I loved making these and was tickled pink when they came out of the kiln looking like little sweeties yesterday morning. This morning I spotted and incompatibly crack in one of the hearts - literally heart breaking.
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.
These rainbow rounds came out of the kiln looking perfect this morning. I made them before the first crack appeared on the hearts and just a few hours later 3 now show small fractures internally too.
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.