They were made over a core of CiM Peace with tiny wraps of bog standard Effetre colours over the top and encased with 006. The cracking is internal, partial and only over the "hot" colours.
So a few hours of pouring over internet and I found a good lot of interesting info on Lampwork Etc which talks about Cadmium based colours being inconsistent under encasement.
So some things to try - making a core of clear glass under white before layering on the rainbow or making the rainbow directly over a core of clear. Or making a similar design using Bullseye glass which is CoE 90.....update coming soon.
Have a super evening, Jolene x