Heating method is the only way to solve it. The only problem is the media used to heat it up.
If the thermometer is somewhere lower than 100degree Celcius, you can use boiling water to achieve it.
In case you have some high temp thermometer, you have to use different media to heat it up.
Glycerin (pure, 290'C) or cooking oil (i.e. olive oil, approx 250'C or higher, but it may has reached burning point) can do the work.
IF you have a thermometer higher than 300-350'C, you'll need special media to get that hot. Sorry I forgot the name of the oil (wax), if you need the name, let me know. I'll have to check it out.
Be aware of the cooling steps. The higher temp, the shorter reacting time to remove it from boiling oil.
You may try din-in (long time) & out (short time) to raise the temp gradually until "splited part" "unsplited".
ALSO, the higher temp, the longer, smoother cooling needed. You can also use dip-in (shorter time) - out(longer time) until it's lower than 100 degree'C. Fast cooling will cause craking.
Good luck.