heat and hold my lotions a double boiler instead of using a microwave to melt our ingredients. There are a few reasons.
I heat my ingredients for lotions in two separate containers: One for the heated water phase and one for the heated oil phase. I heat them to 70˚C for 20 minutes, then remove them and mix well. I heat and hold them because this is how it works best for me. It ensures all the ingredients are well melted - stearic acid melts at 69˚C, for instance - and it ensures that when I combine them, they'll emulsify well.
Remember that a good emulsion requires three things: heat, mixing, and chemistry. The chemistry comes from using a good, all-in-one emulsifier at the proper rate. The mixing comes from your stick blender or hand mixer after you've combined the two phases. The big thing is the heat. When you heat everything up to melted in a microwave, have you added enough heat to get it to emulsify?
Maybe, maybe not. When I teach classes, we don't have time for heat and hold. We heat the oil phase in the microwave until they're at 70˚C, use water that has previously boiled, and mix like silly. We use recipes I have made many many times in the last nine years, and almost once per class, we have a lotion fail with a well written, tested recipe. The only real difference is the heat.
A seasoned participant in my class yesterday heated her oil phase in the microwave. After 45 seconds, the BTMS-50 and cetyl alcohol hadn't melted. After another 30 seconds, they were at 45˚C - too cold. After another 30 seconds, it was boiling and melted the plastic container and tablecloth. Thank goodness we were all so aware of the situation that no one was hurt! Boiling oil on your skin would have meant a hospital trip!
I know there are loads of you out there who are successfully using a microwave to make your products, and I don't want to dismiss those experiences, but these extreme temperature shifts simply don't happen with a double boiler, and I would hate to suggest something that could put you in peril!
On this blog, I'll always suggest you heat and hold your lotions in a double boiler because that's what works for me when I'm creating these recipes in my workshop. If you choose to do differently, the recipes might not work out the same way. And I would hate to see another picture like the one I just posted again!