Hard to imagine your solvent will get hot enough to ruin the MDMA especially when you consider it's refluxed in production for hours and the solvents you use will have a low BP
There's a couple methods, typically only after a wash. Most common and simple being IPA or di water recrystallization by essentially seeding the solution and very, very slowly bringing down the temperature.
Take two jars, straight necked. Put the pulverized, washed and dried MDMA in one (after etching the bottom to seed the crystal) and isopropyl alcohol in the other. Put the alcohol over the stove in a water bath and bring to a gentle boil. Let it cool a bit and cover the MDMA juuust enough that the MDMA can saturate. The slower the alcohol evaporates, the better crystals you'll have. Best to slowly work it down to room temperature then eventually put it in the fridge.
Once it's done you'll have fat, clear crystals that you place in a coffee filter and pour cold acetone over as a final step. Let the crystals dry and voilà.
Fat crystals might look good but aren't always the purest due to contaminents getting stuck in the crystal structure especially if you're doing a final acetone wash. If you want fat crystals make them with your purified product not before.
Must admit I've never got large crystals when I've purified MDMA but then again never aimed for them either tbf
Yeah I love the look of fat crystals but honestly powder will always be easier to work with and I don't sell anything so bag appeal isn't important to me.
But having crystal clear molly is a beautiful thing, at least for the aesthetic.
you can grow giant crystals of mdma (the hydrate) by dissolving in distilled water and letting it evaporate. adjust the size of the evaporation opening depending on how large you want to go and how long you can wait. 2 weeks will get you fat crystals.
Things like excess heat and moisture can and do damage the final product. Common impurities will be mercury, aluminum amalgam, leftover saffrole, other heavy metals. Difficult to strip everything, but your next step would be di water recrystallization.