conclusion - high lead content glass was used. The highest lead content glass is over twice as dense as soda glass, and almost twice as dense as "typical lead crystal". Because these disks are heavy, I've had a hard time using any more than a few in necklaces - otherwise the final result is a neck strain!

From Wikipedia:
The density of soda glass is 2.4 g/cm3 or below, while typical lead crystal has a density of around 3.1 g/cm3 and high-lead glass can be over 4.0 g/cm3 or even up to 5.9 g/cm3.

Lead glass can contain 18% up to 40% by weight lead oxide.

A rough density measurement on one of the largest disks in this photo was 3.4 g/cc. For comparison, I did a cylindrical Venetian fancy bead (2.7 g/cc) and a Hebron disk bead (2.3 g/cc). These values are approximate but shows that the Middle Eastern Hebron glass probably had no lead content, while the Venetian "regular" glass had some. Several bead people have told me that they can judge by feel the higher density of Venetian beads which is one way they decide if they are Venetian.

If you read our book, Wild Beads of Africa, you will see that we propose that these high-lead yellow disk beads may have been the source for yellow glass that composes the exteriors of many old powderglass beads from Ghana. The high lead content would cause the glass to melt at a lower temperature, and would thus be a good source for making a thin exterior layer on a bead. JDA.

They are on several Murano museum sample cards in "A world in a bead" by Augusto Panini.
We have them also on different cards we photographed in the 1980s in that museum.
JP

Re. JP's post of sample cards, and Jamey's "Wild Beads" fig. 8, here is what I have in used beads from the Africa trade. Because of the different shapes, cuts, color variants, and surface luminosities, I had always wondered if these were the same basic bead. The unusual weight and the sample card showing varieties seems to clinch this. Apparently this was quite a popular trade bead in its day, and present scarcity may be due to their use in creating new beads; perhaps the filed-down ones being remnants after material was ground off. Thanks, everyone!