Small beads keep Cracking... but only with a particular design?!

[Dmclax]

Usually I wouldn't make a post about beads cracking as I have read so much about what can cause cracks....
but with this problem, I am very confused!

I have been making some glass studs, using steel stems so I can melt the glass directly on to the metal and this has been really successful apart from with one design. I am using a transparent amber, with a transparent red dot on top and then on top of that is a dot from a goldstone stringer, the glass is then heated and pushed gently so it resembles a disc shape. I assumed it was cooling to quickly but my other studs have been okay AND I have already made one of these studs and I just need to make the other one. The shape is not to thin and I am flame annealing and then putting the glass straight into vermiculite. The crack is horizontal running through the centre of the bead.

Would could be causing it??

[Kaz]

It sounds like thermal shock - if you are using vermiculite and annealing later, the steel of the post will draw the heat out of the glass causing it to crack. It seems you have probably been lucky with the others. They would most likely survive if you were able to place them straight into the kiln.
Kazx

[Dmclax]

Thanks for the response 
So excluding the kiln, because I usually batch anneal, is there a way to avoid the thermal shock?

[Kaz]

You could try heating the vermiculite but I think others on here have had the same problem with the studs and cooling in vermiculite.
Kazx