Hi SP - Yes it's certainly internal stress. This is usually due to how the casting freezes directionally. It freezes from the outside in and creates internal tension and compression due to the shrinkage of the wax over time and distance. Same happens with metal castings. My wife casts beeswax and it freezes slowly when cast in big blocks. Stress relief is usually done in metals by bringing it up to a softening temperature and holding to allow the weakened material to stretch and settle. The issue with wax is thats its an insulator so by the time the middle is warm the outside has melted. Maybe a microwave oven is the go as this will heat from the inside out but it would be touch and go on melting... We melt beeswax in a uwave and it melts from the inside out like a slab of butter nearly same thing. Good luck - Peter
Aluminium plates can warp after machining. Tooling grade plates are stress relieved thermally or stretched. Some rolled plate grades like 5083-H32 can warp due to the internal stresses due to its rolling creating stressed skins on the outside of the plate. 6061-T6 being quenched and aged will have little internal stress. If you over constrain billets when blocking they can warp, then you have to correct them or stress relieve them. All interesting stuff...
In metals the usual rule of thumb for stress relief temp is half the melting temp so steel melts at ~1400C so roughly SR=700C, 650C is a typical SR temp. So if this holds for wax "fileawax" melts at about 116C so 58C is the go? In the kitchen oven? We melt beeswax at 60-65degC...
soak time for metal is 1hr per inch thick, I expect wax to be more than this as its less conductive...