hi Martine,
here's another example of the Nigerian Coral (Ateyun) beads. I think these are indeed made with powderglas, but mixed with water and maybe honey, arabic gum or something else a little bit sticky (wet core technique). I think the cylindrical ones on this photo are made in a horizontal clay mold. Every bead in this necklace has on one side a stain of white glass and on the opposite some scars impressed with a small stick before baking.
What surprised me is the red color, over the years it didn't change much. Maybe the Yoruba used a colorant what was easy available locally over the years.
The beads from your picture could be made "freehand"

best wishes Hans
btw are you visiting "the Duif" on 28 september this year?